scriptum.js

/*
      ___           ___           ___                        ___          ___           ___           ___     
     /\  \         /\  \         /\  \          ___         /\  \        /\  \         /\__\         /\__\    
    /::\  \       /::\  \       /::\  \        /\  \       /::\  \       \:\  \       /:/  /        /::|  |   
   /:/\ \  \     /:/\:\  \     /:/\:\  \       \:\  \     /:/\:\  \       \:\  \     /:/  /        /:|:|  |   
  _\:\~\ \  \   /:/  \:\  \   /::\~\:\  \      /::\__\   /::\~\:\  \      /::\  \   /:/  /  ___   /:/|:|__|__ 
 /\ \:\ \ \__\ /:/__/ \:\__\ /:/\:\ \:\__\  __/:/\/__/  /:/\:\ \:\__\    /:/\:\__\ /:/__/  /\__\ /:/ |::::\__\
 \:\ \:\ \/__/ \:\  \  \/__/ \/_|::\/:/  / /\/:/  /    /:/  \:\/:/  /   /:/  \/__/ \:\  \ /:/  / \/__/~~/:/  /
  \:\ \:\__\    \:\  \          |:|::/  /  \::/__/    /:/  / \::/  /   /:/  /       \:\  /:/  /        /:/  / 
   \:\/:/  /     \:\  \         |:|\/__/    \:\__\   /:/  /   \/__/   /:/  /         \:\/:/  /        /:/  /  
    \::/  /       \:\__\        |:|  |       \/__/   \/__/            \/__/           \::/  /        /:/  /   
     \/__/         \/__/         \|__|                                                 \/__/         \/__/    
*/




/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████
████████████████████████████ CROSS-CUTTING ASPECTS ████████████████████████████
███████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████████ CONSTANTS ██████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


const PREFIX = "$criptum_"; // avoid property name collisions


export const LT = -1;


export const EQ = 0;


export const GT = 1;


/* Used to tag functions that return a fresh instance of their respective data
type so that any later mutations on these values are not shared. */

export const NEW = Symbol("new");


export const NOOP = null; // no operation


export const NOT_FOUND = -1; // native search protocol


export const TAG = Symbol.toStringTag;


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ STATE ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* The monadic types for asynchronous computation used in this library are not
stack safe by default. However, for large asynchronous computations you can run
these types with a special method that utilizes stack safe promises. Every time
the asynchronous counter is greater than a hundred, the monadic asynchronous
type is wrapped in a native promise and the counter is reset to zero. The
counter is shared between all running asynchronous operations. */

let asyncCounter = 0; // upper bound: 100


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████ TAGGED TYPES █████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// create a tagged object with a single property

export const type = (tag, k = tag[0].toLowerCase() + tag.slice(1)) => arg => {
  return Object.defineProperties({}, {
    [TAG]: {value: tag},
    [k]: {value: arg, enumerable: true, writable: true}
  });
};


/* Create a tagged object with multiple properties. The latter are only
accessible via the type's primary property, which enhances type safety.
Usage:

const Coord = product("Coord") ("x", "y", "z");

const point = Coord(2, 3, 4); // Coord {x: 2, y: 3, z: 4}
point.coord.x; // 2 */

export const product = (tag, k = tag[0].toLowerCase() + tag.slice(1)) => (...ks) => (...vs) => {
  return Object.defineProperties({}, {
    [TAG]: {value: tag},

    [k]: {
      value: ks.reduce((acc, k2, i) => (acc[k2] = vs[i], acc), {}),
      enumerable: true,
      writable: true
    }
  });
};


// like product but the first argument is curried

export const product_ = (tag, k = tag[0].toLowerCase() + tag.slice(1)) => (k, ...ks) => (...vs) => v => {
  return Object.defineProperties({}, {
    [TAG]: {value: tag},

    [k]: {
      value: ks.reduce((acc, k2, i) => (acc[k2] = vs[i], acc), {k: v}),
      enumerable: true,
      writable: true
    }
  });
};


// binary product

export const product2 = (tag, k = tag[0].toLowerCase() + tag.slice(1)) => (x, y) => {
  return Object.defineProperties({}, {
    [TAG]: {value: tag},

    [k]: {
      value: Pair(x, y),
      enumerable: true,
      writable: true
    }
  });
};


// ternary product

export const product3 = (tag, k = tag[0].toLowerCase() + tag.slice(1)) => (x, y, z) => {
  return Object.defineProperties({}, {
    [TAG]: {value: tag},

    [k]: {
      value: Triple(x, y, z),
      enumerable: true,
      writable: true
    }
  });
};


/* Create a tagged union type that can instantiate values in different shapes.
Each tagged union has its individual root property where it exposes its values.
The underlying idea is the simple creation of "sum of product" types, i.e. where
tagged unions (sums) contain product types. Usage:

const Option = variant("Option", "opt") (nullary("None"), unary("Some"));

const tx = Option.Some(2), ty = Option.None;

tx.opt.val // 2
tx.opt.tag // "some"

tx.opt.run({some: x => x + 1, none: 0}); // 3
ty.opt.run({some: x => x + 1, none: 0}); // 0

switch (tx.opt.tag) {
  case: "some": {...}
  case: "none": {...}
} */

export const variant = (tag, k = tag[0].toLowerCase() + tag.slice(1)) => (...cases) => {
  return cases.reduce((acc, _case) => {
    acc[_case.name] = _case(tag, k);
    return acc;
  }, {});
};


// nullary constructor (constant)

export const nullary = (_case, k = _case[0].toLowerCase() + _case.slice(1)) => {
  return ({
    [_case]: (tag, k2) => {
      return Object.defineProperties({}, {
        [TAG]: {value: tag},

        [k2]: {
          value: {
            run: ({[_case]: x}) => x,
            val: null,
            tag: k
          },

          enumerable: true,
          writable: true
        }
      });
    }
  }) [_case];
};


// unary constructor

export const unary = (_case, k = _case[0].toLowerCase() + _case.slice(1)) => {
  return ({
    [_case]: (tag, k2) => x => {
      return Object.defineProperties({}, {
        [TAG]: {value: tag},

        [k2]: {
          value: {
            run: ({[_case]: f}) => f(x),
            val: x,
            tag: k
          },

          enumerable: true,
          writable: true
        }
      });
    }
  }) [_case];
};


// binary constructor

export const binary = (_case, k = _case[0].toLowerCase() + _case.slice(1)) => {
  return ({
    [_case]: (tag, k2) => x => y => {
      return Object.defineProperties({}, {
        [TAG]: {value: tag},

        [k2]: {
          value: {
            run: ({[_case]: f}) => f(x) (y),
            val: Pair(x, y),
            tag: k
          },

          enumerable: true,
          writable: true
        }
      });
    }
  }) [_case];
};


export const binary_ = (_case, k = _case[0].toLowerCase() + _case.slice(1)) => {
  return ({
    [_case]: (tag, k2) => (x, y) => {
      return Object.defineProperties({}, {
        [TAG]: {value: tag},

        [k2]: {
          value: {
            run: ({[_case]: f}) => f(x, y),
            val: Pair(x, y),
            tag: k
          },

          enumerable: true,
          writable: true
        }
      });
    }
  }) [_case];
};


// variadic constructor

export const variadic = (_case, k = _case[0].toLowerCase() + _case.slice(1)) => {
  return ({
    [_case]: (tag, k2) => (...args) => {
      return Object.defineProperties({}, {
        [TAG]: {value: tag},

        [k2]: {
          value: {
            run: ({[_case]: f}) => f(...args),
            val: args,
            tag: k
          },

          enumerable: true,
          writable: true
        }
      });
    }
  }) [_case];
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ EFFECTS ███████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// effectively handle effects in a dynamically typed environment


export const Eff = {};


/*
█████ Composition █████████████████████████████████████████████████████████████*/


// functor

Eff.comp = (dict, dict2) => f => ttx => dict.map(dict2.map(f)) (ttx);


// applicative functor

Eff.compA = (dict, dict2) => ttf => ttx =>
  dict.ap(dict.map(dict2.ap) (ttf)) (ttx);


/* Monads cannot be composed in a general way. Use monad transformers instead
or implement your own specific monad composition that suits your needs. */


// Kleisli composition

Eff.komp = dict => fm => gm => x => dict.chain(fm(x)) (gm);


/*
█████ Foldable ████████████████████████████████████████████████████████████████*/


// eliminiate the effect


Eff.Fold = {};


Eff.Fold.array = () => A.foldl;


Eff.Fold.async = tx => tx(id);


Eff.Fold.bottom = tx => {
  if (tx === undefined) throw new Err("received undefined");
  else return tx;
};


Eff.Fold.defer = thunk => thunk();


Eff.Fold.either = tx => tx.either.val


Eff.Fold.except = tx => {
  if (tx?.constructor?.name === "Exception") throw tx;
  else return tx;
};


Eff.Fold.lazy = tx => strict(tx);


Eff.Fold.list = () => L.foldl;


Eff.Fold.option = x => tx => tx === null ? x : tx;


Eff.Fold.tramp = tx => Tramp(tx);


/*
█████ Foldable :: Traversable █████████████████████████████████████████████████*/


/* Sequence effects in a mechanistically applicative way. Usage:

Eff.Trav.array(Eff.dict.either)
  (x => x & 1 === 1 ? Either.Left(x) : Either.Right(x * x))
    ([2,4,6]); // yields Right([4, 16, 36])

Eff.Trav.array(Eff.dict.either)
  (x => x & 1 === 1 ? Either.Left(x) : Either.Right(x * x))
    ([2,5,6]); // yields Left(5) */


Eff.Trav = {};


// Applicative f => (a -> f b) -> [a] -> f [b]
Eff.Trav.array = () => A.mapA;


// Applicative f => [f a] -> f [a]
Eff.Trav.arraySeq = () => A.seqA;


Eff.Trav.either = dict => ft => tx => {
  switch (tx.either.tag) {
    case "left": return dict.of(tx);

    case "right": return dict.map(x =>
      Either.Right(x)) (ft(tx.either.val));

    default: throw new Err("Either expected");
  }
};


Eff.Trav.eitherSeq = dict => ft => tx => {
  switch (tx.either.tag) {
    case "left": return dict.map(Either.Left) (tx);
    case "right": return dict.map(Either.Right) (tx);
    default: throw new Err("Either expected");
  }
};


Eff.Trav.except = dict => ft => tx =>
  tx?.constructor?.name === "Exception" ? dict.of(tx) : fm(tx);


Eff.Trav.exceptSeq = dict => tx =>
  tx?.constructor?.name === "Exception" ? dict.of(tx) : tx;


// Applicative f => (a -> f b) -> List a -> f (List b)
Eff.Trav.list = () => L.mapA;


// Applicative f => List (f a) -> f (List a)
Eff.Trav.listSeq = () => L.seqA;


Eff.Trav.option = dict => ft => tx => tx === null ? dict.of(tx) : fm(tx);


Eff.Trav.optionSeq = dict => tx => tx === null ? dict.of(tx) : tx;


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Eff.F = {};


// zero, one or several results (indeterministic)

Eff.F.array = f => xs => xs.map(f);


// asynchronous function (serially evaluated)

Eff.F.async = f => tf => k => tf(x => k(f(x)))


// bottom type immediately throws an error to avoid silent errors

Eff.F.bottom = f => tx => tx === undefined
  ? _throw("received undefined")
  : f(tx);


// either arbitrary value or valid computation (used for short circuiting)

Eff.F.either = f => tx => {
  switch (tx.either.tag) {
    case "left": return tx;
    case "right": return Either.Right(f(tx.either.val))
    default: throw new Err("Either expected");
  }
};


// defer evaluation with thunks

Eff.F.defer = f => tx => () => f(tx());


// either exception or valid computation (more restricted than either)

Eff.F.except = f => tx => tx?.constructor?.name === "Exception"
  ? tx : f(tx);


// lazy evaluation with thunks (deferred evaluation + sharing)

Eff.F.lazy = f => tx => lazy(() => f(tx));


// computations that might not yield a result

Eff.F.option = f => tx => tx === null ? tx : f(tx);


// implicit argument

Eff.F.reader = f => tx => r => f(tx(r));


// stateful computation

Eff.F.state = f => tx => s => _let(tx(s)).in(([x, s2]) => [f(x), s2]);


/* Trampoline effect to ensure stack safety. You must wrap the whole expression
into a trampoline using `Tramp.bounce` in order for it to work. */

Eff.F.tramp = f => tx => Eff.M.tramp(tx) (x => Eff.tampOf(f(x)));


// logging

Eff.F.writer = f => ([x, w]) => [f(x), w];


/*
█████ Functor :: Alternative ██████████████████████████████████████████████████*/


Eff.Alt = {};


Eff.Alt.option = tx => ty => tx === null ? ty : tx;


Eff.Alt.optionZero = null;


/*
█████ Functor :: Applicative ██████████████████████████████████████████████████*/


Eff.A = {};


Eff.A.array = tf => tx => tx.reduce((acc, x) =>
  tf.reduce((acc2, f) =>
    (acc2.push(f(x)), acc2), acc), []);


Eff.A.arrayOf = x => [x];


Eff.A.async = tf => tx => k => tf(f => tx(x => k(f(x))))


Eff.A.asyncOf = x => k => k(x);


Eff.A.bottom = tf => tx => tf === undefined
  ? _throw("received undefined")
  : tx === undefined
    ? _throw("received undefined")
    : tf(tx);


Eff.A.bottomOf = x => x === null ? _throw("unexpected null") : x;


Eff.A.defer = tf => tx => () => tf() (tx());


Eff.A.deferOf = x => () => x;


Eff.A.either = tf => tx => {
  switch (tf.either.tag) {
    case "left": return tf;

    case "right": {
      switch (tx.either.tag) {
        case "left": return tx;

        case "right": return Either.Right(
          tf.either.val(tx.either.val));

        default: throw new Err("Either expected");
      }
    }

    default: throw new Err("Either expected");
  }
};


Eff.A.eitherOf = x => Either.Right(x);


Eff.A.except = tf => tx => tf?.constructor?.name === "Exception" ? tf
  : tx?.constructor?.name === "Exception" ? tx
  : tf(tx);


Eff.A.exceptOf = x => {
  if (x?.constructor?.name === "Exception")
    throw new Err("unexpected exception");

  else return x;
};


Eff.A.lazy = tf => tx => lazy(() => tf(tx));


Eff.A.lazyOf = x => lazy(() => x);


// see list monad below

Eff.A.listOf = x => [x, []];


Eff.A.option = tf => tx => tf === null ? tf : tx === null ? tx : tf(tx);


Eff.A.optionOf = x => {
  if (x === null) throw new Err("unexpected null");
  else return x;
};


Eff.A.reader = tf => tx => r => tf(r) (tx(r));


Eff.A.readerOf = x => _ => x;


Eff.A.tramp = tf => tx => Eff.M.tramp(tf) (f =>
  Eff.M.tramp(tx) (x => Eff.A.trampOf(f(x))));


Eff.A.state = tf => tx => s => _let(tf(s)).in(([f, s2]) =>
  _let(tx(s2)).in(([x, s3]) => [f(x), s3]));


Eff.A.stateOf = x => s => [x, s];


Eff.A.trampOf = () => Tramp.return;


Eff.A.writer = dict => ([f, w]) => ([x, w2]) => [f(x), dict.append(w) (w2)];


Eff.A.writerOf = dict => x => [x, dict.empty];


/*
█████ Functor :: Applicative :: Monad █████████████████████████████████████████*/


Eff.M = {};


// lawless array monad (arrays are no ADTs because they lack the empty case)

Eff.M.array = mx => fm => mx.flatMap(fm);

/*Eff.arrayM = mx => fm => function go(acc, i) {
  if (i >= mx.length) return acc;
  else return go(A.pushn(fm(mx[i])) (acc), i + 1);
} ([], 0);*/


// even more lawless array "monad" variant that short circuits

Eff.M.array_ = mx => fm => function go(acc, i) {
  if (i >= mx.length) return acc;
  
  else {
    const my = fm(mx[i]);

    if (my?.constructor?.name === "Array")
      return go(A.pushn(my) (acc), i + 1);

    else return acc;
//       ^^^^^^^^^^ short circuit
  }
} ([], 0);


// can only be the outermost monad in a composition

Eff.M.async = mx => fm => k => mx(x => fm(x) (k));


Eff.M.bottom = mx => fm => {
  if (mx === undefined) throw new Err("received undefined");

  else {
    const my = fm(mx);
    if (my === undefined) throw new Err("returned undefined");
    else return my;
  }
};


// can only be the outermost monad in a composition

Eff.M.defer = mx => fm => () => fm(mx()) ();


Eff.M.either = mx => fm => {
  switch (mx.either.tag) {
    case "left": return mx;
    case "right": return fm(mx.either.val);
    default: throw new Err("Either expected");
  }
};


Eff.M.except = mx => fm =>
  mx?.constructor?.name === "Exception" ? mx : fm(mx);


/* Can only be the outermost monad in a composition. Usage:

const tx = Eff.T.list(Eff.dict.lazy)
  (L.fromArr([1,2,3,4,5]))
    (x => lazy(() => L.of(x * x)));

// result list not evaluated yet

L.take(3) (tx); // evaluates the first three elements and yields 9
tx[1] [0]; // evaluates nothing due to sharing and yields 4 */

Eff.M.lazy = mx => fm => lazy(() => fm(mx));


// the lawful array monad is the list monad

Eff.M.list = mx => fm => function go(my) {
  if (my.length === 0) return [];
  else return L.append(fm(my[0])) (go(my[1]));
} (mx);


Eff.M.option = mx => fm => mx === null ? mx : fm(mx);


Eff.M.reader = mx => fm => r => fm(mx(r)) (r);


Eff.M.state = mx => fm => s => _let(mx(s)).in(([x, s2]) => fm(x) (s2));


/* Can only be the outermost monad in a composition. Usage:

Tramp(Eff.T.array(Eff.dict.tramp)
  (Eff.A.trampOf(Array(1e6).fill(1)))
    (Tramp.bounce_(x => Eff.A.trampOf([x + 1]))))); // stack-safe */

Eff.M.tramp = mx => fm => {
  if (mx.constructor === Tramp.bounce)
    return Tramp.bounce(mx.x) (y => Eff.M.tramp(mx.f(y)) (fm));

  else if (mx.constructor === Tramp.return) return fm(mx.x);
  else throw new Err("invalid constructor");
};


Eff.M.writer = dict => ([x, w]) => fm => 
  _let(f(x)).in(([y, w2]) => [y, dict.append(w, w2)]);


/*
█████ Transformer █████████████████████████████████████████████████████████████*/


// compose monadic effects


Eff.T = {};


// not a lawful monad transformer, i.e. won't always behave as expected

// Monad m -> m [a] -> (a -> m [b]) -> m [b]
Eff.T.array = dict => mmx => fmm => function go(acc, i) {
  return dict.chain(mmx) (mx => {
    if (i >= mx.length) return dict.of(acc);
    else return dict.chain(fmm(mx[i])) (my => go(A.pushn(my) (acc), i + 1));
  });
} ([], 0);


Eff.T.bottom = dict => mmx => fmm => dict.chain(mmx) (mx => {
  if (mx === undefined) throw new Err("received undefined");
  else return fmm(mx);
});


Eff.T.either = dict => mmx => fmm => dict.chain(mmx) (mx => {
  switch (mx.either.tag) {
    case "left": return dict.of(mx);
    case "right": return fmm(mx.either.val);
  }  
});


Eff.T.except = dict => mmx => fmm => dict.chain(mmx) (mx => {
  if (mx?.constructor?.name === "Exception") return dict.of(mx);
  else return fmm(mx);
});


// lawful monad transformer

Eff.M.list = mx => fm => function go(my) {
  if (my.length === 0) return [];
  else return L.append(fm(my[0])) (go(my[1]));
} (mx);


// Monad m -> m (List a) -> (a -> (List b)) -> (List b)
Eff.T.list = dict => mmx => fmm => function go(mmy) {
  return dict.chain(mmy) (my => {
    if (my.length === 0) return dict.of([]);
    else return L.append(fmm(my[0])) (go(my[1]));
  });
} (mmx);


Eff.T.option = dict => mmx => fmm => dict.chain(mmx) (mx => {
  if (mx === null) return dict.of(mx);
  else return fmm(mx);
});


/*
█████ Transformer :: Foldable █████████████████████████████████████████████████*/


// eliminiate the inner effect


Eff.T.Fold = {};


// Monad m -> (a -> m b -> m b) -> m b -> m [a] -> m b
Eff.T.Fold.array = dict => fmm => acc => mmx => function go(i) {
  return dict.chain(mmx) (mx => mx.length === 0 ? acc : f(mx[i]) (go(i + 1)));
} (0);


// Monad m -> (a -> m b -> m b) -> m b -> m (List a) -> m b
Eff.T.Fold.list = dict => fmm => acc => function go(mmx) {
  return dict.chain(mmx) (mx => mx.length === 0 ? acc : fmm(mx[0]) (go(mx[1])));
};


/*
█████ Type Class Ops ██████████████████████████████████████████████████████████*/


Eff.liftA = dict => f => tx => ty => dict.ap(dict.map(f) (tx)) (ty);


// map effects but discard values

Eff.mapEff = dict => x => dict.map(_ => x);


// applicative map but discard the second result

Eff.apEff1 = dict => tx => ty => dict.ap(dict.map(_const) (tx)) (ty);


// applicative map but discard the first result

Eff.apEff2 = dict => tx => ty => dict.ap(dict.map(const_) (tx)) (ty);


/* Fold with effects:

Eff.foldM(Eff.dict.option) (acc => x =>
  x === null ? null : acc + x) (0) ([1, 2, 3, null, 5]); // yields null

Eff.foldM(Eff.dict.either) (acc => tx =>
  tx.either.tag === "left"
    ? Either.Left(acc)
    : Either.Right(acc + tx.either.val)) (0) ([
        Either.Right(1),
        Either.Right(2),
        Either.Right(3),
        Either.Left(4),
        Either.Right(5)]); // yields Left(6) */

// Monad m => (b -> a -> m b) -> b -> [a] -> m b
Eff.foldM = dict => fm => init => xs =>
  A.foldr(x => gm => acc =>
    dict.chain(fm(acc) (x)) (gm)) (dict.of) (xs) (init);


// exception

Eff.except = {};


Eff.except.catch = x => tx => {
  if (tx?.constructor?.name === "Exception") return x;
  else return tx;
};


// option

Eff.option = {};


Eff.option.throw = tx => {
  if (tx === null) throw new Err("missing value");
  else return tx;
};


/*
█████ Type Dictionaries ███████████████████████████████████████████████████████*/


Eff.dict = {};


Eff.dict.array = {
  map: Eff.F.array,
  ap: Eff.A.array,
  of: Eff.A.arrayOf,
  chain: Eff.M.array,
  get mapA() {return Eff.Trav.array},
  get seqA() {return Eff.Trav.arraySeq},
  get fold() {return Eff.Fold.array}
};


Eff.dict.async = {
  map: Eff.F.async,
  ap: Eff.A.async,
  of: Eff.A.asyncOf,
  chain: Eff.M.async,
  mapA: Eff.Trav.async,
  seqA: Eff.Trav.asyncSeq,
  fold: Eff.Fold.async
};


Eff.dict.bottom = {
  map: Eff.F.bottom,
  ap: Eff.A.bottom,
  of: Eff.A.bottomOf,
  chain: Eff.M.bottom,
  mapA: Eff.Trav.bottom,
  seqA: Eff.Trav.bottomSeq,
  fold: Eff.Fold.bottom
};


Eff.dict.defer = {
  map: Eff.F.defer,
  ap: Eff.A.defer,
  of: Eff.A.deferOf,
  chain: Eff.M.defer,
  mapA: Eff.Trav.defer,
  seqA: Eff.Trav.deferSeq,
  fold: Eff.Fold.defer
};


Eff.dict.either = {
  map: Eff.F.either,
  ap: Eff.A.either,
  of: Eff.A.eitherOf,
  chain: Eff.M.either,
  mapA: Eff.Trav.either,
  seqA: Eff.Trav.eitherSeq,
  fold: Eff.Fold.either
};


Eff.dict.except = {
  map: Eff.F.except,
  ap: Eff.A.except,
  of: Eff.A.exceptOf,
  chain: Eff.M.except,
  mapA: Eff.Trav.except,
  seqA: Eff.Trav.exceptSeq,
  fold: Eff.Fold.except
};


Eff.dict.lazy = {
  map: Eff.F.lazy,
  ap: Eff.A.lazy,
  of: Eff.A.lazyOf,
  chain: Eff.M.lazy,
  mapA: Eff.Trav.lazy,
  seqA: Eff.Trav.lazySeq,
  fold: Eff.Fold.lazy
};


Eff.dict.list = {
  map: Eff.F.list,
  ap: Eff.A.list,
  of: Eff.A.listOf,
  chain: Eff.M.list,
  get mapA() {return Eff.Trav.list},
  get seqA() {return Eff.Trav.listSeq},
  get fold() {return Eff.Fold.list}
};


Eff.dict.option = {
  map: Eff.F.option,
  ap: Eff.A.option,
  of: Eff.A.optionOf,
  chain: Eff.M.option,
  mapA: Eff.Trav.option,
  seqA: Eff.Trav.optionSeq,
  fold: Eff.Fold.option
};


Eff.dict.tramp = {
  map: Eff.F.tramp,
  ap: Eff.A.tramp,
  get of() {return Eff.A.trampOf},
  chain: Eff.M.tramp,
  mapA: Eff.Trav.tramp,
  seqA: Eff.Trav.trampSeq,
  fold: Eff.Fold.tramp
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████ ERROR/EXCEPTION ███████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const Err = Error; // shortcut


/* Exception denotes an anticipatable error that is not thrown but dynamically
handled, because you can take it into account upfront. An exception can hold a
previous one in order to accumulate all contingent exceptions during a
computation. */

export class Exception extends Error {
  constructor(s, prev = null) {
    super(s);
    this.prev = prev;
  }
};


export const Ex = Exception;


// throw as a first class expression

export const _throw = e => {
  throw e;
};


export const throw_ = e => {
  return {
    inCaseOf: p => x => {
      if (p(x)) throw e(x);
      else return x;
    },

    exceptFor: p => x => {
      if (!p(x)) throw e(x);
      else return x;
    }
  };
};


// try/catch block as an expression

export const _try = f => x => ({
  catch: handler => {
    try {return f(x)}
    catch(e) {return handler(x) (e)};
  }
});


Ex.accum = (...es) => es.reduceRight((acc, e) => {
  e.prev = acc;
  acc = e;
  return acc;
}, null);


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████ LAZY EVALUATION ███████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Lazy evaluation has the following properties:

  * evaluate only when needed
  * evaluate only as far as necessary (to WHNF)
  * evaluate at most once (sharing)

scriptum realizes lazy evaluation using implicit thunks, i.e. thunks like
`() => expr` are hidden behind a proxy and thus are called implicitly:

  * expr + x
  * expr.foo
  * expr < x

All three operations force `expr` as an implicit thunk to be evaluated. You can
also call implicit thunks as if it were explicit ones, i.e. `expr()` works even
if if the evaluation doesn't yield an unary function.

There are some limitations to this Proxy-based thunk approach:

  * `typeof` isn't intercepted by the proxy (thus yields `object`)
  * `===` doesn't trigger evaluation
  * `&&`/`||` doesn't trigger evaluation
  * `throw` doesn't trigger evaluation

These limitations are considered throughout the library. */


/*
█████ Constants ███████████████████████████████████████████████████████████████*/


const DETHUNK = PREFIX + "dethunk";


const EVAL = PREFIX + "eval";


const NULL = PREFIX + "null";


const THUNK = PREFIX + "thunk";


/*
█████ API █████████████████████████████████████████████████████████████████████*/


/* Create an implicit thunk. Just like with `typeof`, tag introspection should
not force evaluation. This isn't always possible in an untyped setting, though.
If wihtin an operation the tag for a lazy expression is known upfront, it can
be passed to the thunk through the `tag` argument. Otherwise, `null` is provided
via the `lazy` partially applied auxliliary function. In case of `null`, tag
introspection forces evaluation. When a specific tag is provided, it is up to
the user that it corresponds to the type of the eventually evaluated thunk. */

export const lazy_ = tag => thunk =>
  new Proxy(thunk, new Thunk(tag));


// evaluate an expression to weak head normal form (WHNF) provided it is a thunk

export const strict = x => {
  if (x?.[THUNK]) return x[DETHUNK];
  else return x;
};


/*
█████ Implementation Details ██████████████████████████████████████████████████*/


class Thunk {
  constructor(tag) {
    this.tag = tag;
    this.memo = NULL;
  }

  apply(f, that, args) {

    // enforce evalutation to WHNF

    if (this.memo === NULL) evaluate(this, f);

    if (typeof this.memo === "function") {
      this.memo = this.memo(...args);
      return this.memo;
    }

    // allow implicit thunks to be called explicitly

    else if (args.length === 0) return this.memo;
    else throw Err("calling a non-callable thunk");
  }

  get(f, k, p) {

    // prevent evaluation in case of introspection
    
    if (k === THUNK) return true;
    else if (k === "constructor") return Thunk;

    // enforce evaluation of a single layer

    else if (k === EVAL) {
      if (this.memo === NULL) this.memo = f();
      else if (this?.memo?.[THUNK] === true) this.memo = this.memo[EVAL];
      
      return this.memo;
    }

    // enforce evaluation to WHNF

    else if (k === DETHUNK) {
      if (this.memo === NULL) evaluate(this, f);
      return this.memo;
    }

    // avoid evaluation due to tag introspection

    else if (k === Symbol.toStringTag) {
      if (this.tag === null) {
        if (this.memo === NULL) evaluate(this, f);
        this.tag = this.memo ? this.memo[TAG] : undefined;
      }

      return this.tag;
    }

    // intercept implicit type casts

    else if (k === Symbol.toPrimitive
      || k === "valueOf"
      || k === "toString") {
        if (this.memo === NULL) evaluate(this, f);
        
        if (this.memo === null) throw new Err("implicit type cast on null");
        else if (k === "valueOf") return () => this.memo.valueOf();
        else if (k === "toString") return () => this.memo.toString();
        
        else if (k === Symbol.toPrimitive && this.memo?.[k]) 
          return () => this.memo[k] ();

        else return undefined;
    }

    // enforce evaluation to WHNF due to array context

    else if (k === Symbol.isConcatSpreadable) {
      if (this.memo === NULL) evaluate(this, f);
      if (this.memo?.[Symbol.isConcatSpreadable]) return true;
      else return false;
    }

    // enforce evaluation to WHNF due to property access

    else {
      if (this.memo === NULL) evaluate(this, f);

      // take method binding into account

      if (Object(this.memo) === this.memo 
        && this.memo[k]
        && this.memo[k].bind)
          return this.memo[k].bind(this.memo);

      else {
        const r = this.memo[k];

        if (r?.[THUNK]) {
          const x = r[DETHUNK];
          
          // replace thunk with result value

          this.memo[k] = x;
          return x;
        }

        else return r;
      }
    }
  }

  getOwnPropertyDescriptor(f, k) {

    // force evaluation to WHNF

    if (this.memo === NULL) evaluate(this, f);
    return Reflect.getOwnPropertyDescriptor(this.memo, k);
  }

  has(f, k) {

    // prevent evaluation in case of thunk introspection

    if (k === THUNK) return true;

    // enforce evaluation to WHNF

    if (this.memo === NULL) evaluate(this, f);
    return k in this.memo;
  }

  ownKeys(o) {

    // enforce evaluation to WHNF

    if (this.memo === NULL) evaluate(this, f);
    return Reflect.ownKeys(this.memo);
  }

  set(o) {
    throw new Err("set operation on immutable value");
  }
}


const evaluate = (_this, f) => {
  _this.memo = f();
  
  while (_this.memo?.[THUNK] === true) _this.memo = _this.memo[EVAL];

  // throw on undefined program state

  if (_this.memo === undefined)
    throw new Err("thunk evaluated to undefined");
  
  // enforce tag consistency

  else if (_this.tag !== null && _this.memo?.[TAG] !== _this.tag)
    throw new Err("tag argument deviates from actual value");
};


/*
█████ Resolve Deps ████████████████████████████████████████████████████████████*/


export const lazy = lazy_(null);


/*█████████████████████████████████████████████████████████████████████████████
█████████████████████████████████ MEMOIZATION █████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Memoization of recursive functions with a single argument. You can transform
them into CPS form in order to derive memoized and a non-memoized versions from
them:

  const app = f => x => f(x);

  const cpsFib = k => {
    const rec = k(n => n <= 1 ? n : rec(n - 1) + rec(n - 2));
    return rec;
  };

  const fibMemo = cpsFib(memo)
    fib = cpsFib(app);

  fib(40); // yields 102334155
  fibMemo(40); // yields 102334155 much faster

Or simply rely on reference mutation of the original recursive function with
`let fib = n => {...}; fib = memo(fib)` */


const memo = f => {
  const m = new Map();

  return x => {
    if (m.has(x)) return m.get(x);
    
    else {
      const r = f(x);
      m.set(x, r);
      return r;
    }
  };
};


// more general version where the key is derived from the value

const memo_ = f => g => {
  const m = new Map();

  return x => {
    const k = f(x);

    if (m.has(k)) return m.get(k);
    
    else {
      const y = g(x), k2 = f(y);
      m.set(k2, y);
      return y;
    }
  };
};


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████ OVERLOADED OPERATORS █████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Javascript built-in overloaded operators as functions. Some of them only
make sense with implicit/explicit thunks. For example, with `and` you would
lose the lazy evaluation property of the `&&` operator, if you pass them as
expressions not wrapped in a thunk/function. */


/*
█████ Short Circuiting ████████████████████████████████████████████████████████*/


// g might never be evaluated

export const and = f => g => f() && g();


// g might never be evaluated

export const or = f => g => f() || g();


/*
█████ Boolean Logic ███████████████████████████████████████████████████████████*/


export const between = ({lower, upper}) => x => x >= lower && x <= upper;


export const notBetween = ({lower, upper}) => x => x < lower || y > upper;


export const lt = x => y => x < y;


export const lte = x => y => x <= y;


export const eq = x => y => x === y;


/* Since `===` cannot be intercepted by proxies, implicit thunks are not forced
to WHNF. Hence the strict evaluation of operands. Works with implicit and
explicit thunks. */

export const eq_ = f => g => f() === g();


export const neq = x => y => x !== y;


/* Since `===` cannot be intercepted by proxies, implicit thunks are not forced
to WHNF. Hence the strict evaluation of operands. Works with implicit and
explicit thunks. */

export const neq_ = f => g => f() !== g();


export const gt = x => y => x > y;


export const gte = x => y => x >= y;


export const imply = ({true: t, false: f}) => x => y => {
  if (x) {
    if (y) return t;
    else return f;
  }

  else return t;
};


export const min = x => y => x <= y ? x : y;


export const max = x => y => x >= y ? x : y;


export const nand = ({t, f}) => x => y => !(x && y) ? t : f;


export const nor = ({t, f}) => x => y => !(x || y) ? t : f;


export const notF = x => f => !f(x);


export const notF_ = f => x => !f(x);


export const xor = ({t, f}) => x => y => {
  if (x && !y) return t;
  else if (!x && y) return t;
  else return f;
};


export const xor_ = xor({t: true, f: false});


export const xnor = ({t, f}) => x => y => // aka iff
  (x && y) || (!x && !y) ? t : f;


/*
█████ Ordering ████████████████████████████████████████████████████████████████*/


export const asc = x => y => x - y;


export const asc_ = (x, y) => x - y;


export const desc = y => x => x - y;


export const desc_ = (y, x) => x - y;


export const compareOn = order => compBoth(order);


export const compareOn_ = order => f => x => y => order(f(x), f(y));


export const ordering = n => n < 0 ? -1 : n > 0 ? 1 : 0;


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████ PATTERN MATCHING ███████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Pattern matching for the poor man with first class switch expressions and a
default case check. You can simply return in each case block instead of having
to rely on the awkward `break` statement. Usage:

match(o => {
  switch(o.tag) {
    case "foo": {return ...}
    case "bar": {return ...}
    default: {return ...}
  }
}).pattern({tag: ...})

match(o => {
  switch(typeof o) {
    case "function": {return ...}
    case "object": {return ...}
    default: {return ...}
  }
}).pattern(x => {...})

match(xs => {
  switch(xs.length) {
    case 1: {return ...}
    case 2: {return ...}
    default: {return ...}
  }
}).pattern([...]) */

const match = f => ({
  pattern: o => {
    const r = f(o);
    if (r === undefined) throw new Err("missing default case");
    else return r;
  }
});


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████ STACK SAFETY █████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*
█████ Tail Recursion ██████████████████████████████████████████████████████████*/


/* Stack-safe tail-recursion and mutual tail-recursion using a trampoline. The
`next` and `done` constructors are used to encode the recursive case and base
cases, respectively. Additionally, the `call` constructor is used to defer
function invocations. */


export const Loop = f => x => {
  let o = f(x);

  while (o?.constructor === Loop.rec) {
    switch (o.constructor) {
      case Loop.call: {
        o = o.f(o.x);
        break;
      }

      case Loop.rec: {
        o = f(o.x);
        break;
      }

      case Thunk: {
        o = strict(o);
        break;
      }

      default: throw new Err("invalid constructor");
    }
  }

  if (o?.constructor === Loop.base) return o.x;
  else return o;
};


export const Loop2 = f => (x, y) => {
  let o = f(x, y);

  while (o?.constructor === Loop2.rec) {
    switch (o.constructor) {
      case Loop2.call: {
        o = o.f(o.x, o.y);
        break;
      }

      case Loop2.rec: {
        o = f(o.x, o.y);
        break;
      }

      case Thunk: {
        o = strict(o);
        break;
      }

      default: throw new Err("invalid tag");
    }
  }

  if (o?.constructor === Loop2.base) return o.x;
  else return o;
};


export const Loop3 = f => (x, y, z) => {
  let o = f(x, y, z);

  while (o?.constructor === Loop3.rec) {
    switch (o.constructor) {
      case Loop3.call: {
        o = o.f(o.x, o.y, o.z);
        break;
      }

      case Loop3.rec: {
        o = f(o.x, o.y, o.z);
        break;
      }

      case Thunk: {
        o = strict(o);
        break;
      }

      default: throw new Err("invalid tag");
    }
  }

  if (o?.constructor === Loop3.base) return o.x;
  else return o;
};


// constructors


Loop.call = function call(f, x) {
  return {[TAG]: "Loop", constructor: Loop.call, f, x};
};


Loop.rec = function rec(x) {
  return {[TAG]: "Loop", constructor: Loop.rec, x};
};


Loop.base = function base(x) {
  return {[TAG]: "Loop", constructor: Loop.base, x};
};


Loop2.call = function call(f, x, y) {
  return {[TAG]: "Loop2", constructor: Loop2.call, f, x, y};
};


Loop2.rec = function rec(x, y) {
  return {[TAG]: "Loop2", constructor: Loop2.rec, x, y};
};


Loop2.base = function base(x) {
  return {[TAG]: "Loop2", constructor: Loop2.base, x};
};


Loop3.call = function call(f, x, y, z) {
  return {[TAG]: "Loop3", constructor: Loop3.call, f, x, y, z};
};


Loop3.rec = function rec(x, y, z) {
  return {[TAG]: "Loop3", constructor: Loop3.rec, x, y, z};
};


Loop3.base = function base(x) {
  return {[TAG]: "Loop3", constructor: Loop3.base, x};
};


/*
█████ Tail Recurson Modulo Cons & Beyond ██████████████████████████████████████*/


/* Stack-safe recursion not in tail position using a trampoline. It is capable
of mimicking tail recursion modulo cons and more complex operations not in
tail position like the original Fibbonacci algorithm:

  const fib_ = n =>
    n <= 1 ? n
      : fib_(n - 1) + fib_(n - 2);

Transformed into the trampoline version it becomes:

  const add = x => y => x + y;

  const fib = Stack(n =>
    n <= 1
      ? Stack.base(n)
      : Stack.call2(
          add,
          Stack.rec(n - 1),
          Stack.rec(n - 2))); */


export const Stack = f => x => {
  const stack = [f(x)];

  while (stack.length > 1 || stack[0].constructor !== Stack.base) {
    let o = stack[stack.length - 1];

    switch (o.constructor) {
      case Stack.call:
      case Stack.call2: {

        // 1st x of call/call2 and 2nd x of nested constructor

        o = f(o.x.x);
        stack.push(o);
        break;
      }

      case Stack.base: {
        while (stack.length > 1 && stack[stack.length - 1].constructor === Stack.base) {
          const p = (stack.pop(), stack.pop());

          switch (p.constructor) {
            case Stack.call: {
              o = Stack.base(p.f(o.x));
              stack.push(o);
              break;
            }

            case Stack.call2: {
              o = Stack.call(p.f(o.x), p.y);
              stack.push(o);
              break;
            }

            default: throw new Err("invalid trampoline constructor");
          }
        }

        break;
      }

      case Thunk: {
        o = strict(o);
        break;
      }

      default: throw new Err("invalid trampoline constructor");
    }
  }

  return stack[0].x;
};


export const Stack2 = f => (x, y) => {
  const stack = [f(x, y)];

  while (stack.length > 1 || stack[0].constructor !== Stack2.base) {
    let o = stack[stack.length - 1];

    switch (o.constructor) {
      case Stack2.call:      
      case Stack2.call2: {

        // 1st x of call/call2 and 2nd x/y of nested constructor
        
        o = f(o.x.x, o.x.y);
        stack.push(o);
        break;
      }

      case Stack2.base: {
        while (stack.length > 1 && stack[stack.length - 1].constructor === Stack2.base) {
          const p = (stack.pop(), stack.pop());

          switch (p.constructor) {
            case Stack2.call: {
              o = Stack2.base(p.f(o.x, o.y));
              stack.push(o);
              break;
            }

            case Stack2.call2: {
              o = Stack2.call(p.f(o.x, o.y), p.y);
              stack.push(o);
              break;
            }

            default: throw new Err("invalid trampoline constructor");
          }
        }

        break;
      }

      case Thunk: {
        o = strict(o);
        break;
      }

      default: throw new Err("invalid trampoline constructor");
    }
  }

  return stack[0].x;
};


// constructors


Stack.call = function call(f, x) {
  return {[TAG]: "Stack", constructor: Stack.call, f, x};
};


Stack.call2 = function call2(f, x, y) {
  return {[TAG]: "Stack", constructor: Stack.call2, f, x, y};
};


Stack.rec = function rec(x) {
  return {[TAG]: "Stack", constructor: Stack.rec, x};
};


Stack.base = function base(x) {
  return {[TAG]: "Stack", constructor: Stack.base, x};
};


Stack2.call = function call(f, x) {
  return {[TAG]: "Stack2", constructor: Stack2.call, f, x};
};


Stack2.call2 = function call2(f, x, y) {
  return {[TAG]: "Stack2", constructor: Stack2.call2, f, x, y};
};


Stack2.rec = function rec(x) {
  return function rec(y) {
    return {[TAG]: "Stack2", constructor: Stack2.rec, x, y};
  };
};


Stack2.base = function base(x) {
  return {[TAG]: "Stack2", constructor: Stack2.base, x};
};



/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ THIS █████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// make the receiving object explicit and consequently `this` redundant

export const This = t => ({
  app: x => This(t(x)), // applies the boxed fun
  app_: x => This(y => t(y) (x)), // applies the 2nd arg of the boxed fun
  map: f => This(f(t)),  // applies the fun
  map_: f => This(x => f(x) (t)), // applies the 2nd arg of the fun
  unbox: t // retrieves the boxed value
});


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████ VALUE OBJECTS ████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Value objects are objects but with value identity just like primitives, i.e.
comparing two value objects is equivalent to comparing two numbers. Equality
solely depends on values not references. Value objects don't create a memory
leak because they use weak references. Usage:

  const Tuple = ValObj("Tuple") ("1", "2");

  const tx = Tuple(123, true),
    ty = Tuple(123, true),
    tz = Tuple(123, false),
    ta = Tuple(tx, ty),
    tb = Tuple(tx, ty),
    tc = Tuple(ty, tz);

  tx === ty // true
  tx === tz // false
  ta === tb // true
  ta === tc // false

Internally, the value objects depend on JSON serialization as a hashing
mechanism. Hence, functions and symbols are invalid values. Please note
that a performance penalty only exists at value object creation time, not
afterwards. */

export const ValObj = (tag, store = new Map())  => (...ks) => (...vs) => {
  let hash = "";

  for (let i = 0; i < ks.length; i++) {
    hash += "|" + JSON.stringify(vs[i], function replacer(k, v) {
      const tag2 = Object.prototype.toString.call(v).slice(8, -1);

      switch (tag2) {
        case "Function":
        case "Symbol":
        case "Undefined": throw new Err(`invalid value of type "${tag2}"`);
        default: return v;
      }
    });
  }

  let o = store.get(hash)?.deref();

  if (o === undefined) {
    o = Object.defineProperty({}, TAG, {value: tag});
    for (let i = 0; i < ks.length; i++) o[ks[i]] = vs[i];
    store.set(hash, new WeakRef(o));
    return o;
  }

  else return o;
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████
███████████████████████████ TYPE CLASS COMBINATORS ████████████████████████████
███████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████████ FOLDABLE ███████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const foldAll = Foldable => p => Foldable.foldr(x => acc =>
  p(x) ? acc : false) (true);


export const foldAnd = Foldable => Foldable.foldr(b => acc => b && acc) (true);


export const foldAny = Foldable => p => Foldable.foldr(x => acc =>
  p(x) ? true : acc) (false);


// (Foldable t, Monoid m) => (a -> m) -> t a -> m
export const foldMapl = (Foldable, Monoid) => f =>
  Foldable.foldl(compSnd(Monoid.append) (f)) (Monoid.empty);


// (Foldable t, Monoid m) => (a -> m) -> t a -> m
export const foldMapr = (Foldable, Monoid) => f =>
  Foldable.foldr(comp(Monoid.append) (f)) (Monoid.empty);


export const foldMax = (Foldable, Order) => tx => Foldable.foldl1(Order.max) (tx);


export const foldMin = (Foldable, Order) => tx => Foldable.foldl1(Order.min) (tx);


export const foldOr = Foldable => Foldable.foldr(b => acc => b || acc) (false);


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ FUNCTOR ███████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// perform effects but discard result

export const mapEff = Functor => x => Functor.map(_ => x);


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████ FUNCTOR :: ALTERNATIVE ████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Usage of guard:

  A.chain([1,2,3]) (x =>
    A.chain([4,5,6]) (y =>
      A.chain(guard(A.Alternative) (x * y === 8)) (_ =>
        A.of(Pair(x, y))))); // yields [[2, 4]]

This is backtracking with left-biased conjunctions. */

export const guard = Alternative => x =>
  x ? Alternative.of(null) : Alternative.zero;


export const some = Alternative => tx =>
  Alternative.ap(Alternative.map(A.Cons) (tx)) (many(Alternative) (tx));
  

export const many = Alternative => tx =>
  Alternative.alt(some(Alternative) (tx)) (Alternative.of([]));


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████ FUNCTOR :: APPLY ███████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const apEff1 = Apply => tx => ty => Apply.ap(Apply.map(_const) (tx)) (ty);


export const apEff2 = Apply => tx => ty => Apply.ap(mapEff(Apply) (id) (tx)) (ty);


export const liftA2 = Apply => f => tx => ty => Apply.ap(Apply.map(f) (tx)) (ty);


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████ FUNCTOR :: APPLY :: APPLICATIVE ███████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// Applicative m => (a -> m Boolean) -> [a] -> m [a]
export const filterM = Applicative => p => mx =>
  A.foldr(x => 
    liftA2(Applicative) (b =>
      b ? A.cons(x) : id) (p(x))) (Applicative.of([]));


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████ FUNCTOR :: APPLY :: CHAIN ██████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* It is possible to chain several monads of the same type in a principled
fashion while maintaining a flat composition syntax, provided all subsequent
invocations of the partially applied Kleisli action are wrapped in a minimal
functiorial context.

The approach leaks into the call side and adds some syntactical noise but there
is no better alternative:

  const chain2_ = chain2(Monad);
                         ^^^^^
                       type class

  chain2_([1,2,3]) ([4,5,6]) (x => x & 1 ? of(y => [x + y]) : []);
          ^^^^^^^   ^^^^^^^                ^^^^^^^^^^^^^^^^
           monad     monad         next computation depends on x */


export const chain2 = Chain => mx => my => fm =>
  Chain.chain(mx) (x => Chain.chain(fm(x)) (gm => Chain.chain(my) (gm)));


export const chain3 = Chain => mx => my => mz => fm =>
  Chain.chain(mx) (x =>
    Chain.chain(fm(x)) (gm =>
      Chain.chain(my) (y =>
        Chain.chain(gm(y)) (hm =>
          Chain.chain(mz) (hm)))));


export const chainn = Chain => (...ms) => fm => function go(gm, i) {
  if (i === ms.length) return gm;
  else return Chain.chain(ms[i]) (x => Chain.chain(gm(x)) (hm => go(hm, i + 1)));
} (fm, 0);


// collapse two monadic contexts (of the same type)

export const join = Chain => mmx => Chain.chain(mmx) (id);


/*
█████ Kleisli Composition █████████████████████████████████████████████████████*/


// composing of monadic actions: `a -> m a`


export const komp = Chain => fm => gm => x => Chain.chain(fm(x)) (gm);


export const kipe = Chain => gm => fm => x => Chain.chain(fm(x)) (gm);


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ MONAD ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// (b -> a -> m b) -> b -> t a -> m b
export const foldlM = (Foldable, Monad) => fm => init => mx =>
  Foldable.foldr(x => gm => acc =>
    Monad.chain(fm(acc) (x)) (gm)) (Monad.of) (mx) (init);


// (a -> b -> m b) -> b -> t a -> m b
export const foldrM = (Foldable, Monad) => fm => init => mx =>
  Foldable.foldl(gm => x => acc =>
    Monad.chain(fm(x) (acc)) (gm)) (Monad.of) (mx) (init);


// Monad -> (s -> m (Maybe [a, s])) -> s -> m [a]
export const unfoldrM = Monad => fm => function go(seed) {
  return Monad.chain(fm(seed)) (pair => {
    const t = intro(pair);
    
    if (t === null) return [];

    else return liftA2(Monad) (A.cons)
      (Monad.of(pair[0]))
        (lazy(() => go(pair[1])));
  });
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ TYPES ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████████ FUNCTION ███████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const F = {}; // shortcut


/*
█████ Applicators █████████████████████████████████████████████████████████████*/


export const app = f => x => f(x);


export const app_ = x => f => f(x);


export const appr = (f, y) => x => f(x) (y);


export const applyObj = f => o => Object.values(o)
  .reduce((acc, v) => acc(v), f);


export const applyObj_ = f => o => f(...Object.values(o));


export const applyTuple = f => xs => xs.reduce((acc, x) => acc(x), f);


export const applyTuple_ = f => xs => f(...xs);


export const cont = f => x => k => k(f(x));


export const curry = f => x => y => f(x, y);


export const flip = f => y => x => f(x) (y);


// enable let bindings as expressions in a succinct form

export const _let = (...args) => ({in: f => f(...args)});


/* Avoid function call nesting. There are two forms depending on the `first`
argument:

  true:  (x, f, y, g, z) => g(f(x) (y)) (z)
  false: (x, f, y, g, z) => g(z) (f(x) (y))

Classic function composition composes operators `comp`/`pipe` compose in their
first argument just like applicative `ap`. Functorial `map`, however, composes
in its second argument. */

const _infix = first => (...args) => {
  if (args.length === 0) throw new Err("no argument found");

  let i = 1, x = args[0];

  while (i < args.length) {
    if (i === 1) x = args[i++] (x) (args[i++]);
    else if (first) x = args[i++] (x) (args[i++]);
    else x = args[i++] (args[i++]) (x);
  }

  return x;
};


export const infix = _infix(true);


export const infix_ = _infix(false);


// more readable immediately invoked functon expression

export const scope = f => f();


// make a thunk out of a function

const thunkify = f => x => f.bind(undefined, x);


const thunkify_ = (f, x) => f.bind(undefined, x);


export const uncurry = f => (x, y) => f(x) (y);


/*
█████ Category ████████████████████████████████████████████████████████████████*/


export const id = x => x;


F.Category = () => {
  comp,
  id
};


/*
█████ Composition █████████████████████████████████████████████████████████████*/


export const comp = f => g => x => f(g(x));


export const comp3 = f => g => h => x => f(g(h(x)));


export const compSnd = f => g => x => y => f(x) (g(y));


export const compThd = f => g => x => y => z => f(x) (y) (g(z));


export const compBin = f => g => x => y => f(g(x) (y));


export const compBoth = f => g => x => y => f(g(x)) (g(y));


export const liftFst = f => g => x => g(f(x)) (x); // chain


export const liftSnd = f => g => x => f(x) (g(x)); // ap


export const liftBoth = f => g => h => x => f(g(x)) (h(x));


export const pipe = g => f => x => f(g(x));


export const pipe3 = h => g => f => x => f(g(h(x)));


/* Create a queue of functions that can be applied in sequence in a stack-safe
manner. You can not only get the final result but recover all intermediate
results by consuming the generator with `Array.from`. */

export const seq = fs => function* (x) {
  for (let i = 0; i < fs.length; i++) {
    x = fs[i] (x);
    yield x;
  }

  return x;
};


// variant implemented as a stack, not a queue

export const seq_ = fs => function* (x) {
  for (let i = fs.length - 1; i >= 0; i--) {
    x = fs[i] (x);
    yield x;
  }

  return x;
};


/*
█████ Contravariant ███████████████████████████████████████████████████████████*/


F.contramap = () => pipe;


F.Contra = () => {contramap: F.contramap};


/*
█████ Debugging ███████████████████████████████████████████████████████████████*/


// debug after a (sub-)expression is evaluated

export const debug = expr => {
  debugger;
  return expr;
};


// debug before a function call

export const debugf = f => (...args) => {
  debugger;
  return f(...args);
};


export const debugIf = p => expr => {
  if (p(expr)) debugger;
  return expr;
};


export const debugIfF = p => f => (...args) => {
  if (p(...args)) debugger;
  return f(...args);
};


export const log = (x, tag = "") => {
  if (tag) console.log(tag, x);
  else console.log(x);
  return x;
};


export const trace = x => {
  console.log(x);
  console.log("JSON:", JSON.stringify(x));
  return x;
};


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


F.map = comp;


F.Functor = {map: F.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


// identical to Chain


F.ap = liftSnd;


F.Apply = {
  ...F.Functor,
  ap: F.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


export const _const = x => y => x;


export const const_ = x => y => y;


F.of = _const;


F.Applicative = {
  ...F.Apply,
  of: F.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


/* Encodes the effect of implicitly threading an argument through a large
function composition. */ 


F.chain = liftFst;


F.Chain = {
  ...F.Apply,
  chain: F.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


F.join = f => x => f(x) (x);


F.Monad = {
  ...F.Applicative,
  chain: F.chain
};


/*
█████ Functor :: Extend ███████████████████████████████████████████████████████*/


F.extend = Semigroup => f => g => x => f(y => g(Semigroup.append(x) (y)));


F.Extend = {
  ...F.Functor,
  extend: F.extend
};


/*
█████ Functor :: Extend :: Comonad ████████████████████████████████████████████*/


F.extract = Monoid => f => f(Monoid.empty);


F.Comonad = {
  ...F.Extend,
  extract: F.extract
};


/*
█████ Functor :: Profunctor ███████████████████████████████████████████████████*/


/* bimap/dimap comparison:

  bimap :: (a -> b) -> (c -> d) -> bifunctor  a c -> bifunctor  b d
            ^^^^^^
  dimap :: (b -> a) -> (c -> d) -> profunctor a c -> profunctor b d
  (phrase -> [word]) -> ([word] -> phrase) -> ([word] -> [word]) -> (phrase -> phrase) */


F.dimap = h => g => f => x => g(f(h(x)));
          

F.lmap = pipe;


F.rmap = comp;


F.Profunctor = {
  ...F.Functor,
  dimap: F.dimap,
  lmap: F.lmap,
  rmap: F.rmap
};


/*
█████ Impure ██████████████████████████████████████████████████████████████████*/


export const eff = f => x => (f(x), x);


/* Takes an arbitrary number of expressions or statements and returns an array
of evaluated values of each one. Useful if statements are mixed with expressions
or destructive operations return a value but also modify the initial reference
value (e.g. `effs(xs.pop(), xs)`). */

export const enumEffs = (...exps) => exps;


// like `effs` but only returns the first evaluated value

export const enumEffsFirst = (...exps) => exps[0];


// like `effs` but only returns the last evaluated value

export const enumEffsLast = (...exps) => exps[exps.length - 1];


// introspection

export const intro = x =>
  Object.prototype.toString.call(x).slice(8, -1);


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


F.append = Semigroup => f => g => x => Semigroup.append(f(x)) (g(x));


F.Semigroup = {append: F.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


F.empty = Monoid => _ => Monoid.empty;


F.Monoid = {
  ...F.Semigroup,
  empty: F.empty
};


/*
█████ Transducer ██████████████████████████████████████████████████████████████*/


export const drop = n => append => { 
  let m = 0;

  return acc => x =>
    m < n
      ? (m++, acc)
      : append(acc) (x);
};


export const dropk = n => append => { 
  let m = 0;

  return acc => x => k =>
      m < n
        ? (m++, k(acc))
        : append(acc) (x) (k)
};


export const dropr = n => append => { 
  let m = 0;

  return x => acc =>
    m < n
      ? (m++, acc)
      : append(x) (acc);
};


export const dropWhile = p => append => {
  let drop = true;

  return acc => x => 
    drop && p(x)
      ? acc
      : (drop = false, append(acc) (x));
};


export const dropWhilek = p => append => {
  let drop = true;

  return acc => x => k =>
    drop && p(x)
      ? k(acc)
      : (drop = false, append(acc) (x) (k))
};


export const dropWhiler = p => append => {
  let drop = true;

  return x => acc =>
    drop && p(x)
      ? acc
      : (drop = false, append(x) (acc));
};


export const filter = p => append => acc => x =>
  p(x)
    ? append(acc) (x)
    : acc;


export const filterk = p => append => acc => x => k =>
  p(x)
    ? append(acc) (x) (k)
    : k(acc);


export const filterr = p => append => x => acc =>
  p(x)
    ? append(x) (acc)
    : acc;


export const map = f => append => acc => x =>
  append(acc) (f(x));


export const mapk = f => append => acc => x => k =>
  append(acc) (f(x)) (k);


export const mapr = f => append => x => acc =>
  append(f(x)) (acc);


export const take = n => append => { 
  let m = 0;

  return acc => x =>
    m < n
      ? (m++, append(acc) (x))
      : acc;
};


export const takek = n => append => { 
  let m = 0;

  return acc => x => k =>
    m < n
      ? (m++, append(acc) (x) (k))
      : acc;
};


export const taker = n => append => { 
  let m = 0;

  return x => acc =>
    m < n
      ? (m++, append(x) (acc))
      : acc;
};


export const takeWhile = p => append => acc => x =>
  p(x)
    ? append(acc) (x)
    : acc;


export const takeWhilek = p => append => acc => x => k =>
  p(x)
    ? append(acc) (x)(k)
    : acc;


export const takeWhiler = p => append => x => acc =>
  p(x)
    ? append(x) (acc)
    : acc;


export const transducel = (Semigroup, Foldable) => f =>
  Foldable.foldl(f(Semigroup.append));


export const transducer = (Semigroup, Foldable) => f =>
  Foldable.foldr(f(Semigroup.append));


/*
█████ Resolve Deps ████████████████████████████████████████████████████████████*/


F.Category = F.Category();


F.contramap = F.contramap();


F.Contra = F.Contra();


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ ARRAY ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Enocodes the effect of computations that may have no, one or several results.
Array is not a functional data type, because it has a non recursive definition.
While it has a valid monad instance, there is no valid transformer. Use list or
streams instead. */


export const A = {}; // shortcut


/*
█████ Cloning █████████████████████████████████████████████████████████████████*/


A.clone = xs => xs.concat();


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


A.fromCsv = ({sep, header}) => csv => {
  const table = csv.trim()
    .replace(/"/g, "")
    .split(/\r?\n/)
    .map(row => row.split(sep));

  let names;

  if (header) names = table.shift();

  return header
    ? table.map(cols => cols.reduce((acc, col, i) => (acc[names[i]] = col, acc), {}))
    : table;
};


A.fromIt = ix => {
  const xs = [];
  for (const x of ix) xs.push(x);
  return xs;
};


A.fromItKeys = ix => {
  const xs = [];
  for (const [k] of ix) xs.push(k);
  return xs;
};


A.fromItValues = ix => {
  const xs = [];
  for (const [, v] of ix) xs.push(v);
  return xs;
};


A.fromList = () => L.foldl(acc => x => (acc.push(x), acc))
  (Object.defineProperty(() => [], NEW, {value: true}));


A.fromListF = f => L.foldl(acc => x => (acc.push(f(x)), acc)) ([]);


A.fromQueue = () => ([fw, bw]) => {
  const fw2 = L.foldl(acc => x => (acc.push(x), acc)) ([]) (fw),
    bw2 = L.foldl(acc => x => (acc.unshift(x), acc)) ([]) (bw);

  return A.pushn(bw2) (fw2);
};


A.fromQueueF = f => ([fw, bw]) => {
  const fw2 = L.foldl(acc => x => (acc.push(f(x)), acc)) ([]) (fw),
    bw2 = L.foldl(acc => x => (acc.unshift(f(x)), acc)) ([]) (bw);

  return A.pushn(bw2) (fw2);
};


A.fromStr = s => s.split("");


A.fromTable = xss => xss.flat();


A.fromTableBy = f => xs => xs.reduce((acc, x) => f(acc) (x), []);


/*
█████ Con-/Deconstruction █████████████████████████████████████████████████████*/


A.cons = x => xs => [x].concat(xs);


A.cons_ = xs => x => [x].concat(xs);


A.head = xs => xs.length === 0 ? null : xs[0];


A.headOr = x => xs => xs.length === 0 ? x : xs[0];


A.index = i => xs => (i in xs) ? xs[i] : null;


A.indexOr = x => i => xs => (i in xs) ? xs[i] : x;


A.init = xs => xs.length === 0 ? null : xs.slice(0, -1);


A.last = xs => xs.length === 0 ? null : xs[xs.length - 1];


A.lastOr = x => xs => xs.length === 0 ? x : xs[xs.length - 1];


A.push = x => xs => (xs.push(x), xs);


A.pushn = ys => xs => (xs.push.apply(xs, ys), xs);


A.push_ = xs => x => (xs.push(x), xs);


A.pushn_ = xs => ys => (xs.push.apply(xs, ys), xs);


A.pop = xs => Pair(xs.length === 0 ? null : xs.pop(), xs);


A.shift = xs => Pair(xs.length === 0 ? null : xs.shift(), xs);


A.singleton = x => [x];


A.snoc = x => xs => xs.concat([x]);


A.snoc_ = xs => x => xs.concat([x]);


A.tail = xs => xs.length === 0 ? null : xs.slice(1);


A.uncons = xs => Pair(xs.length === 0 ? null : xs[0], xs.slice(1));


A.unshift = x => xs => (xs.unshift(x), xs);


A.unshiftn = ys => xs => (xs.unshift.apply(xs, ys), xs);


A.unshift_ = xs => x => (xs.unshift(x), xs);


A.unshiftn_ = xs => ys => (xs.unshift.apply(xs, ys), xs);


A.unsnoc = xs => Pair(
  xs.length === 0 ? null : xs[xs.length - 1],
  xs.slice(-1));


/*
█████ Eq ██████████████████████████████████████████████████████████████████████*/


A.eq = Eq => xs => ys => {
  if (xs.length !== ys.length) return false;

  else return Loop(i => {
    if (i === xs.length) return Loop.base(true);

    else {
      const b = Eq.eq(xs[i]) (ys[i]);
      return b === true ? Loop.rec(i + 1) : Loop.base(false);
    }
  }) (0);
};


A.Eq = {eq: A.eq};


/*
█████ Focus ███████████████████████████████████████████████████████████████████*/


/* Defines a focus over n elements of an existing array. The resulting virtual
array behaves like an ordinary one. You can apply the iterable protocol or the
standard array functions on it. */

A.focus = (i, j = null) => xs => {
  if (j === null) j = xs.length - 1;

  return new Proxy(xs, {
    get(_, k, p) {
      switch (typeof k) {
        case "string": {
          if (k === "length") return j - i + 1;
          
          else {
            const i2 = Number(k);

            if (String(i2) === k) {
              if (i + i2 > j) return undefined;
              else return xs[i + i2];
            }

            else return xs[k];
          }
        }

        default: return xs[k];
      }
    },

    has(_, k) {
      switch (typeof k) {
        case "string": {
          const i2 = Number(k);

          if (String(i2) === k) {
            if (i + i2 > j) return false;
            else return i + i2 in xs;
          }

          else k in xs;
        }

        default: k in xs;
      }
    },

    set(_, k, v, p) {
      switch (typeof k) {
        case "string": {
          const i2 = Number(k);

          if (String(i2) === k) {
            if (i + i2 > j) return undefined;
            else return xs[i + i2] = v;
          }

          else xs[k] = v;
        }

        default: xs[k] = v;
      }
    }
  });
};


/*
█████ Filterable ██████████████████████████████████████████████████████████████*/


A.filter = p => xs => xs.filter(x => p(x));


A.Filterable = {filter: A.filter};


/*
█████ Foldable ████████████████████████████████████████████████████████████████*/


A.foldl = f => init => xs => {
  let acc = init?.[NEW] ? init() : init;

  for (let i = 0; i < xs.length; i++)
    acc = f(acc) (xs[i]);

  return acc;
};


// uncurried

A.foldl_ = f => acc => xs => {
  for (let i = 0; i < xs.length; i++)
    acc = f(acc, xs[i]);

  return acc;
};


A.foldl1 = f => xs => {
  let acc = xs.length === 0
    ? _throw(new Err("empty array")) : xs[0];

  for (let i = 1; i < xs.length; i++)
    acc = f(acc) (xs[i]);

  return acc;
};


// variant that incorporates the running index

A.foldi = f => init => xs => {
  let acc = init;

  for (let i = 0; i < xs.length; i++)
    acc = f(acc) (xs[i], i);

  return acc;
};


/* A fold with `f` encoded in continuation passing style so that the function
argument can decide whether to continue or abort the right-associative fold. */

A.foldk = f => init => xs =>
  Loop2((acc, i) =>
    i === xs.length
      ? Loop2.base(acc)
      : f(xs[i]) (acc) (acc2 => Loop2.rec(acc2, i + 1)))
        (init, 0);


// eager, right-associative and yet stack-safe fold

A.foldr = f => acc => xs => Stack(i => {
  if (i === xs.length) return Stack.base(acc);

  else return Stack.call(
    f(xs[i]),
    Stack.rec(i + 1));
}) (0);


// not stack-safe

A.foldr_ = f => acc => xs => function go(i) {
  if (i === xs.length) return acc;
  else return f(xs[i]) (go(i + 1));
} (0);


// only stack-safe if `f` is non-strict in its second argument

A._foldr = f => acc => xs => function go(i) {
  if (i === xs.length) return acc;
  else return f(xs[i]) (lazy(() => go(i + 1)));
} (0);


A.foldr1 = f => xs => Stack(i => {
  let acc = xs.length === 0
    ? _throw(new Err("empty array")) : xs[0];

  if (i === xs.length) return Stack.base(acc);

  else return Stack.call(
    f(xs[i]),
    Stack.rec(i + 1));
}) (0);


// fold pairwise

A.foldBin = f => acc => xs => {
  for (let i = 0, j = 1; j < xs.length; i++, j++)
    acc = f(acc) (Pair(xs[i], xs[j]));

  return acc;
};


// uncurried

A.foldBin_ = f => acc => xs => {
  for (let i = 0, j = 1; j < xs.length; i++, j++)
    acc = f(acc, xs[i], xs[j]);

  return acc;
};


// map followed by folding the resulting monoid

A.foldMap = Monoid => f => xs => {
  let acc = Monoid.empty;

  for (let i = 0; i < xs.length; i++)
    acc = Monoid.append(acc) (f(xs[i]));

  return acc;
};


A.sum = acc => A.fold((m, n) => m + n) (acc);


A.Foldable = {
  foldl: A.foldl,
  foldl1: A.foldl1,
  foldr: A.foldr,
  foldr1: A.foldr1
};


/*
█████ Foldable :: Traversable █████████████████████████████████████████████████*/


A.mapA = Applicative => ft => {
  const liftA2_ = liftA2(Applicative);

  return A.foldl(acc => x =>
    liftA2_(A.push) (ft(x)) (acc))
      (Applicative.of([]));
};


A.seqA = Applicative =>
  A.foldl(liftA2(Applicative) (A.push_)) (Applicative.of([]));


A.Traversable = () => ({
  ...A.Foldable,
  ...A.Functor,
  mapA: A.mapA,
  seqA: A.seqA
});


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


A.map = f => tx => tx.map(f);


A.Functor = {map: A.map};


/*
█████ Functor :: Alt ██████████████████████████████████████████████████████████*/


A.alt = () => A.append;


A.Alt = () => ({
  ...A.Functor,
  alt: A.alt
});


/*
█████ Functor :: Alt :: Plus ██████████████████████████████████████████████████*/


Object.defineProperty(A, "zero", { // to avoid mutation sharing
  get() {return []},
  configurable: true,
  enumerable: true
});


A.Plus = {
  ...A.Alt
};


Object.defineProperty(A.Plus, "zero", { // to avoid mutation sharing
  get() {return []},
  configurable: true,
  enumerable: true
});


/*
█████ Functor :: Apply (Non-Determinism) ██████████████████████████████████████*/


A.ap = fs => xs =>
  fs.reduce((acc, f) =>
    (acc.push.apply(acc, xs.map(x => f(x))), acc), []);


A.Apply = {
  ...A.Functor,
  ap: A.ap
};


/*
█████ Functor :: Apply (ZipArr) ███████████████████████████████████████████████*/


A.ZipArr = {}; // namespace


A.ZipArr.ap = () => A.zipWith(app);


/*
█████ Functor :: Apply :: Applicative (Non-Determinism) ███████████████████████*/


/* There is no applicative `ZipArr` instance because we cannot construct an
infinite array for the minimal context. */


A.of = A.singleton;


A.Applicative = {
  ...A.Apply,
  of: A.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


A.chain = xs => fm =>
  xs.reduce((acc, x) =>
    (acc.push.apply(acc, fm(x)), acc), []);


A.Chain = {
  ...A.Apply,
  chain: A.chain
};


/*
█████ Functor :: Apply :: Applicative :: Alternative ██████████████████████████*/


A.Alternative = {
  ...A.Plus,
  ...A.Applicative
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


A.Monad = {
  ...A.Applicative,
  chain: A.chain
};


/*
█████ Functor :: Extend ███████████████████████████████████████████████████████*/


// there seems to be no useful instance


/*
█████ Functor :: Extend :: Comonad ████████████████████████████████████████████*/


// there seems to be no useful instance


/*
█████ Generators ██████████████████████████████████████████████████████████████*/


A.entries = m => m[Symbol.iterator] ();


A.keys = function* (m) {
  for (let [k] of m) {
    yield k;
  }
};


A.values = function* (m) {
  for (let [, v] in m) {
    yield v;
  }
};


/*
█████ Getter ██████████████████████████████████████████████████████████████████*/


A.at = i => xs => xs[i]; // curried `at` non-reliant on `this`


/*
█████ Recursion Schemes ███████████████████████████████████████████████████████*/


// eager arrays don't comply with lazy recursion schemes


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


A.append = xs => ys => xs.concat(ys);


A.Semigroup = {append: A.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


Object.defineProperty(A, "empty", { // to avoid mutation sharing
  get() {return []},
  configurable: true,
  enumerable: true
});


A.Monoid = {
  ...A.Semigroup
};


Object.defineProperty(A.Monoid, "empty", { // to avoid mutation sharing
  get() {return []},
  configurable: true,
  enumerable: true
});


/*
█████ Special Folds ███████████████████████████████████████████████████████████*/


/* mapAccum isn't required for arrays because the last element representing the
final value can be easily accessed through its index. */


A.scanl = f => init => A.foldl(acc => x =>
  (acc.push(f(acc[acc.length - 1]) (x)), acc)) ([init]);


A.scanr = f => init => A.foldr(x => acc =>
  (acc.unshift(f(x) (acc[0])), acc)) ([init]);


/*
█████ Set Operations ██████████████████████████████████████████████████████████*/


A.dedupe = xs => Array.from(new Set(xs));


// ignores duplicates 

A.diff = xs => ys => {
  const s = new Set(xs),
    s2 = new Set(ys),
    acc = new Set();

  for (const x of s)
    if (!s2.has(x)) acc.add(x);

  for (const y of s2)
    if (!s.has(y)) acc.add(y);

  return Array.from(acc);
};


// left biased difference (ignores duplicates)

A.diffl = xs => ys => {
  const s = new Set(xs),
    s2 = new Set(ys),
    acc = [];

  for (const x of s)
    if (!s2.has(x)) acc.push(x);

  return acc;
};


// ignores duplicates 

A.intersect = xs => ys => {
  const s = new Set(ys);

  return Array.from(xs.reduce(
    (acc, x) => s.has(x) ? acc.add(x) : acc, new Set()));
};


// ignores duplicates 

A.union = xs => ys => Array.from(new Set(xs.concat(ys)));


/*
█████ Subarrays ███████████████████████████████████████████████████████████████*/


// `slice` method is used instead of `A.drop`


A.dropWhile = p => xs => Loop2((acc, i) => {
  if (i === xs.length) return Loop2.base(acc);
  else return p(xs[i]) ? Loop2.rec(acc, i + 1) : Loop2.base(xs.slice(i));
}) ([], 0);


// `slice` method is used instead of `A.take`


A.takeWhile = p => xs => Loop2((acc, i) => {
  if (i === xs.length) return Loop2.base(acc);
  else return p(xs[i]) ? Loop2.rec((acc.push(xs[i]), acc), i + 1) : Loop2.base(acc);
}) ([], 0);


/*
█████ Transformation ██████████████████████████████████████████████████████████*/


// binary parition function

A.collate = p => xs => xs.reduce((pair, x)=> {
  if (p(x)) return (pair[0].push(x), pair);
  else return (pair[1].push(x), pair);
}, Pair([], []));


/* A more general partition function that allows dynamic key generation and
value combination. */

A.collateBy = f => g => xs => xs.reduce((acc, x) => {
  const k = f(x);
  return acc.set(k, g(acc.has(k) ? acc.get(k) : null) (x));
}, new Map());


// collect all consecutive elements of a certain length

A.consecs = chunkLen => f => xs => {
  const ys = [];

  if (chunkLen > xs.length) return ys;

  else {
    for (let i = 0; i + chunkLen <= xs.length; i++)
      ys.push(f(xs.slice(i, i + chunkLen)));
  }

  return ys;
};


/* Groups all consecutive elements by applying a binary predicate to the
pervious/current element. If the predicate fails, a new subgroup is created
otherwise the element is pushed on the current subgroup. */

A.groupBy = p => xs => Loop2((acc, i) => {
  if (i >= xs.length) return Loop2.base(acc);
  if (acc.length === 0) acc.push([xs[0]]);

  if (p(xs[i - 1]) (xs[i])) {
    acc[acc.length - 1].push(xs[i]);
    return Loop2.rec(acc, i + 1);
  }
  
  else {
    acc.push([xs[i]]);
    return Loop2.rec(acc, i + 1);
  }
}) ([], 1);


/* Determines permutations of a given array. If an array includes duplicates,
so will the list of array permutations. Stack-safety isn't required because a
large array would exhaust the heap before the call stack. */

A.perms = xs => {
  if (xs.length === 0) return [[]];
  
  else return xs.flatMap((x, i) =>
    A.perms(xs.filter((y, j) => i !== j))
      .map(ys => [x, ...ys]));
};


/* Collects all possible subsequences of an array. If it includes duplicates,
so will the list of array subsequences. Stack-safety isn't required because a
large array would exhaust the heap before the call stack. */

A.subseqs = xs => function go(i) {
  if (i === xs.length) return [[]];
  
  else {
    const yss = go(i + 1);

    const zss = yss.map(ys => {
      const zs = [xs[i]];
      return (zs.push.apply(zs, ys), zs);
    });

    return (zss.push.apply(zss, yss), zss);
  }
} (0);


A.transpose = xss =>
  xss.reduce((acc, xs) =>
    xs.map((x, i) => {
      const ys = acc[i] || [];
      return (ys.push(x), ys);
    }), []);


/*
█████ Unfoldable ██████████████████████████████████████████████████████████████*/


// left associative, eager unfold due to non-recursive array data type

A.unfold = f => seed => {
  let acc = [], x = seed;

  while (true) {
    const r = f(x);

    if (strict(r) === null) break;

    else {
      const [y, z] = r;

      x = z;
      acc.push(y);
    }
  }

  return acc;
};


A.Unfoldable = {unfold: A.unfold};


/*
█████ Zipping █████████████████████████████████████████████████████████████████*/


A.zip = () => xs => ys => Loop2((acc, i) => {
  if (i === xs.length) return Loop2.base(acc);
  else if (i === ys.length) return Loop2.base(acc);

  else {
    acc.push(Pair(xs[i], ys[i]));
    return Loop2.rec(acc, i + 1);
  }
}) ([], 0);


A.zipWith = f => xs => ys => Loop2((acc, i) => {
  if (i === xs.length) return Loop2.base(acc);
  else if (i === ys.length) return Loop2.base(acc);
  else return Loop2.rec((acc.push(f(xs[i]) (ys[i])), acc), i + 1);
}) ([], 0);


A.unzip = () => A.foldl(([x, y]) => ([xs, ys]) =>
  Pair((xs.push(x), xs), (ys.push(y), ys))) (Pair([], []));


/*
█████ Resolve Deps ████████████████████████████████████████████████████████████*/


A.alt = A.alt();


A.Alt = A.Alt();


A.Traversable = A.Traversable();


A.ZipArr.ap = A.ZipArr.ap();


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████ ARRAY :: LIST ████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// immutable stack (FILO) implemented as single linked list


export const List = {}


export const L = List;


L.cons = head => tail => [head, tail];


L.cons_ = tail => head => [head, tail];


L.nil = [];


/* TODO:
take
takeWhile
drop
dropWhile
span
break
zip
zipWith
unzip
*/


/*
█████ Con-/Deconstruction █████████████████████████████████████████████████████*/


L.head = xs => xs[0];


L.headOr = x => xs => xs.length === 0 ? x : xs[0];


L.tail = xs => xs.length === 0 ? [] : xs[1];


L.uncons = xs => xs.length === 0 ? [null, []] : [xs[0], xs[1]];


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


L.fromArr = xs => {
  let ys = [];

  for (let i = xs.length - 1; i >= 0; i--) ys = [xs[i], ys];
  return ys;
};


L.fromIt = ix => {
  let xs = [];
  const root = xs;

  for (const x of ix) (xs[0] = x, xs[1] = [], xs = xs[1]);
  return root;
};


L.fromQueue = ([xs, ys]) => L.append(xs) (L.reverse(ys));


/*
█████ Foldable ████████████████████████████████████████████████████████████████*/


/* Mind the side effect in the accumulator, which is performed for the sake of
performance. */

L.foldl = f => init => xs => {

  // use a thunk if you don't want to share mutations

  let acc = init?.[NEW] ? init() : init;

  while (true) {
    if (xs.length === 0) break;

    else {
      acc = f(acc) (xs[0]);
      xs = xs[1];
    }
  }

  return acc;
};


// stack-safe even if the passed function is strict in its second argument

L.foldr = f => acc => Stack(xs => {
  if (xs.length === 0) return Stack.base(acc);
  else return Stack.call(f(xs[0]), Stack.rec(xs[1]));
});


// not stack-safe

L.foldr_ = f => acc => function go(xs) {
  if (xs.length === 0) return acc;
  else return f(xs[0]) (go(xs[1]));
};


// only stack-safe if `f` is non-strict in its second argument

L._foldr = f => acc => function go(xs) {
  if (xs.length === 0) return acc;
  else return f(xs[0]) (lazy(() => go(xs[1])));
};


L.Foldable = {
  foldl: L.foldl,
  foldr: L.foldr
};


/*
█████ Foldable :: Traversable █████████████████████████████████████████████████*/


L.mapA = Applicative => {
  const liftA2_ = liftA2(Applicative) (L.cons_);
  return f => L.foldr(x => acc => liftA2_(f(x)) (acc)) (Applicative.of([]));
};


L.mapA_ = Applicative => {
  const liftA2_ = liftA2(Applicative) (L.cons_);
  return f => L.foldr_(x => acc => liftA2_(f(x)) (acc)) (Applicative.of([]));
};


L.seqA = Applicative => {
  const liftA2_ = liftA2(Applicative) (L.cons_);
  return L.foldr(x => acc => liftA2_(x) (acc)) (Applicative.of([]));
};


L.seqA_ = Applicative => {
  const liftA2_ = liftA2(Applicative) (L.cons_);
  return L.foldr_(x => acc => liftA2_(x) (acc)) (Applicative.of([]));
};


L.Traversable = () => ({
  ...L.Foldable,
  ...L.Functor,
  mapA: L.mapA,
  seqA: L.seqA
});


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


L.map = f => L.foldr(x => acc => [f(x), acc]) ([]);


L.Functor = {map: L.map};


/*
█████ Functor :: Alt ██████████████████████████████████████████████████████████*/


L.alt = () => L.append;


L.Alt = () => ({
  ...L.Functor,
  alt: L.alt
});


/*
█████ Functor :: Alt :: Plus ██████████████████████████████████████████████████*/


Object.defineProperty(L, "zero", { // to avoid mutation sharing
  get() {return []},
  configurable: true,
  enumerable: true
});


L.Plus = {
  ...L.Alt
};


Object.defineProperty(L.Plus, "zero", { // to avoid mutation sharing
  get() {return []},
  configurable: true,
  enumerable: true
});


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


L.ap = tf => tx =>
  L.foldr(f => acc =>
    L.append(L.map(f) (tx)) (acc)) ([]) (tf);


L.Apply = {
  ...L.Functor,
  ap: L.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


L.of = x => [x, []];


L.Applicative = {
  ...L.Apply,
  of: L.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


L.chain = mx => fm => L.foldr(x => acc =>
  L.append(fm(x)) (acc)) ([]) (mx);


L.Chain = {
  ...L.Apply,
  chain: L.chain
};


/*
█████ Functor :: Apply :: Applicative :: Alternative ██████████████████████████*/


L.Alternative = {
  ...L.Plus,
  ...L.Applicative
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


L.Monad = {
  ...L.Applicative,
  chain: L.chain
};


/*
█████ Functor :: Extend ███████████████████████████████████████████████████████*/


L.extend = () => L.tails;


L.Extend = {
  ...L.Functor,
  extend: L.extend
};


/*
█████ Functor :: Extend :: Comonad ████████████████████████████████████████████*/


L.extract = xs => xs[0];


L.Comonad = {
  ...L.Extend,
  extract: L.extract
};


/*
█████ Infinity ████████████████████████████████████████████████████████████████*/


L.iterate = f => function go(x) {
  return [x, lazy(() => go(f(x)))];
};


L.repeat = x => [x, lazy(() => repeat(x))];


L.replicate = n => x => function go(m) {
  if (m === 0) return [x, L.Nil];
  else return [x, lazy(() => go(m - 1))];
} (n);


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


L.append = flip(L.foldr(L.cons));


L.Semigroup = {append: L.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


Object.defineProperty(L, "empty", { // to avoid mutation sharing
  get() {return []},
  configurable: true,
  enumerable: true
});


L.Monoid = {
  ...L.Semigroup
};


Object.defineProperty(L.Monoid, "empty", { // to avoid mutation sharing
  get() {return []},
  configurable: true,
  enumerable: true
});


/*
█████ Special Folds ███████████████████████████████████████████████████████████*/


// like a fold but additionally holds the intermediate results

// (b -> a -> b) -> b -> [a] -> [b]
L.scanl = f => init => xs => {
  let acc = [init, []], prev = acc;
  const root = acc;
  acc = acc[1];

  while (true) {
    if (xs.length === 0) break;

    else {
      const x = f(prev[0]) (xs[0]);

      acc[0] = x;
      acc[1] = [];
      prev = acc;
      acc = acc[1];
      xs = xs[1];
    }
  }

  return root;
};


// stack-safe right associative version

// (a -> b -> b) -> b -> [a] -> [b]
L.scanr = f => acc => Stack(xs => {
  if (xs.length === 0) return Stack.base([acc, []]);

  else return Stack.call(
    ys => [f(xs[0]) (ys[0]), ys],
    Stack.rec(xs[1]));
});


L.mapAccuml = f => init => xs => {
  let acc = [init, []], prev = acc, final;
  const root = acc;
  acc = acc[1];

  while (true) {
    if (xs.length === 0) {
      final = prev[0];

      // mutation to get rid of the last cons

      prev.length = 0;
      break;
    }

    else {
      const x = f(prev[0]) (xs[0]);

      acc[0] = x;
      acc[1] = [];
      prev = acc;
      acc = acc[1];
      xs = xs[1];
    }
  }

  return Pair(final, root);
};


// L.mapAccumr isn't supplied since you can just compose `L.scanr` with `L.decons`


L.tails = Stack(xs => {
  if (xs.length === 0) return Stack.base([]);

  else return Stack.call(
    ys => [xs, ys],
    Stack.rec(xs[1]));
});


/*
█████ Unfoldable ██████████████████████████████████████████████████████████████*/


// lazy unfold

L.unfold = f => function go(y) {
  const pair = strict(f(y));

  if (pair === null) return [];
  else return new [pair[0], lazy(() => go(pair[1]))];
};


L.Unfoldable = {unfold: L.unfold};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


L.take = n => tx => {
  let acc = [];
  const root = acc;

  while (tx.length === 2) {
    acc[0] = tx[0];
    acc[1] = [];
    acc = acc[1];

    if (--n === 0) break;
    else tx = tx[1];
  }

  return root;
};


L.takeWhile = p => tx => {
  let acc = [];
  const root = acc;

  while (tx.length === 2) {
    if (!p(tx[0])) break;

    else {
      acc[0] = tx[0];
      acc[1] = [];
      acc = acc[1];
      tx = tx[1];
    }
  }

  return root;
};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


L.reverse = L.foldl(x => y => [y, x])
  (Object.defineProperty(() => [], NEW, {value: true}));


/*
█████ Resolve Deps ████████████████████████████████████████████████████████████*/


L.alt = L.alt();


L.Alt = L.Alt();


L.extend = L.extend();


L.Traversable = L.Traversable();


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████ ARRAY :: LIST :: DLIST ████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// difference list for more efficient append operation


export const DList = f => ({
  [TAG]: "DList",
  run: f
});


/*
█████ Con-/Deconstruction █████████████████████████████████████████████████████*/


DList.cons = x => xss => DList(comp(L.Cons(x)) (xss));


DList.singleton = comp(DList) (L.Cons_);


DList.snoc = x => xss => DList(comp(xss) (L.Cons_(x)));


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


DList.fromList = xss => comp(DList) (L.append);


DList.fromArr = xs => comp(DList) (A.append);


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


DList.append = xss => yss => DList(comp(xss) (yss));


DList.Semigroup = {append: DList.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


DList.empty = DList(id);


DList.Monoid = {
  ...DList.Semigroup,
  empty: DList.empty
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████ ARRAY :: QUEUE ████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// immutable queue (FIFO) implemented as two single linked lists


export const Queue = {};


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


Queue.fromArr = xs => [L.fromArr(xs), []];


/*
█████ Queue Operations ████████████████████████████████████████████████████████*/


Queue.en = x => ([fw, bw]) => {
  if (bw.length === 0 && fw.length === 0) return [[x, fw], bw];
  else return [fw, [x, bw]];
};


Queue.de = ([fw, bw]) => {
  if (fw.length === 2) return [fw[0], [fw[1], bw]];
  else if (bw.length === 0) return null;

  else {
    const fw2 = L.reverse(bw);
    return [fw2[0], [fw2[1], []]];
  }
};


/*
█████ Deque Operations ████████████████████████████████████████████████████████*/


// use the queue as a combined immutable stack/queue


Queue.push = x => ([fw, bw]) => [fw, [x, bw]];


Queue.pop = x => ([fw, bw]) => {
  if (bw.length === 2) return [bw[0], [fw, bw[1]]];

  else {
    const bw2 = L.reverse(fw);
    return [bw2[0], [[], bw2[1]]]
  }
};


Queue.unshift = x => ([fw, bw]) => {
  if (bw.length === 0 && fw.length === 0) return [[x, fw], bw];
  else return [fw, [x, bw]];
};


Queue.shift = ([fw, bw]) => {
  if (fw.length === 2) return [fw[0], [fw[1], bw]];
  else if (bw.length === 0) return null;

  else {
    const fw2 = L.reverse(bw);
    return [fw2[0], [fw2[1], []]];
  }
};


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Queue.map = f => tx => [
  L.map(f) (tx[0]),
  L.map(f) (tx[1])
];


Queue.Functor = {map: Queue.map};


/*
█████ Foldable ████████████████████████████████████████████████████████████████*/


Queue.foldl = f => acc => tx => {
  const ty = L.foldl(f) (acc) (tx[0]);
  return L.foldl(f) (ty) (tx[1]);
};


Queue.foldr = f => acc => tx => {
  const ty = L.foldr(f) (acc) (tx[0]),
    tz = L.foldr(f) (acc) (tx[1]);

  return f(ty) (f(tz) (acc));
};


Queue.Foldable = {
  foldl: Queue.foldl,
  foldr: Queue.foldr
};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


Queue.ap = tf => tx => [
  L.ap(tf[0]) (tx[0]),
  L.ap(tf[1]) (tx[1])
];


Queue.Apply = {
  ...Queue.Functor,
  ap: Queue.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


Queue.of = x => [[x], []];


Queue.Applicative = {
  ...Queue.Apply,
  of: Queue.of
};


/*
█████ Functor :: Apply :: Applicative :: Alternative ██████████████████████████*/


Queue.Alternative = {
  ...Queue.Plus,
  ...Queue.Applicative
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


Queue.chain = mx => fm => [
  L.chain(fm) (mx[0]),
  L.chain(fm) (mx[1])
];


Queue.Chain = {
  ...Queue.Apply,
  chain: Queue.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


Queue.Monad = {
  ...Queue.Applicative,
  chain: Queue.chain
};


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


Queue.append = tx => ty => [
  L.append(
    L.append(
      L.append(tx[0]) (L.reverse(tx[1])))
        (ty[0]))
          (L.reverse(ty[1])),
  []
];


Queue.Semigroup = {append: Queue.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


Queue.empty = [[], []];


Queue.Monoid = {
  ...Queue.Semigroup,
  empty: Queue.empty
};


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████████ BEHAVIOR ███████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Encode time changing values. Such a value is asynchronously mutated over time
by the events it has subscribed to. As opposed to `Observable`, a `Behavior` has
a continuous value over its entire lifespan. It has the following properties:

  * multicast
  * async
  * push
  * lazy

A `Behavior` takes an initial value and a function that in turn takes this
initial value and returns an intermediate object. The intermediate object (A)
must have two properties holding the state (A) and a nullary cancellation
function (B). Here is a simple example:

  const secCounter = Behavior(0) (init => {
    const state = {run: init}, // (B)
      id = setInterval(state2 => state2.run++, 1000, state); // event source

    return {state, cancel: () => clearInterval(id)}; // (A)
  });

As behaviors are multicast, cancelation is an issue because other parts of the
code might rely on them. Usually, cancelation just means the behavior keeps
holding the value of the last processed event. It is more safe to throw an
exception in case of post cancellation access, though. This can be easily
defined inside the nullary `cancel` function. */


// smart constructor

export const Behavior = init => behave => {
  const o = {
    get run() {
      delete this.run;
      const  {state, cancel} = behave(init);

      Object.defineProperty(this, "run", {
        get() {return state.run}
      });
      
      this.cancel = cancel;
      return init;
    },

    cancel() {}
  };

  Object.defineProperty(o, TAG, {value: "Behavior"});
  return o;
};


export const Be = Behavior; // shortcut


/*█████████████████████████████████████████████████████████████████████████████
█████████████████████████████████████ BIT █████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


const popCount = n => {
  let bits = 0;

  while (n !== 0n) {
    bits += Number(n & 1n);
    n >>= 1n;
  }

  return bits;
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ COMPOSE ███████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// encodes the composition of functors


export const Comp = type("Comp");


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Comp.map = (Functor, Functor2) => f => ttx =>
  Comp(Functor.map(Functor2.map(f)) (ttx));


Comp.Functor = {map: Comp.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


Comp.ap = (Apply, Apply2) => ttf => ttx =>
  Comp(Apply.ap(Apply.map(Apply2.ap) (ttf)) (ttx));


Comp.Apply = {
  ...Comp.Functor,
  ap: Comp.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


Comp.of = (Applicative, Applicative2) => x =>
  Comp(Applicative.of(Applicative2.of(x)));


Comp.Applicative = {
  ...Comp.Apply,
  of: Comp.of
};


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ CONST ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// encodes constant behavior in the realm of functors/monads


export const Const = type("Const");


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Const.map = const_;


Const.Functor = {map: Const.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


Const.ap = Semigroup => tf => tx =>
  Const(Semigroup.append(tf.const) (tx.const));


Const.Apply = {
  ...Const.Functor,
  ap: Const.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


Const.of = Monoid => _ => Const(Monoid.empty);


Const.Applicative = {
  ...Const.Apply,
  of: Const.of
};


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████ CONTINUATION █████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// encode stack-safe continuation passing style using a trampoline


// TODO: remove trampoline at less expensive combinators


export const Cont = type("Cont");


Cont.eval = tx => tx.cont(id);


Cont.run = f => tx => tx.cont(f);


/*
█████ Binary ██████████████████████████████████████████████████████████████████*/


Cont.binary = mx => my => Cont(k => mx.cont(x => my.cont(y => k(x, y))));


/*
█████ Category ████████████████████████████████████████████████████████████████*/


Cont.comp = f => g => Cont(k => x =>
  g(x).cont(f).cont(Cont.Tramp.call_(k)));


Cont.id = tx => tx.cont(id);


Cont.Category = {
  comp: Cont.comp,
  id: Cont.id
};


/*
█████ Composition █████████████████████████████████████████████████████████████*/


// (r -> r) -> Cont r t -> Cont r t
Cont.mapCont = f => tx => Cont(k => f(tx.cont(Cont.Tramp.call_(k))));


Cont.pipe = g => f => Cont(k => x =>
  g(x).cont(f).cont(Cont.Tramp.call_(k)));


// ((s -> r) -> t -> r) -> Cont r t -> Cont r s
Cont.withCont = f => tx => Cont(k => tx.cont(f(Cont.Tramp.call_(k))));


/*
█████ Delimited Continuations █████████████████████████████████████████████████*/


/* Delimited continuation expecting a monad in their codomain: (a => m r) => m r
If you only need pure values just pass the identity monad's type dictionary. */


// Cont r r -> Cont s r
Cont.reset = tx => Cont(k => k(tx.cont(id)));

// ((t -> r) -> Cont r r) -> Cont r t
Cont.shift = ft => Cont(k => ft(k).cont(id));


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Cont.map = f => tx => Cont(k => tx.cont(x => Cont.Tramp.call(k, f(x))));


Cont.Functor = {map: Cont.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


Cont.ap = tf => tx => Cont(k =>
  tf.cont(f => tx.cont(x => Cont.Tramp.call(k, f(x)))));


Cont.Apply = {
  ...Cont.Functor,
  ap: Cont.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


// applicative map but discard the second result


Cont.apEff1 = tx => ty => Cont(k =>
  Cont.Tramp.call(k, Cont.ap(Cont.map(_const) (tx)) (ty)));


// applicative map but discard the first result

Cont.apEff2 = tx => ty => Cont(k =>
  Cont.Tramp.call(k, Cont.ap(Cont.map(const_) (tx)) (ty)));


Cont.of = x => Cont(k => Cont.Tramp.call(k, x));


Cont.Applicative = {
  ...Cont.Apply,
  of: Cont.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


/* The mother of all chains: Take a continuation (the previous computation) and
a CPS function (the next computation yielding another continuation) and feed
the CPS function into the previous continuation as soon as the next continuation
is provided. */

Cont.chain = mx => fm => Cont(k => mx.cont(x => fm(x).cont(Cont.Tramp.call_(k))));


Cont.Chain = {
  ...Cont.Apply,
  chain: Cont.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


Cont.join = mmx => Cont(mmx.cont(id));


Cont.Monad = {
  ...Cont.Applicative,
  chain: Cont.chain
};


/*
█████ Lifting █████████████████████████████████████████████████████████████████*/


Cont.lift = f => x => Cont(k => k(f(x)));


Cont.lift2 = f => x => y => Cont(k => k(f(x) (y)));


/*
█████ Profunctor ██████████████████████████████████████████████████████████████*/


Cont.dimap = h => g => f => Cont(k =>
  x => h(x).cont(f).cont(g).cont(Cont.Tramp.call_(k)));


Cont.lmap = Cont.pipe;


Cont.rmap = Cont.comp;


Cont.Profunctor = {
  ...Cont.Functor,
  dimap: Cont.dimap,
  lmap: Cont.lmap,
  rmap: Cont.rmap
};


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


Cont.append = Semigroup => tx => ty => Cont(k =>
  tx.cont(x => ty.cont(y => Cont.Tramp.call(k, Semigroup.append(x) (y)))));


Cont.Semigroup = {append: Cont.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


Cont.empty = Monoid => Cont(k => Cont.Tramp.call(k, Monoid.empty));


Cont.Monoid = {
  ...Cont.Semigroup,
  empty: Cont.empty
};


/*
█████ Short Circuiting █████████████████████████████████████████████████████████*/


Cont.abrupt = x => Cont(k => x);


/* Short circuit mechanism (unwinds the whole stack). Usage:

  Cont.callcc(shortCircuit => Cont.chain(shortCircuit(0))
    (x => Cont.of(x * x))).cont(x => x); */

Cont.callcc = f => Cont(k => f(x => Cont(_ => k(x))).cont(k));


/*
█████ Trampoline ██████████████████████████████████████████████████████████████*/


// stack-safe invocation of the deferred function call tree using a trampoline

Cont.Tramp = {};


// TODO: consider call2/call2_

// strictly call the deferred function call tree

Cont.Tramp.pogo = tx => {
  while (tx?.tag === "call") {
    tx = tx.f(tx.x);
  }

  return tx;
};


// non-strictly call the deferred function call tree

Cont.Tramp.step = function* interpret_(tx) {
  while (tx?.tag === "call") {
    yield tx.x;
    tx = tx.f(tx.x);
  }

  yield tx;
};


Cont.Tramp.call = function call(f, x) {
  return {[TAG]: "Cont.Tramp", constructor: Cont.Tramp.call, f, x};
};


Cont.Tramp.call2 = function call2(f, x, y) {
  return {[TAG]: "Cont.Tramp", constructor: Cont.Tramp.call2, f, x, y};
};


Cont.Tramp.call_ = function call_(f) {
  return function call_(x) {
    return {[TAG]: "Cont.Tramp", constructor: Cont.Tramp.call, f, x};
  };
};


Cont.Tramp.call2_ = function call2_(f) {
  return function call2_(x) {
    return function call2_(y) {
      return {[TAG]: "Cont.Tramp", constructor: Cont.Tramp.call2, f, x, y};
    };
  };
};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


Cont.get = tx => Cont(k => Cont.Tramp.call(k, tx.cont));


Cont.reify = k => x => Cont(_ => k(x));


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ DATE █████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const DateTime = {}; // namespace


export const D = DateTime; // shortcut


/*
█████ Constants ███████████████████████████████████████████████████████████████*/


D.minInMs = 60000;


D.hourInMs = 3600000;


D.dayInMs = 86400000;


D.weekInMs = 604800000;


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


/* Expects the following ISO date fragments/strings:

* YY-MM-DD
* YYYY-MM-DD
* YYYY-MM-DDTHH:MM:SS.SSSZ
* YYYY-MM-DDTHH:MM:SS.SSS+HH:MM
* YYYY-MM-DDTHH:MM:SS.SSS-HH:MM

Returns a date object either with UTC time or the timezone offset. */

D.fromStr = s => {
  let s2 = "", offset = 0;

  if (s.length === 8 && Rex.iso.dates.date8.test(s)) {
    s2 = "20" + s;
  }

  else if (s.length === 10 && Rex.iso.dates.date10.test(s)) s2 = s;
  else if (s.length === 24 && Rex.iso.dates.datetimeUTC.test(s)) s2 = s;

  else if (s.length === 29 && Rex.iso.dates.datetimeLoc.test(s)) {
    s2 = s.slice(0, 23) + "Z";
    offset = Number(s.slice(23, -3)) * D.hourInMs;
  }

  else throw new Err(`invalid date string "${s}"`);

  const d = new Date(s2);

  if (Number.isNaN(d.getTime()))
    throw new Err(`invalid date string "${s}"`);
  
  else {
    const d2 = new Date(
      d.getTime()
        + d.getTimezoneOffset()
        * D.minInMs
        + offset);

    return d2;
  }
};


/*
█████ Serialization ███████████████████████████████████████████████████████████*/


D.format = sep => (...fs) => d =>
  fs.map(f => f(d))
    .join(sep);


D.formatDay = digits => d => {
  switch (digits) {
    case 1: return String(d.getDate());
    case 2: return String(d.getDate()).padStart(2, "0");
    default: throw new Err("invalid number of digits");
  }
};


D.formatMonth = ({names = [], digits}) => d => {
  switch (digits) {
    case 1: return String(d.getMonth() + 1);
    case 2: return String(d.getMonth() + 1).padStart(2, "0");
    case 3: return names[String(d.getMonth())];
    default: throw new Err("invalid number of digits");
  }
};


D.formatWeekday = ({names = [], digits}) => d => {
  switch (digits) {
    case 1: return String(d.getDay());
    case 2: return String(d.getDay()).padStart(2, "0");
    case 3: return names[String(d.getDay())];
    default: throw new Err("invalid number of digits");
  }
};


D.formatYear = digits => d => {
  switch (digits) {
    case 2: return String(d.getFullYear()).slice(2);
    case 4: return String(d.getFullYear());
    default: throw new Err("invalid number of digits");
  }
};


D.formatMilliSec = digits => d => {
  switch (digits) {
    case 1: return String(d.getMilliseconds());
    case 3: return String(d.getMilliseconds()).padStart(3, "0");
    default: throw new Err("invalid number of digits");
  }
};


D.formatSecond = digits => d => {
  switch (digits) {
    case 1: return String(d.getSeconds());
    case 2: return String(d.getSeconds()).padStart(2, "0");
    default: throw new Err("invalid number of digits");
  }
};


D.formatMinute = digits => d => {
  switch (digits) {
    case 1: return String(d.getMinutes());
    case 2: return String(d.getMinutes()).padStart(2, "0");
    default: throw new Err("invalid number of digits");
  }
};


D.formatHour = digits => d => {
  switch (digits) {
    case 1: return String(d.getHours());
    case 2: return String(d.getHours()).padStart(2, "0");
    default: throw new Err("invalid number of digits");
  }
};


// time zone

D.formatTz = digits => d => {
  if (digits === 0) return "Z";

  else {
    let offset = d.getTimezoneOffset() / 60, sign = "";

    if (offset < 0) {
      sign = "-";
      offset = String(offset).slice(1);
    }

    else if (offset > 0) sign = "+";

    else return "Z";

    switch (digits) {
      case 1: return sign + offset + ":0";
      case 2: return sign + offset.padStart(2, "0") + ":00";
      default: throw new Err("invalid number of digits");
    }
  }
};


// DD.MM.YY

D.formatDe8 = D.format(".") (
  D.formatDay(2),
  D.formatMonth({digits: 2}),
  D.formatYear(2));


// DD.MM.YYYY

D.formatDe10 = D.format(".") (
  D.formatDay(2),
  D.formatMonth({digits: 2}),
  D.formatYear(4));


// YY-MM-DD

D.formatIso8 = D.format("-") (
  D.formatYear(2),
  D.formatMonth({digits: 2}),
  D.formatDay(2));


// YYYY-MM-DD

D.formatIso10 = D.format("-") (
  D.formatYear(4),
  D.formatMonth({digits: 2}),
  D.formatDay(2));


// HH:MM

D.formatTimeIso5 = D.format(":") (
  D.formatHour(2),
  D.formatMinute(2));


// HH:MM:SS

D.formatTimeIso8 = D.format(":") (
  D.formatHour(2),
  D.formatMinute(2),
  D.formatSecond(2));


// HH:MM:SS.SSS

D.formatTimeIso12 = d =>
  [D.formatTimeIso8, D.formatMilliSec(3)]
    .map(f => f(d)).join(".");


// YYYY-MM-DDTHH:MM:SS.SSSZ

D.formatIso24 = d => 
  [D.formatIso10, D.formatTimeIso12]
    .map(f => f(d)).join("T")
      + D.formatTz(0) (d);


// YYYY-MM-DDTHH:MM:SS.SSS+|-HH:MM

D.formatIso29 = d => 
  [D.formatIso10, D.formatTimeIso12]
    .map(f => f(d)).join("T")
      + D.formatTz(2) (d);


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


// daylight saving time

D.isDst = d => {
  const jan = new Date(d.getFullYear(), 0, 1).getTimezoneOffset(),
    jul = new Date(d.getFullYear(), 6, 1).getTimezoneOffset();

  return Math.max(jan, jul) !== d.getTimezoneOffset();    
};


D.lastDayOfMonth = ({m, y}) => {
  const d = new Date(y, m, 1);
  return new Date(d - 1).getDate();
};


D.numDaysOfMonth = (y, m) => new Date(y, m, 0).getDate();


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ EITHER ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// either left or right (used for short circuiting)


export const Either = variant("Either") (unary("Left"), unary("Right"));


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ EXCEPT ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Short circuit computations if they fail and stack contingent exceptions
along the way. */


export const Except = {}; // namespace


export const E = Except; // shortcut


E.cata = x => tx => tx?.constructor?.name === "Exception" ? x : tx;


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


/* Since the type isn't defined as a sum type some imperative introspection is
required. */

E.map = f => tx => tx?.constructor?.name === "Exception" ? tx : f(tx);


E.Functor = {map: E.map};


/*
█████ Functor :: Alt ██████████████████████████████████████████████████████████*/


// pick with left bias

E.alt = tx => ty => {
  if (tx?.constructor?.name === "Exception") {
    if (ty?.constructor?.name === "Exception")
      return (ty.prev = tx, ty);

    else return ty;
  }

  else return tx;
};


E.Alt = {
  ...E.Functor,
  alt: E.alt
};


/*
█████ Functor :: Alt :: Plus ██████████████████████████████████████████████████*/


E.zero = new Ex();


E.Plus = {
  ...E.Alt,
  zero: E.zero
};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


E.ap = tf => tx => {
  if (tf?.constructor?.name === "Exception") {
    if (tx?.constructor?.name === "Exception") return (tx.prev = tf, tx);
    else return tf;
  }

  else if (tx?.constructor?.name === "Exception") return tx;
  else return tf(tx);
};


E.Apply = {
  ...E.Functor,
  ap: E.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


E.of = x => {
  if (x?.constructor?.name === "Exception")
    throw new Err("invalid value");

  else return x;
}


E.Applicative = {
  ...E.Apply,
  of: E.of
};


/*
█████ Functor :: Apply :: Applicative :: Alternative ██████████████████████████*/


E.Alternative = {
  ...E.Plus,
  ...E.Applicative
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


E.chain = mx => fm => mx?.constructor?.name === "Exception" ? mx : fm(mx);


E.Chain = {
  ...E.Apply,
  chain: E.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


E.Monad = {
  ...E.Applicative,
  chain: E.chain
};


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


E.append = Semigroup => tx => ty => {
  if (tx?.constructor?.name === "Exception") {
    if (ty?.constructor?.name === "Exception")
      return (ty.prev = tx, ty);
    
    else return tx;
  }

  else if (ty?.constructor?.name === "Exception") return ty;
  else return Semigroup.append(tx) (ty);
};


E.Semigroup = {append: E.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


E.empty = Monoid => Monoid.empty;


E.Monoid = {
  ...E.Semigroup,
  empty: E.empty
};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


E.throwOnErr = tx => {
  if (tx?.constructor?.name === "Exception") throw tx;
  else return tx;
};


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ FREE █████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Of course you wanna be free, who wouldn't? `Free` separates program
construction from its evaluation. Usage:

// Monad m -> ListT m a
const nil = dict => dict.of([]);

// Monad m -> a -> ListT m a -> ListT m a
const cons = dict => head => tail => dict.of([head, tail]);

// Number -> a -> ListT Free a
const replicate = n => x => n
  ? cons(Free.Monad) (x) (Free.thunk(() => replicate(n - 1) (x)))
  : nil(Free.Monad);

// Number -> Free Number -> Free Number
const add = x => Free.map(y => x + y);

// Free Number
const tx = Eff.T.Fold.list(Free.Monad) (add)
  (Free.of(0))
    (replicate(1000000) (1));

Free.interpret(tx); // yields 1000000 */


export const Free = {};


/*
█████ Constructors ████████████████████████████████████████████████████████████*/


// (() -> Free a) -> Free a
Free.thunk = eval_ => ({constructor: Free.thunk, eval: eval_});


/*
█████ Interpretation ██████████████████████████████████████████████████████████*/


// Free a -> a
Free.interpret = expression => {
  let expr = expression, stack = null;

  while (true) {
    switch (expr.constructor) {
      case Free.of: {
        if (stack === null) return expr.value;
        expr = stack.fm(expr.value);
        stack = stack.stack;
        break;
      }

      case Free.chain: {
        stack = { fm: expr.fm, stack };
        expr = expr.monad;
        break;
      }

      // deferring thunks

      case Free.thunk: {
        expr = expr.eval();
        break;
      }

      // lazy evaluated thunks

      case Thunk: {
        expr = strict(expr);
        break;
      }

      default: throw new Err(
        `unknown constructor "${expr?.constructor?.name}"`);
    }
  }
};


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


// (a -> b) -> Free a -> Free b
Free.map = f => mx => Free.chain(mx) (x => Free.of(f(x)));


Free.Functor = {map: Free.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


// Free (a -> b) -> Free a -> Free b
Free.ap = mf => mx => Free.chain(mf) (f =>
  Free.chain(mx) (x => f(t)));


Free.Apply = {
  ...Free.Functor,
  ap: Free.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


// a -> Free a
Free.of = value => ({constructor: Free.of, value});


Free.Applicative = {
  ...Free.Apply,
  of: Free.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


// Free a -> (a -> Free b) -> Free b
Free.chain = mx => fm => ({constructor: Free.chain, monad: mx, fm});


Free.Chain = {
  ...Free.Apply,
  chain: Free.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


Free.Monad = {
  ...Free.Applicative,
  chain: Free.chain
};


/*█████████████████████████████████████████████████████████████████████████████
█████████████████████████████████████ ID ██████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// encodes the absence of any effects in the realm of functors/monads


export const Id = type("Id");


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Id.map = f => tx => Id(f(tx.id));


Id.Functor = {map: Id.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


Id.ap = tf => tx => Id(tf.id(tx.id));


Id.Apply = {
  ...Id.Functor,
  ap: Id.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


Id.of = Id;


Id.Applicative = {
  ...Id.Apply,
  of: Id.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


Id.chain = mx => fm => fm(mx.id);


Id.Chain = {
  ...Id.Apply,
  chain: Id.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


Id.Monad = {
  ...Id.Applicative,
  chain: Id.chain
};


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████████ ITERATOR ███████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Iterators defer execution which isn't sufficient for lazy evaluation because
it lacks sharing. You cannot share an iterator since every invocation of `next`
destructively changes the iterator's state. For this reason, scriptum provides
a wrapper that renders native iterators idempotent.

Some basic rules on working with impure iterators:

* it is the responsibiliy of the mapper to maintain the outermost structure of
  the return value across generator function calls, e.g. with `[k, v]` the key
  or value may change inside `[]` but not the array itself
* strict functions must not be passed to `comp`/`pipe` as an argument but always
  be the outermost function call
* folds require only strict exhaustion with `It.toAccum` to obtain an accumulated
  result value because they operate their own internal accumulator */


export const It = {};


/*
█████ Alternation █████████████████████████████████████████████████████████████*/


It.intercalate = x => function* (ix) {
  let initial = true;

  while (true) {
    const o = ix.next();
    if (o.done) return undefined;
    else if (initial) (initial = false, yield o.value);
    else (yield x, yield o.value);
  }
};


It.interleave = ix => function* (iy) {
  while (true) {
    const o = ix.next(), p = iy.next();
    if (o.done || p.done) return undefined;
    else (yield o.value, yield p.value);
  }
};


/*
█████ Cloning █████████████████████████████████████████████████████████████████*/


/* Mimic a cloned iterator by keeping track of the original iterator's consumed
values. Interferes with garbage collection and thus only be used cautiously. */

It.cloneable = ix => {
  const xs = [];

  return function make(n) {
    return {
      next(arg) {
        const len = xs.length;
        if (n >= len) xs[len] = it.next(arg);
        return xs[n++];
      },

      clone() {return make(n)},
      throw(e) {if (it.throw) it.throw(e)},
      return(v) {if (it.return) it.return(v)},
      [Symbol.iterator]() {return this}
    };
  } (0);
};


/*
█████ Combining/Dividing ██████████████████████████████████████████████████████*/


// consolidate consecutive pairs

It.combine = f => acc => function* (ix) {
  const o = ix.next(), p = ix.next();

  if (o.done && p.done) return undefined;
  else if (o.done) yield f(acc) (p.value);
  else if (p.done) yield f(o.value) (acc);
  else yield f(o.value) (p.value);
};


// uncurried

It.combine_ = f => acc => function* (ix) {
  const o = ix.next(), p = ix.next();

  if (o.done && p.done) return undefined;
  else if (o.done) yield f(acc, p.value);
  else if (p.done) yield f(o.value, acc);
  else yield f(o.value, p.value);
};


It.divide = f => acc => function* (ix) {
  while (true) {
    const o = ix.next();
    if (o.done) return undefined;
    else for (x of f(o.value)) yield x;
  }
};


/*
█████ Con-/Deconstruction █████████████████████████████████████████████████████*/


It.cons = x => function* (ix) {
  yield x;

  while (true) {
    const o = ix.next();
    if (done) return undefined;
    else yield o.value;
  }
};


It.cons_ = ix => function* (x) {
  yield x;

  while (true) {
    const o = ix.next();
    if (o.done) return undefined;
    else yield o.value;
  }
};


It.head = function* (ix) {
  const o = ix.next();
  if (o.done) return undefined;
  else yield o.value;
};


It.snoc = x => function* (ix) {
  while (true) {
    const o = ix.next();
    if (o.done) yield x;
    else yield o.value;
  }
};


It.snoc_ = ix => function* (x) {
  while (true) {
    const o = ix.next();
    if (o.done) yield x;
    else yield o.value;
  }
};


It.tail = function* (ix) {
  ix.next();

  while (true) {
    const o = ix.next();
    if (o.done) return undefined;
    else yield o.value;
  }
};


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


// from iterable

It.from = x => x[Symbol.iterator] ();


It.fromList = function* (xs) {
  while (xs.length) {
    yield xs[0]
    xs = xs[1];
  }
};


/*
█████ Conjunction █████████████████████████████████████████████████████████████*/


/* Yield the current element provided it and the next one satisfy the given
predicate or short circuit the stream. */

It.and = pred => function* (ix) {
  let o = ix.next();

  if (o.done || !p(o.value)) return undefined;

  while (true) {
    const p = ix.next();

    if (p.done || !pred(p.value)) return undefined;
  
    else {
      yield o.value;
      o = p;
    }
  }
};


/*
█████ Disjunction █████████████████████████████████████████████████████████████*/


/* Yield either the current or the next element provided one of them satisfies
the given predicate or short circuit the stream. */

It.or = pred => function* (ix) {
  while (true) {
    const o = ix.next();

    if (o.done) return undefined;
    else if (p(o.value)) yield o.value;

    else {
      const p = ix.next();

      if (p.done || !pred(p.value)) return undefined;
      else yield p.value;
    }
  }
};


/*
█████ Filterable ██████████████████████████████████████████████████████████████*/


It.filter = p => function* (ix) {
  const o = ix.next();
  if (o.done) return undefined;
  else if (p(o.value)) yield o.value;
};


It.Filterable = {filter: It.filter};


/*
█████ Foldable ████████████████████████████████████████████████████████████████*/


// exhaustive left-associative fold

It.foldl = f => acc => ix => {
  while (true) {
    const o = ix.next();
    if (o.done) return undefined;
    else acc = f(acc) (o.value);
  }

  return acc;
};


// uncurried

It.foldl_ = f => acc => ix => {
  while (true) {
    const o = ix.next();
    if (o.done) return undefined;
    else acc = f(acc, o.value);
  }

  return acc;
};


// there is no right-accosiative fold for contingently infinite streams


// exhaustive map followed by folding the resulting monoid

It.foldMap = Monoid => f => ix => {
  let acc = Monoid.empty;

  while (true) {
    const o = ix.next();
    if (o.done) return undefined;
    else acc = Monoid.append(acc) (f(o.value));
  }

  return acc;
};


// exhaustive sum

It.sum = acc => ix => {
  while (true) {
    const o = ix.next();
    if (o.done) return undefined;
    else acc = acc + o.value;
  }

  return acc;
};


It.Foldable = {
  foldl: It.foldl
};


/*
█████ Foldable :: Traversable █████████████████████████████████████████████████*/


// no meaningful implementation


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


It.map = f => function* (ix) {
  const o = ix.next();
  if (o.done) return undefined;
  else yield f(o.value);
};


// map effects but discard values

It.mapEff = f => function* (ix) {
  const o = ix.next();
  if (o.done) return undefined;
  else (f(o.value), yield o.value);
};


It.Functor = {map: It.map};


/*
█████ Functor :: Alt ██████████████████████████████████████████████████████████*/


It.alt = ix => function* (iy) {
  const o = ix.next(), p = iy.next();
  if (o.done === false) yield o.value;
  else if (p.done === false) yield p.value;
  else return undefined;
};


It.Alt = {
  ...It.Functor,
  alt: It.alt
};


/*
█████ Functor :: Alt :: Plus ██████████████████████████████████████████████████*/


It.zero = function* () {} ();


It.Plus = {
  ...It.Alt,
  zero: It.zero
};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


It.ap = _if => function* (ix) {
  while (true) {
    const o = _if.next(), p = ix.next();
    if (o.done || p.done) return undefined;
    else yield o.value(p.value);
  }
};


It.Apply = {
  ...It.Functor,
  ap: It.ap
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


It.chain = mx => function* (fm) {
  while (true) {
    const o = mx.next();
    if (o.done) return undefined;
    else yield* fm(o.value);
  }
};


It.Chain = {
  ...It.Apply,
  chain: It.chain
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


It.of = function* (x) {yield x};


It.Applicative = {
  ...It.Apply,
  of: It.of
};


/*
█████ Functor :: Apply :: Applicative :: Alternative ██████████████████████████*/


It.Alternative = {
  ...It.Plus,
  ...It.Applicative
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


// kleisli composition


// (b -> Iterator c) -> (a -> Iterator b) -> a -> Iterator c
It.komp = f => g => function* (x) {
  for (let y of g(x)) {
    yield* f(y);
  }
};


// (b -> Iterator c) -> (a -> Iterator b) -> Iterator a -> Iterator c
It.komp_ = f => g => function* (ix) {
  for (let x of ix) {
    for (let y of g(x)) {
      yield* f(y);
    }
  }
};


It.Monad = {
  ...It.Applicative,
  chain: It.chain
};


/*
█████ Infinite ████████████████████████████████████████████████████████████████*/


It.cycle = function* (xs) {
  while (true) {
    yield* xs[Symbol.iterator]();
  }
};


It.iterate = f => function* (x) {
  while (true) {
    yield x;
    x = f(x);
  }
};


It.repeat = function* (x) {
  while (true) yield x;
};


/*
█████ Recursion Schemes ███████████████████████████████████████████████████████*/


// lazy recursion schemes are suitable to be encoded using native iterators


// anamorphism

It.ana = () => It.unfold;


// apomorhism: anamorphism plus extra short circuit mechanism

It.apo = f => function* (seed) {
  let x = seed;

  while (true) {
    const pair = f(x);
    
    if (strict(pair) === null) return undefined;
    
    else {
      const [y, z] = pair;

      if (intro(z) === "Error") {
        yield y;
        return undefined;
      }

      else {
        x = z;
        yield y;
      }
    }
  }
};


// catamorphism

It.cata = It.foldr;


It.para = f => source => acc => function* (ix) {
  while (true) {
    const o = ix.next();

    if (o.done) return undefined;
    
    else {
      acc = f(o.value) (source) (acc);
      yield acc;
    }
  } 
};


// hylomorphism: anamorphism and immediately following catamorphism

It.hylo = f => g => function* (seed) {
  const ix = It.ana(f) (seed);

  for (const x of ix) yield* It.cata(g) (x);
};


/* Zygomorphism: fold that depends on another fold. Example: Check whether the
length of the list is even or odd and how long it actual is at the same time. */

It.zygo = f => g => acc => acc2 => function* (ix) {
  for ([value, done] of It.cata(x => pair =>
    Pair(f(x) (pair[0]), g(x) (pair[0]) (pair[1]))) (Pair(acc, acc2))) {
      yield value[1];
  }
};



// mutumorphism: two folds that depend on each other (mutual recursion asa fold)

It.mutu = f => g => acc => acc2 => function* (ix) {
  for ([value, done] of It.cata(x => pair =>
    Pair(f(x) (pair[0]) (pair[1]), g(x) (pair[0]) (pair[1]))) (Pair(acc, acc2))) {
      yield value[1];
  }
};


// TODO: It.histo


// TODO: It.futu


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


It.append = ix => function* (iy) {
  while (true) {
    const o = ix.next();
    if (o.done) break;
    else yield o.value;
  }

  while (true) {
    const p = iy.next();
    if (p.done) return undefined;
    else yield p.value;
  }
};


It.Semigroup = {append: It.append};


/*
█████ Semigroup (Alignment) ███████████████████████████████████████████████████*/


It.Align = {};


It.Align.append = Semigroup => ix => function* (iy) {
  while (true) {
    const o = ix.next(), p = iy.next();
    if (o.done || p.done) return undefined;
    else yield Semigroup.append(o.value) (p.value);
  }
};


It.Align.Semigroup = {append: It.Align.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


It.empty = function* () {} ();


It.Monoid = {
  ...It.Semigroup,
  empty: It.empty
};


/*
█████ Semigroup :: Monoid (Alignment) █████████████████████████████████████████*/


It.Align.empty = function* (Monoid) {yield Monoid.empty};


It.Align.Monoid = {
  ...It.Align.Semigroup,
  empty: It.Align.empty
};


/*
█████ Sublists ████████████████████████████████████████████████████████████████*/


/* sublist combinators are supplied in strict and non-strict form. Non-scrict
combinators only specify the quantity but not the data structure that ultimately
contains it. */


It.drop = n => ix => {
  const acc = [];

  while (n-- > 0) {
    const o = ix.next();
    if (o.done) return acc;
  };

  while (true) {
    const o = ix.next();
    if (o.done) break;
    else acc.push(o.value);
  }

  return acc;
};


// non-strict

It.drop_ = n => function* (ix) {
  while (n-- > 0) {
    const o = ix.next();
    if (o.done) return undefined;
  };

  while (true) {
    const o = ix.next();

    if (o.done) return undefined;
    else yield o.value;
  }
};


It.dropWhile = p => ix => {
  const acc = [];

  while (true) {
    const o = ix.next();

    if (o.done) return acc;

    else if (!p(o.value)) {
      acc.push(o.value);
      break;
    }
  };

  while (true) {
    const o = ix.next();
    if (o.done) return acc;
    else acc.push(o.value);
  }

  return acc;
};


// non-strict

It.dropWhile_ = p => function* (ix) {
  while (true) {
    const o = ix.next();

    if (o.done) return undefined;

    else if (!p(o.value)) {
      yield o.value;
      break;
    }
  };

  while (true) {
    const o = ix.next();

    if (o.done) return undefined;
    else yield o.value;
  }
};


It.take = n => ix => {
  const acc = [];

  while (n-- > 0) {
    const o = ix.next();
    if (o.done) return acc;
    else acc.push(o.value);
  }

  return acc;
};


// non-strict

It.take_ = n => function* (ix) {
  while (n-- > 0) {
    const o = ix.next();
    if (o.done) return undefined;
    else yield o.value;
  }
};


It.takeWhile = p => ix => {
  const acc = [];

  while (true) {
    const o = ix.next();
    if (o.done) return acc;
    else if (p(o.value)) acc.push(o.value);
    else return acc;
  }

  return acc;
};


// non-strict

It.takeWhile_ = p => function* (ix) {
  while (true) {
    const o = ix.next();
    if (o.done) return undefined;
    else if (p(o.value)) yield o.value;
    else return undefined;
  }
};


/*
█████ Transformation ██████████████████████████████████████████████████████████*/


It.collate = p => ix => {
  const iy = It.cloneable(ix),
    iz = iy.clone();

  return Pair(
    function* () {
      while (true) {
        const o = iy.next();

        if (o.done) return undefined;
        else if (!p(o.value)) yield o.value;
      }
    } (),

    function* () {
      while (true) {
        const o = iz.next();

        if (o.done) return undefined;
        else if (p(o.value)) yield o.value;
      }
    } ()
  );
};


It.flatten = function* (iix) {
  while (true) {
    const o = iix.next();

    if (o.done) return undefined;

    while (true) {
      const p = o.value.next();

      if (p.done) break;
      else yield p.value;
    }
  }
};


/*
█████ Unfoldable ██████████████████████████████████████████████████████████████*/


/* Lazy, potentially infinite unfold that always needs to be at the beginning
of the iterator chain. */

It.unfold = f => function* (seed) {
  let x = seed;

  while (true) {
    const pair = f(x);
    
    if (strict(pair) === null) return undefined;
    
    else {
      const [y, z] = pair;

      x = z;
      yield y;
    }
  }
};


/*
█████ Unzipping ███████████████████████████████████████████████████████████████*/


It.unzip = ix => function* (iy) {
  while (true) {
    const o = ix.next();

    if (o.done) return undefined;
    
    else {
      yield o.value[0];
      yield o.value[1];
    }
  }
};


/*
█████ Zipping █████████████████████████████████████████████████████████████████*/


It.zip = ix => function* (iy) {
  while (true) {
    const o = ix.next(), p = iy.next();
    if (o.done || p.done) return undefined;
    else yield Pair(o.value, p.value);
  }
};


It.zipWith = f => ix => function* (iy) {
  while (true) {
    const o = ix.next(), p = iy.next();
    if (o.done || p.done) return undefined;
    else yield f(o.value) (p.value);
  }
};


/*
█████ Resolve Deps ████████████████████████████████████████████████████████████*/


It.ana = It.ana();


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████ ITERATOR :: ASYNC ██████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Smart constructor for an asynchronous iterator delivering meaningfully sized
chunks of data that can be processed in isolation from each other. This way, you
can process large data sources in a divide and conquer approach. */

export const Ait = ({stor, threshold = 0}) => ix => {
  let chunks = [], buffer = "";

  return async function* go() {
    for await (const value of ix) {
      chunks = (buffer + value.toString()).split(stor);
      buffer = chunks.pop();

      if (threshold > 0 && buffer.length > threshold)
        throw new Err("buffer overflow");

      else if (chunks.length === 0) yield* go();

      else {
        yield* async function* () {
          do {
            const chunk = chunks.shift();
            yield chunk;
          } while (chunks.length);

          yield* go();
        } ();
      }
    }

    if (buffer.length) {
      yield* async function* () {
        yield buffer;
      } ();
    }
  } ();
};


/*
█████ Alternation █████████████████████████████████████████████████████████████*/


Ait.intercalate = y => async function* (ix) {
  let initial = true;

  for await (const x of ix) {
    if (initial) (initial = false, yield x);
    else (yield y, yield x);
  }
};


Ait.interleave = ix => async function* (iy) {
  for await (const x of ix) {
    for await (const y of iy) {
      yield x;
      yield y;
    }    
  }
};


/*
█████ Combining/Dividing ██████████████████████████████████████████████████████*/


// combine consecutive pairs

Ait.combine = f => acc => async function* (ix) {
  const o = await ix.next(), p = await ix.next();

  if (o.done || p.done) return undefined;
  else if (o.done) yield f(acc) (p.value);
  else if (p.done) yield f(o.value) (acc);
  else yield f(o.value) (p.value);
};


// uncurried

Ait.combine_ = f => acc => async function* (ix) {
  const o = await ix.next(), p = await ix.next();

  if (o.done || p.done) return undefined;
  else if (o.done) yield f(acc, p.value);
  else if (p.done) yield f(o.value, acc);
  else yield f(o.value, p.value);
};


Ait.divide = f => acc => async function* (ix) {
  for await (const x of ix) {
    for (y of f(x)) yield y;
  }
};


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


Ait.from = x => x[Symbol.asyncIterator] ();


/*
█████ Filterable ██████████████████████████████████████████████████████████████*/


Ait.filter = p => async function* (ix) {
  for await (const x of ix) if (p(x)) yield x;
};


Ait.Filterable = {filter: Ait.filter};


/*
█████ Foldable ████████████████████████████████████████████████████████████████*/


// exhaustive left-associative fold

Ait.foldl = f => init => ix => S(k => function go(acc) {
  return ix.next().then(({value, done}) => {
    if (done) return k(acc);
    else return go(f(acc) (value));
  });
} (init));


// uncurried

Ait.foldl_ = f => init => ix => S(k => function go(acc) {
  return ix.next().then(({value, done}) => {
    if (done) return k(acc);
    else return go(f(acc, value));
  });
} (init));


// there is no right-accosiative fold for contingently infinite streams


// exhaustive map followed by folding the resulting monoid

Ait.foldMap = Monoid => f => ix => S(k => function go(acc) {
  return ix.next().then(({value, done}) => {
    if (done) return k(acc);
    else return go(Monoid.append(acc) (f(value)));
  });
} (Monoid.empty));


Ait.Foldable = {
  foldl: Ait.foldl
};


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Ait.map = f => async function* (ix) {
  for await (const x of ix) yield f(x);
};


// bifunctor-like

Ait.bimap = p => f => g => async function* (ix) {
  for await (const x of ix) {
    if (p(x)) yield f(x);
    else yield g(x);
  }
};


// map effects but discard values

Ait.mapEff = f => async function* (ix) {
  for await (const x of ix) (f(x), yield x);
};


Ait.Functor = {map: Ait.map};


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


Ait.append = ix => async function* (iy) {
  for await (const x of ix) {
    yield f(x);
  }

  for await (const y of iy) {
    yield f(y);
  }
};


Ait.Semigroup = {append: Ait.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


Ait.empty = async function* () {} ();


Ait.Monoid = {
  ...Ait.Semigroup,
  empty: Ait.empty
};


/*
█████ Sublists ████████████████████████████████████████████████████████████████*/


/* sublist combinators are supplied in strict and non-strict form. Non-scrict
combinators only specify the quantity but not the data structure that ultimately
contains it. */


Ait.drop = n => ix => S(k => function go(acc, m) {
  return ix.next().then(({value, done}) => {
    if (done) return k(acc);
    else if (m > 0) return go(acc, m - 1);
    else return go((acc.push(value), acc), 0);
  });
} ([], n));


// non-strict

Ait.drop_ = n => async function* (ix) {
  for (let i = n; i > 0; i--) {
    const o = await ix.next();
    if (o.done) return undefined;
  }

  for await (const x of ix) yield x;
};


Ait.dropWhile = p => ix => S(k => function go(acc) {
  return ix.next().then(({value, done}) => {
    if (done) return k(acc);
    else if (p(value)) return go(acc);
    else return go((acc.push(value), acc));
  });
} ([]));


// non-strict

Ait.dropWhile_ = p => async function* (ix) {
  for await (const x of ix) {
    if (!p(x)) {
      yield x;
      break;
    }
  }

  for await (const x of ix) yield x;
};


Ait.take = n => ix => S(k => function go(acc, m) {
  return ix.next().then(({value, done}) => {
    if (done) return k(acc);
    else if (m === 0) return k(acc);
    else return go((acc.push(value), acc), m - 1);
  });
} ([], n));


// non-strict

Ait.take_ = n => async function* (ix) {
  for (let i = 0; i < n; i++) {
    const o = await ix.next();
    if (o.done) return undefined;
    else yield o.value;
  }
};


Ait.takeWhile = p => ix => S(k => function go(acc) {
  return ix.next().then(({value, done}) => {
    if (done) return k(acc);
    else if (p(value)) return go((acc.push(value), acc));
    else return k(acc);
  });
} ([]));


// non-strict

Ait.takeWhile_ = p => async function* (ix) {
  for await (const x of ix) {
    if (p(x)) yield x;
    else break;
  }
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████ ITERATOR :: IDEMPOTENT ████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Idempotent iterators mimic immutable ones by rendering the `next` method
idempotent. The following sequence of invocations advances the iterator only
once:
  
  ix.next();
  ix.next();
  ix.next();

In order to advance it further, invocations need to be recursive:
  
  ix.next().next();

You must not separate the next method from its receiver, as with native
iterators:

  const next = ix.next;
  next(); // will advance the iterator as a side effect

Iterator results are nested and can be traversed backwards via the `prev`
property:

  const iy = ix.next().next().next();
  iy.prev.prev; // yields the first iterator result

*/


export const Iit = ix => {
  let iy = {
    value: null,
    done: false,

    next() {
      const iz = ix.next();

      Object.defineProperty(iz, TAG, {value: "IdempotentIterator"});
      iz.next = iy.next;
      iz.prev = iy;
      delete iy.next;
      iy.next = () => iz;
      iy = iz;
      return iz;
    }
  };

  Object.defineProperty(iy, TAG, {value: "IdempotentIterator"});
  return iy;
};


/*
█████ Sublists ████████████████████████████████████████████████████████████████*/


// resumable after the generator is forwarded n times

Iit.take = n => function* (ix) {
  while (true) {
    const iy = ix.next();

    if (iy.done) return undefined;
    else if (n <= 0) return ix;
    else yield iy.value;
    ix = iy;
    n--;
  }
};


// resumable after the generator is forwarded x times

Iit.takeWhile = p => function* (ix) {
  while (true) {
    const iy = ix.next();

    if (iy.done) return undefined;
    else if (p(iy.value)) yield ix.value;
    else return ix;
    ix = iy;
  }
};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


// clear the previous iterator result chain to avoid memeory leakage

Iit.clear = ix => (ix.prev = null, ix);


/*█████████████████████████████████████████████████████████████████████████████
█████████████████████████████████████ MAP █████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const _Map = {}; // namespace


/*
█████ Clone ███████████████████████████████████████████████████████████████████*/


_Map.clone = m => new Map(m);


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


_Map.fromIt = ix => {
  const m = new Map();
  for (const [k, v] of ix) m.set(k, v);
  return m;
};


// `k` must be unique

_Map.fromTable = k => xs => xs.reduce(
  (acc, o) => acc.set(o[k], o), new Map());


// key is dynamically generated by the passed function

_Map.fromTableBy = f => xs => xs.reduce(
  (acc, o) => acc.set(f(o), o), new Map());


_Map.interconvert = f => m => new Map(f(Array.from(m)));


_Map.interconvertBy = f => g => m => new Map(f(Array.from(m).map(g)));


/*
█████ Generators ██████████████████████████████████████████████████████████████*/


_Map.entries = m => m[Symbol.iterator] ();


_Map.keys = function* (m) {
  for (let [k] of m) {
    yield k;
  }
};


_Map.values = function* (m) {
  for (let [, v] in m) {
    yield v;
  }
};


/*
█████ Getters/Setters █████████████████████████████████████████████████████████*/


_Map.get = k => m => m.has(k) ? m.get(k) : null;


_Map.getOr = x => k => m => m.has(k) ? m.get(k) : x;


_Map.has = k => m => m.has(k);


_Map.set = k => v => m => m.set(k, v);


_Map.del = k => m => m.delete(k);


_Map.upd = k => f => m => {
  if (m.has(k)) return m.set(k, f(m.get(k)));
  else return m;
};


_Map.updOr = x => k => f => m => {
  if (m.has(k)) return m.set(k, f(m.get(k)));
  else return m.set(k, x);
};


/*
█████ Mappings ████████████████████████████████████████████████████████████████*/


_Map.monthsFullDe = new Map([
  ["Januar", "01"],
  ["Februar", "02"],
  ["März", "03"],
  ["Maerz", "03"],
  ["April", "04"],
  ["Mai", "05"],
  ["Juni", "06"],
  ["Juli", "07"],
  ["August", "08"],
  ["September", "09"],
  ["Oktober", "10"],
  ["November", "11"],
  ["Dezember", "12"]
]);


_Map.monthsShortDe = new Map([
  ["Jan", "01"],
  ["Feb", "02"],
  ["Mär", "03"],
  ["Mar", "03"],
  ["Apr", "04"],
  ["Mai", "05"],
  ["Jun", "06"],
  ["Jul", "07"],
  ["Aug", "08"],
  ["Sep", "09"],
  ["Okt", "10"],
  ["Nov", "11"],
  ["Dez", "12"]
]);


/*
█████ Union ███████████████████████████████████████████████████████████████████*/


_Map.union = m => n => {
  new Map(n).forEach((v, k) => m.set(k, v));
};


// destructive

_Map.union_ = m => n => {
  n.forEach((v, k) => m.set(k, v));
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████ MAP :: MULTIMAP ███████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Map variant maintaining 1:n-relations between its key/value pairs and thus
introducing ambiguity on the value side. Some accessor functions expect a
predicate as an additional argument. This predicate returns the first value it
is satisfied by. */


export class MultiMap extends Map {
  constructor() {
    super();
    Object.defineProperty(this, TAG, {value: "MultiMap"});
  }


  /*
  █████ Conversion ████████████████████████████████████████████████████████████*/


  static fromIt = ix => {
    const m = new MultiMap();
    for (const [k, v] of ix) m.addItem(k, v);
    return m;
  };


  // `k` must be unqiue

  static fromTable(xss, k) {
    const m = new MultiMap();

    for (const cols of xss) m.addItem(cols[k], cols);
    return m;
  }


  // key is dynamically generated by the passed function

  static fromTableBy(xss, f) {
    const m = new MultiMap();

    for (const cols of xss) m.addItem(f(cols), cols);
    return m;
  }


  /*
  █████ Getters/Setters ███████████████████████████████████████████████████████*/


  // rely on reference identity

  addItem(k, v) {
    if (this.has(k)) {
      const s = this.get(k);
      s.add(v);
      return this;
    }

    else {
      this.set(k, new Set([v]));
      return this;
    }
  }


  delItem(k, pred) {
    const s = this.get(k);

    for (const v of s) {
      if (pred(v)) {
        s.delete(v);
        break;
      }
    }

    if (s.size === 0) this.delete(k);
    return this;
  }


  getItem(k, pred) {
    const s = this.get(k);

    if (s === undefined) return s;

    else {
      for (const v of s) {
        if (pred(v)) return v;
      }

      return undefined;
    }
  }


  setItem(k, v, pred) {
    const s = this.get(k);
    let exists = false;

    for (const v2 of s) {
      if (pred(v2)) {
        s.delete(v2);
        s.add(v);
        exists = true;
        break;
      }
    }

    if (!exists) s.add(v);
    return this;
  }


  upd(k, f) {
    const s = this.get(k),
      s2 = new Set();

    if (s === undefined) return s;

    else {
      for (const v of s) s2.add(f(v));
      this.set(k, s2);
      return this;
    }
  }


  updItem(k, f, pred) {
    const s = this.get(k);

    if (s === undefined) return s;

    else {
      for (const v of s) {
        if (pred(v)) {
          s.delete(v);
          s.add(f(v));
          break;
        }
      }

      return this;
    }
  }


  /*
  █████ Iterator ██████████████████████████████████████████████████████████████*/


  // default

  *[Symbol.iterator]() {
    for (const [k, s] of super[Symbol.iterator]()) {
      for (const v of s) yield Pair(k, v);
    }
  }

  // do not abstract from multiple mapped datasets

  *iterate() {
    for (const [k, s] of super[Symbol.iterator]()) yield Pair(k, s);
  }
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ NUMBER ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const Num = {}; // namespace


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


// expects an ISO number string

Num.fromStr = s => {
  if (Rex.iso.numbers.num.test(s)) return Number(s);
  else throw new Err(`invalid number string: "${s}"`);
};


// more restrictive version (excludes 01.234 etc.)

Num.fromStr_ = s => {
  const n = Number(s);

  if (String(n) === s) return Number(s);
  else throw new Err(`invalid number string: "${s}"`);
};


/*
█████ Decimal Places ██████████████████████████████████████████████████████████*/


/* Note: All combinators may fail for large positive numbers encoded in
scientific notation. */


Num.ceil = places => n => {
  const n2 = Math.abs(n),
    sign = n < 0 ? "-" : "";

  const n3 = n2 * Number(1 + "0".repeat(places)),
    frac = n3 % 1;

  let s;

  if (n3 < Number("0." + "0".repeat(places) + "1")) return 0;
  else if (frac === 0) s = String(n3).split(".") [0];
  
  else {
    if (sign === "-") s = String(n3).split(".") [0];
    else s = String(n3 + 1).split(".") [0];
  }

  if (s[0] === "-") s = s.slice(1);

  if (s.length < places) return Number(sign + "0." + s.padStart(places, "0"));
  else if (s.length === places) return Number(sign + "0." + s);

  else {
    const intLen = String(n2).split(".") [0].length,
      s2 = s.slice(0, intLen) + "." + s.slice(intLen, intLen + places);

    return Number(sign + s2);
  }
};


Num.floor = places => n => {
  const n2 = Math.abs(n),
    sign = n < 0 ? "-" : "";

  const n3 = n2 * Number(1 + "0".repeat(places)),
    frac = n3 % 1;

  let s;

  if (n3 < Number("0." + "0".repeat(places) + "1")) return 0;
  else if (frac === 0) s = String(n3).split(".") [0];
  
  else {
    if (sign === "-") s = String(n3 + 1).split(".") [0];
    else s = String(n3).split(".") [0];
  }

  if (s[0] === "-") s = s.slice(1);

  if (s.length < places) return Number(sign + "0." + s.padStart(places, "0"));
  else if (s.length === places) return Number(sign + "0." + s);

  else {
    const intLen = String(n2).split(".") [0].length,
      s2 = s.slice(0, intLen) + "." + s.slice(intLen, intLen + places);

    return Number(sign + s2);
  }
};


Num.round = places => n => {
  const n2 = Math.abs(n),
    sign = n < 0 ? "-" : "";

  const n3 = n2 * Number(1 + "0".repeat(places)),
    frac = n3 % 1;

  let s;

  if (n3 < Number("0." + "0".repeat(places) + "5")) return 0;
  else if (frac < 0.5) s = String(n3).split(".") [0];
  else s = String(n3 + 1).split(".") [0];

  if (s[0] === "-") s = s.slice(1);

  if (s.length < places) return Number(sign + "0." + s.padStart(places, "0"));
  else if (s.length === places) return Number(sign + "0." + s);

  else {
    const intLen = String(n2).split(".") [0].length,
      s2 = s.slice(0, intLen) + "." + s.slice(intLen, intLen + places);

    return Number(sign + s2);
  }
};


Num.round2 = Num.round(2);


// equivalent to `floor` for positive numbers

Num.trunc = places => n => {
  const [int, frac] = String(n).split(".");

  if (frac.length < places)
    return n;

  else if (frac.length > places)
    return Number(int + "." + frac.slice(0, places));
    
  else return n;
};


/*
█████ Serialization ███████████████████████████████████████████████████████████*/


Num.format = (...fs) => n =>
  fs.map(f => f(n))
    .join("");


Num.formatFrac = digits => n =>
  String(n)
    .replace(/^[^.]+\.?/, "")
    .padEnd(digits, "0");


Num.formatInt = sep => n =>
  String(Math.trunc(n))
    .replace(/^-/, "")
    .replace(new RegExp("(\\d)(?=(?:\\d{3})+$)", "g"), `$1${sep}`);


Num.formatSign = ({pos, neg}) => n =>
  n > 0 ? pos : n < 0 ? neg : "";


Num.formatSep = sep => n => sep;


Num.formatIso = Num.format(
  Num.formatSign({pos: "", neg: "-"}),
  Num.formatInt(""),
  Num.formatSep("."),
  Num.formatFrac(2));


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████ NUMBER :: NATURAL ██████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/



export const Natural = {}; // namespace


export const Nat = Natural; // shortcut


/* Educational implementation of a catamorphism as the eleminaton rule of the
natural number type. In this fashion, you can almost eliminate any type in a
stack-safe manner:

  const fib = comp(Pair.fst) (Nat.cata({
    zero: Pair(0, 1),
    succ: ([a, b]) => Pair(b, a + b)
  }));

  fib(10); // yields 55 */


Nat.cata = ({zero, succ}) => n => {
  let r = zero;

  while (n > 0) {
    r = succ(r);
    n -= 1;
  }

  return r;
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ OBJECT ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const O = {};


/*
█████ Cloning █████████████████████████████████████████████████████████████████*/


// cloning without losing getters/setters

O.clone = o => {
  const p = {};

  for (const k of objKeys(o))
    Object.defineProperty(
      p, k, Object.getOwnPropertyDescriptor(o, k));

  return p;
};


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


O.fromArr = header => xs => {
  const o = {};

  for (let i = 0; i < xs.length; i++)
    if (header.has(i)) o[header.get(i)] = xs[i];

  return o;
};


O.fromIt = ix => {
  const o = {};
  for (const [k, v] of ix) o[k] = v;
  return o;
};


O.fromPairs = pairs => pairs.reduce((acc, [k, v]) => (acc[k] = v, acc), {});


/*
█████ Getters/Setters █████████████████████████████████████████████████████████*/


O.del = k => o => (delete o[k], o);


O.del_ = k => o => {
  const p = Object.assign({}, o);
  delete p[k];
  return p;
};


O.get = k => o => k in o ? o[k] : null;


O.getOr = x => k => o => k in o ? o[k] : x;


O.getDeepOr = x => (...keys) => o => {
  for (const k of keys) {
    if (o) o = o[k];
    else return x;
  }

  return o === undefined ? x : o;
};


O.getDeep = O.getDeepOr(undefined);


// more general version

O.getDeepOr_ = x => (...getters) => o => {
  for (const getter of getters) {
    if (o) o = getter(o);
    else return x;
  }

  return o === undefined ? x : o;
};


O.getDeep_ = O.getDeepOr_(undefined);


O.set = k => v => o => (o[k] = v, o);


O.set_ = k => v => o => Object.assign({}, o, {[k]: v});


O.upd = k => f => o => {
  if (k in o) return (o[k] = f(o[k]), o);
  else return o;
};


O.upd_ = k => f => o => {
  if (k in o) return Object.assign({}, o, {[k]: f(o[k])});
  else return o;
};


O.updOr = x => k => f => o => {
  if (k in o) return (o[k] = f(o[k]), o);
  else return (o[k] = x, o);
};


O.updOr_ = x => k => f => o => {
  if (k in o) return Object.assign({}, o, {[k]: f(o[k])});
  else return Object.assign({}, o, {[k]: x});
};


/*
█████ Iterable ████████████████████████████████████████████████████████████████*/


O.entries = function* (o) {
  for (let prop in o) {
    yield [prop, o[prop]];
  }
};


O.keys = function* (o) {
  for (let prop in o) {
    yield prop;
  }
};


O.values = function* (o) {
  for (let prop in o) {
    yield o[prop];
  }
};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


// create lazy property that shares its result

O.lazyProp = k => thunk => o => {
  return Object.defineProperty(o, k, {
    get() {delete o[k]; return o[k] = thunk()},
    configurable: true,
    enumerable: true
  });
};


O.lazyProps = dtors => o => Object.defineProperties(o, ...dtors);


// self referencing during object creation

export const thisify = f => f({});


O.thisify = thisify;


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ STREAM ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Data stream type with meaningful chunks of data. The stream implementation
is supplied as a formal parameter to avoid a hard dependency. */


export const Stream_ = stream => ({stor, threshold, skipRest}) => {
  return new stream.Transform({
    transform(chunk, encoding, k) {
      const lines = ((this.buffer ? this.buffer : "")
        + chunk.toString()).split(stor);

      this.buffer = lines.pop();

      if (threshold > 0 && this.buffer.length > threshold)
        k(new Err("buffer overflow"));

      for (const line of lines) k(null, line);
    },

    flush(k) {
      if (skipRest) k(null, "");
      else k(null, this.buffer ? this.buffer : "");
    }
  });
};


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ OPTIC ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Defines a focus inside a data structure using composable pairs of getters
and setters to allow deep modifications without losing the reference to the data
structure itself. Normal function composition is used to define foci several
layers deep. The type implicitly holds a description how to reconstruct the data
structure up to its outer layer. It depends on the used setters whether an optic
is destructive or non-destructive.

const o = {foo: {bar: 5}};

// set operation

const tx = comp(
  Optic.focus(O.get("bar"), O.set("bar")))
    (Optic.focus(O.get("foo"), O.set("foo")))
      (Optic.of(o));

Optic.defocus(Optic.set(55) (tx)); // {foo: {bar: 55}}

// update operation

const ty = comp(
  Optic.focus(O.get("bar"), O.upd("bar")))
    (Optic.focus(O.get("foo"), O.set("foo")))
      (Optic.of(o));

Optic.defocus(Optic.set(x => x * x) (ty)); // {foo: {bar: 25}}

// delete operation

const tz = comp(
  Optic.focus(O.get("bar"), _const(O.del("bar"))))
    (Optic.focus(O.get("foo"), O.set("foo")))
      (Optic.of(o));

Optic.defocus(tz); // {foo: {}} */


export const Optic = type("Optic", "opt");


/*
█████ Defocus █████████████████████████████████████████████████████████████████*/


// reconstruct the data structure with the specified modifications

Optic.defocus = tx =>
  tx.parent === null ? tx : Optic.defocus(tx.parent(tx.opt));


// only reconstructs a single layer

Optic.defocus1 = tx =>
  tx.parent === null ? tx : tx.parent(tx.opt);


/*
█████ Focus ███████████████████████████████████████████████████████████████████*/


// set a composable focus on a sub element of a data structure

Optic.focus = (getter, setter) => tx => Optic(
  getter(tx.opt),
  x => Optic(setter(x) (tx.opt), tx.parent));


// try to focus on the specified element or use a default value

Optic.tryFocus = x => (getter, setter) => tx => Optic(
  tx.opt === null ? getter(x) : getter(tx.opt),
  y => Optic(setter(y) (tx.opt === null ? y : tx.opt), tx.parent));


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Optic.map = f => tx => Optic(f(tx.opt), tx.parent);


Optic.Functor = {map: Optic.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


/* Lift a binary function into an applicative functor. There is an ambiguity
regarding which object propert is to be updated with the final result value. */


Optic.ap = tf => tx => Optic(tf.opt(tx.opt), tf.parent); // left-biased


Optic.ap_ = tf => tx => Optic(tf.opt(tx.opt), tx.parent); // right-biased


Optic.Apply = {
  ...Optic.Functor,
  ap: Optic.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


Optic.of = x => Optic(x, null);


Optic.Applicative = {
  ...Optic.Apply,
  of: Optic.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


/* Discards the parent of the next monadic computation but takes the current
one. Dunno whether this is meaningful in any way. */

Optic.chain = mx => fm => {
  const my = fm(mx.opt);
  return Optic(my.opt, mx);
}


Optic.Chain = {
  ...Optic.Apply,
  chain: Optic.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


Optic.Monad = {
  ...Optic.Applicative,
  chain: Optic.chain
};


/*
█████ Getters/Setters █████████████████████████████████████████████████████████*/


/* Auxiliary helpers to avoid lambdas when running an optic. delete operation
is skipped because it doesn't need a lambda in the first place. */


Optic.add = Semigroup => x => tx =>
  Optic(Semigroup.append(tx.opt) (x), tx.parent);


Optic.set = x => tx => Optic(x, tx.parent);


Optic.upd = f => tx => Optic(f(tx.opt), tx.parent);


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ OPTION ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// handle computations that may yield no result


export const Option = {}; // namespace


export const Opt = Option; // shortcut


Opt.cata = x => tx => tx === null ? x : tx;


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Opt.map = f => tx => tx === null ? null : f(tx);


Opt.Functor = {map: Opt.map};


/*
█████ Functor :: Alt ██████████████████████████████████████████████████████████*/


Opt.alt = tx => ty => tx === null ? ty : tx;


Opt.Alt = {
  ...Opt.Functor,
  alt: Opt.alt
};


/*
█████ Functor :: Alt :: Plus ██████████████████████████████████████████████████*/


Opt.zero = null;


Opt.Plus = {
  ...Opt.Alt,
  zero: Opt.zero
};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


Opt.ap = tf => tx =>
  tf === null ? null
    : tx === null ? null
    : tf(tx);


Opt.Apply = {
  ...Opt.Functor,
  ap: Opt.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


/* Since the type isn't defined as a sum type some imperative introspection is
required. */

Opt.of = x => x === null ? _throw("invalid value") : x;


Opt.Applicative = {
  ...Opt.Apply,
  of: Opt.of
};


/*
█████ Functor :: Apply :: Applicative :: Alternative ██████████████████████████*/


Opt.Alternative = {
  ...Opt.Plus,
  ...Opt.Applicative
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


Opt.chain = mx => fm => mx === null ? null : fm(mx);


Opt.Chain = {
  ...Opt.Apply,
  chain: Opt.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


Opt.Monad = {
  ...Opt.Applicative,
  chain: Opt.chain
};


/*
█████ List Operations █████████████████████████████████████████████████████████*/


Opt.cat = xs => {
  const acc = [];

  for (const x of xs) {
    if (x === null) continue;
    else acc.push(x);
  }
};


Opt.mapCat = f => xs => {
  const acc = [];

  for (const x of xs) {
    if (x === null) continue;
    else acc.push(f(x));
  }
};


Opt.singleton = xs => xs.length === 0 ? [] : [xs[0]];


Opt.singleton_ = xs => xs.length === 0 ? [] : [xs[xs.length - 1]];


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


Opt.append = Semigroup => tx => ty =>
  tx === null ? tx
    : ty === null ? tx
    : Semigroup.append(tx) (ty);


Opt.Semigroup = {append: Opt.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


Opt.empty = null;


Opt.Monoid = {
  ...Opt.Semigroup,
  empty: Opt.empty
};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


Opt.run = x => f => tx => tx === null ? x : f(tx);


Opt.toNecessary = tx => {
  if (tx === null) throw new Err("missing value");
  else return tx;
}


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████████ PARALLEL ███████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Just like `Serial` but evaluated in parallel. See `Serial` for more
comprehensive information. */


// smart constructor

export const Parallel = k => {
  const o = {
    par: k,

    parSafe: f => { // stack-safe
      if (asyncCounter > 100) {
        asyncCounter = 0;
        return Promise.resolve(null).then(_ => k(f));
      }

      else {
        asyncCounter++;
        return k(f);
      }
    }
  };

  Object.defineProperty(o, TAG, {value: "Parallel"});
  return o;
};


export const P = Parallel; // shortcut


/*
█████ Category ████████████████████████████████████████████████████████████████*/


P.comp = f => g => P(k => x => g(x).opt.par(f).par(k));


P.id = tx => tx.par(id);


P.Category = {
  comp: P.comp,
  id: P.id
};


/*
█████ Composition █████████████████████████████████████████████████████████████*/


// (r -> r) -> P r t -> P r t
P.mapCont = f => tx => P(k => f(tx.par(k)));


P.pipe = g => f => P(k => x =>
  g(x).par(f).par(k));


// ((s -> r) -> t -> r) -> P r t -> P r s
P.withCont = f => tx => P(k => tx.par(f(k)));


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


P.fromSerial = tx => P(tx.ser);


/*
█████ Conjunction █████████████████████████████████████████████████████████████*/


P.and = tx => ty => {
  return P(k => {
    const pair = Array(2);
    let i = 0;

    return [tx, ty].map((tz, j) => {
      return tz.par(z => {
        if (i < 2) {
          if (pair[j] === undefined) {
            pair[j] = z;
            i++;
          }

          if (i === 2) return k(Pair(pair[0], pair[1]));
          else return null;
        }

        else return k(Pair(pair[0], pair[1]));
      });
    });
  });
};


P.allArr = () =>
  A.seqA({
    map: P.map,
    ap: P.ap,
    of: P.of});


P.allObj = o => {
  const keys = Object.keys(o);

  return P(k => {
    const xs = Array(keys.length),
      p = Object.assign({}, o); // preserve prop order

    let i = 0;

    return keys.map((key, j) => {
      return o[key].par(x => {
        if (i < keys.length) {
          if (xs[j] === undefined) {
            p[key] = x;
            xs[j] = null;
            i++;
          }

          if (i === keys.length) return k(p);
          else return null;
        }

        else return null;
      });
    });
  });
};


/*
█████ Debugging ███████████████████████████████████████████████████████████████*/


P.async = f => msecs => x => P(k => setTimeout(comp(k) (f), msecs, x));


/*
█████ Disjunction █████████████████████████████████████████████████████████████*/


P.or = tx => ty => {
  return P(k => {
    let done = false;

    return [tx, ty].map(tz => {
      return tz.par(z => {
        if (!done) {
          done = true;
          return k(z);
        }

        else return null;
      });
    });
  });
};


P.anyArr = () =>
  A.foldl(acc => tx =>
    P.Race.append(acc) (tx))
      (P.Race.empty);


P.anyObj = o =>
  A.foldl(acc => tx =>
    P.Race.append(acc) (tx))
      (P.Race.empty)
        (Object.values(o));


/*
█████ Excaption Handling ██████████████████████████████████████████████████████*/


P.tryCatch = k2 => tx => P(k => tx.par(x => {
  if (x?.constructor?.name === "Exception") return k2(x);
  else return k(x);
}));


P.tryThrow = tx => P(k => tx.par(x => {
  if (x?.constructor?.name === "Exception") throw x;
  else return k(x);
}));


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


P.map = f => tx =>
  P(k => tx.par(x => k(f(x))));


P.Functor = {map: P.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


P.ap = tf => tx => P(k =>
  P.and(tf) (tx).par(([f, x]) =>
    k(f(x))));


P.Apply = {
  ...P.Functor,
  ap: P.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


P.of = x => P(k => k(x));


P.Applicative = {
  ...P.Apply,
  of: P.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


// please note that Chain diverges from Apply in terms of evaluation order


P.chain = mx => fm =>
  P(k => mx.par(x => fm(x).par(k)));


P.Chain = {
  ...P.Apply,
  chain: P.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


// please note that Monad diverges from Applicative in terms of evaluation order


P.Monad = {
  ...P.Applicative,
  chain: P.chain
};


/*
█████ Profunctor ██████████████████████████████████████████████████████████████*/


P.dimap = h => g => f => P(k => x => h(x).par(f).par(g).par(k));


P.lmap = P.pipe;


P.rmap = P.comp;


P.Profunctor = {
  ...P.Functor,
  dimap: P.dimap,
  lmap: P.lmap,
  rmap: P.rmap
};


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


P.append = Semigroup => tx => ty => P(k =>
  P.and(tx) (ty).par(([x, y]) =>
    k(Semigroup.append(x) (y))));


P.Semigroup = {append: P.append};

  
/*
█████ Semigroup (Race) ████████████████████████████████████████████████████████*/


P.Race = {};


P.Race.append = P.or;


P.Race.Semigroup = {append: P.Race.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


P.empty = Monoid => P(k => k(Monoid.empty));


P.Monoid = {
  ...P.Semigroup,
  empty: P.empty
};


/*
█████ Semigroup :: Monoid (Race) ██████████████████████████████████████████████*/


P.Race.empty = P(k => null);


P.Race.Monoid = {
  ...P.Race.Semigroup,
  empty: P.Race.empty
};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


P.reify = k => x => P(_ => k(x));


/*
█████ Resolve Deps ████████████████████████████████████████████████████████████*/


P.allArr = P.allArr();


P.anyArr = P.anyArr();


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ PARSER ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Stateless parser combinators meant to be used with idempotent iterators. Look
into the respetive section on `Iit` to get more information. Features:

* parse forward
* parse backward
* look behind
* look ahead

For the time being parser results are combined using monoids. I don't know if
this is the optimal approach, though.

If you need a stateful parser built your own one suitable for the task at hand.
You can examine examples in the stateful subsection of the parser composition
section. */


export const Parser = type("Parser", "parse");
  

export const Parsed = variant("Parsed") (binary_("Valid"), binary_("Invalid"));


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████ PARSER :: FIRST ORDER PRIMITIVES ███████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Each first order parser combinator comes in a forward and backward consuming
variant. */


/*
█████ Characters ██████████████████████████████████████████████████████████████*/


// parse a desired character

Parser.char = c => Parser(ix => {
  const iy = ix.next();
  if (iy.done) throw new Err("end of input");
  else if (c === iy.value) return Parsed.Valid(iy.value, iy);
  else return Parsed.Invalid(new Ex(`character ${c} expected`), ix);
});


Parser.charBehind = c => Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) throw new Err("beginning of input");
  else if (c === iy.value) return Parsed.Valid(iy.value, iy.prev);
  else return Parsed.Invalid(new Ex(`character ${c} expected`), ix);
});


// parse a desired character case insensitive

Parser.charCi = c => Parser(ix => {
  const iy = ix.next();
  if (iy.done) throw new Err("end of input");
  else if (c === iy.value.toLowerCase()) return Parsed.Valid(iy.value, iy);
  else return Parsed.Invalid(new Ex(`character ${c} expected`), ix);
});


Parser.charCiBehind = c => Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) throw new Err("beginning of input");
  else if (c === iy.value.toLowerCase()) return Parsed.Valid(iy.value, iy.prev);
  else return Parsed.Invalid(new Ex(`character ${c} expected`), ix);
});


/*
█████ No Consumption ██████████████████████████████████████████████████████████*/


// reject any input

Parser.reject = msg => Parser(ix => {
  const iy = ix.next();
  if (iy.done) throw new Err("end of input");
  else return Parsed.Invalid(new Ex(msg), ix);
});


Parser.rejectBehind = msg => Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) throw new Err("beginning of input");
  else return Parsed.Invalid(new Ex(msg), ix);
});


/*
█████ Predicate Based █████████████████████████████████████████████████████████*/


// parse input that satisfies a predicate

Parser.satisfy = (p, msg = "predicate unmet") => Parser(ix => {
  const iy = ix.next();
  if (iy.done) throw new Err("end of input");
  else if (p(iy.value)) return Parsed.Valid(iy.value, iy);
  else return Parsed.Invalid(new Ex(msg), ix);
});


Parser.satisfyBehind = (p, msg = "predicate unmet") => Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) throw new Err("beginning of input");
  else if (p(iy.value)) return Parsed.Valid(iy.value, iy.prev);
  else return Parsed.Invalid(new Ex(msg), ix);
});


/*
█████ Set Based ███████████████████████████████████████████████████████████████*/


/* Parse any character that is in the given set. Use `_Set.atoz` etc. to define
character ranges. */

Parser.includes = s => Parser(ix => {
  const iy = ix.next();
  if (iy.done) throw new Err("end of input");
  else if (s.has(iy.value)) return Parsed.Valid(iy.value, iy);
  else return Parsed.Invalid(new Ex("input out of character class"), ix);
});


Parser.includesBehind = s => Parser(ix => {
  const iy = ix.prev;
  if (iy.done) throw new Err("end of input");
  else if (s.has(iy.value)) return Parsed.Valid(iy.value, iy.prev);
  else return Parsed.Invalid(new Ex("input out of character class"), ix);
});


/*
█████ Unconditional ███████████████████████████████████████████████████████████*/


// drop any input

Parser.drop = Parser(ix => {
  const iy = ix.next();
  if (iy.done) throw new Err("end of input");
  else return Parsed.Valid(null, iy);
});


Parser.dropBehind = Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) throw new Err("beginning of input");
  else return Parsed.Valid(null, iy.prev);
});


// accept any input

Parser.take = Parser(ix => {
  const iy = ix.next();
  if (iy.done) throw new Err("end of input");
  else return Parsed.Valid(iy.value, iy);
});


// accept any previous input

Parser.takeBehind = Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) throw new Err("beginning of input");
  else return Parsed.Valid(iy.value, iy.prev);
});


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████ PARSER :: HIGHER ORDER PRIMITIVES ██████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*
█████ Combining (Logical) █████████████████████████████████████████████████████*/


// try both parsers and succeed if one succeeds

Parser.or = tx => ty => Parser(iw => {
  return tx.parse(iw).parsed.run({
    Valid: (v, ix) => Parsed.Valid(v, ix),

    Invalid: (e, ix) => ty.parse(ix).parsed.run({
      Valid: (v, iy) => Parsed.Valid(v, iy),
      Invalid: (e2, iz) => Parsed.Invalid(Ex.accum(e, e2), iz)
    })
  })
});


// indeterministic or

Parser.any = ts => Parser(ix => {
  for (const tx of ts) {
    const o = tx.parse(ix);
    if (o.parsed.tag === "valid") return o;
  }

  return Parsed.Invalid(Ex("no parser matched"), ix);
});


// try both parsers and fail if one fails

Parser.and = Semigroup => tx => ty => Parser(iw => { // aka seq
  return tx.parse(iw).parsed.run({
    Valid: (v, ix) => ty.parse(ix).parsed.run({
      Valid: (v2, iy) => Parsed.Valid(Semigroup.append(v) (v2), iy),
      Invalid: (e, iz) => Parsed.Invalid(e, iw)
    }),

    Invalid: (e, ix) => Parsed.Invalid(e, ix)
  })
});


// indeterministic and

Parser.all = Monoid => ts => Parser(ix => {
  let o = Parsed.Valid(null, ix),
    acc = Monoid.empty;

  for (const tx of ts) {
    o = tx.parse(o.parsed.val[1]);

    if (o.parsed.tag === "invalid") return Parsed.Invalid(
      o.parsed.val[0], ix);

    else acc = Monoid.append(acc) (o.parsed.val[0]);
  }

  return Parsed.Valid(acc, o.parsed.val[1]);
});


/* Negate a parser result by either returning the parsed value as an exception
or by returning the empty element of the desired monoid. */

Parser.not = Monoid => tx => Parser(ix => {
  return tx.parse(ix).parsed.run({
    Valid: (v, iy) => Parsed.Invalid(new Ex(v), ix),
    Invalid: (e, iz) => Parsed.Valid(Monoid.empty, iz)
  })
});


// try the first and second parser and fail if both fail or succeed

Parser.xor = tx => ty => Parser(iw => {
  return tx.parse(iw).parsed.run({
    Valid: (v, ix) => ty.parse(ix).parsed.run({
      Valid: (v2, iy) => Parsed.Invalid(Ex.accum(new Ex(v), new Ex(v2)), iw),
      Invalid: (e, iz) => Parsed.Valid(v, iz)
    }),

    Invalid: (e, ix) => ty.parse(ix).parsed.run({
      Valid: (v, iy) => Parsed.Valid(v, iy),
      Invalid: (e2, iz) => Parsed.Invalid(Ex.accum(e, e2), iz)
    })
  });
});


/* Try the first and second parser and fail if both fail or succeed (xnor a.k.a.
if and only if). */

Parser.iff = Monoid => tx => ty => Parser(iw => {
  return tx.parse(iw).parsed.run({
    Valid: (v, ix) => ty.parse(ix).parsed.run({
      Valid: (v2, iy) => Parsed.Valid(Monoid.append(v) (v2), iy),
      Invalid: (e, iz) => Parsed.Invalid(Ex.accum(new Ex(v), e), iw)
    }),

    Invalid: (e, ix) => ty.parse(ix).parsed.run({
      Valid: (v, iy) => Parsed.Invalid(Ex.accum(e, new Ex(v)), iw),
      Invalid: (e2, iz) => Parsed.Valid(Monoid.empty, iz)
    })
  });
});


/*
█████ Combining (Quantitative) ████████████████████████████████████████████████*/


// parse the given pattern at least min or more times

// min..n (n=dynamic)
Parser.min = Monoid => n => tx => Parser(iw => {
  const acc = [];
  let ix = iw;

  while (true) {
    const o = tx.parse(iy).parsed.run({
      Valid: (v, iy) => Parsed.Valid(v, iy),
      Invalid: (e, iz) => Parsed.Invalid(e, iz)
    });

    if (o.parsed.tag === "invalid") break;
    
    else {
      acc.push(o.parsed.val[0]);
      ix = o.parsed.val[1];
    }
  }

  if (acc.length < n) return Parsed.Invalid(
    new Ex(`pattern less than ${n} times received`), iw);

  else return Parsed.Valid(
    acc.reduce((acc2, v) => Monoid.append(acc2) (v), Monoid.empty), ix);
});


// parse the given pattern at least once or more times

// 1..n (n=dynamic)
Parser.min1 = Monoid => Parser.min(Monoid) (1);


// parse the given pattern zero or max times

// 0..max
Parser.max = Monoid => n => tx => Parser(iw => {
  const acc = [];
  let ix = iw;

  while (acc.length <= n) {
    const o = tx.parse(iy).parsed.run({
      Valid: (v, iy) => Parsed.Valid(v, iy),
      Invalid: (e, iz) => Parsed.Invalid(e, iz)
    });

    if (o.parsed.tag === "invalid") break;
    
    else {
      acc.push(o.parsed.val[0]);
      ix = o.parsed.val[1];
    }
  }

  if (acc.length > n) return Parsed.Invalid(
    new Ex(`pattern more than ${n} times received`), iw);

  else return Parsed.Valid(
    acc.reduce((acc2, v) => Monoid.append(acc2) (v), Monoid.empty), ix);
});


// parse the given pattern zero times or once

// 0..1
Parser.max1 = Monoid => Parser.max(Monoid) (1);


// parse the given pattern between m and n times

// m..n (m=static, n=static)
Parser.times = Monoid => (m, n = m) => tx => Parser(ix => {
  const acc = [];
  let o = Parsed.Valid(null, ix);

  for (let i = 0; i < n;i++) {
    o = tx.parse(o.parsed.val[1]);

    if (o.parsed.tag === "valid") acc + o.parsed.val[0];
    else if (i < m) return Parsed.Invalid(new Ex(`pattern less than ${m} times received`), ix);
    else break;
  }

  return Parsed.Valid(acc.reduce((acc2, v) =>
    Semigroup.append(acc2) (v)), o.parsed.val[1]);
});


/* Parse the given pattern between m and n times but only if it doesn't occur
n + 1 times. */

// m..n (m=static, n=static)
Parser.timesOnly = Monoid => (m, n = m) => tx => Parser(ix => {
  const acc = [];
  let o = Parsed.Valid(null, ix);

  for (let i = 0; i < n;i++) {
    o = tx.parse(o.parsed.val[1]);

    if (o.parsed.tag === "valid") acc + o.parsed.val[0];
    else if (i < m) return Parsed.Invalid(new Ex(`pattern less than ${m} times received`), ix);
    else break;
  }

  o = tx.parse(o.parsed.val[1]);

  if (o.parsed.tag === "valid")
    return Parsed.Invalid(new Ex(`pattern more than ${m} times received`), ix);

  else return Parsed.Valid(acc.reduce((acc2, v) =>
    Semigroup.append(acc2) (v)), o.parsed.val[1]);
});


/* Parse the given pattern once. Isn't derived from `Parser.times` because
of the redundant monoid constraint. */

// 1
Parser.once = tx => Parser(iw => {
  return tx.parse(iw).parsed.run({
    Valid: (v, ix) => Parsed.Valid(v, ix),
    Invalid: (e, ix) => Parsed.Invalid(new Ex("pattern not once received"), ix)
  });
});


// parse the pattern once and only once

// 1
Parser.onceOnly = tx => Parser(iw => {
  return tx.parse(iw).parsed.run({
    Valid: (v, ix) => tx.parse(ix).parsed.run({
      Valid: (v2, iy) => Parsed.Invalid(new Ex("pattern more than once received"), iw),
      Invalid: (e, iz) => Parsed.Valid(v, ix)
    }),

    Invalid: (e, ix) => Parsed.Invalid(new Ex("pattern not once received"), ix)
  });
});


// parse the given pattern not once

// 0
Parser.none = tx => Parser(ix => {
  return tx.parse(ix).parsed.run({
    Valid: (v, iy) => Parsed.Invalid(new Ex("unexpected pattern"), ix),
    Invalid: (e, iz) => Parsed.Valid(true, iz)
  });
});


// parse as many repetitions of the pattern as possible

// 0..n (n=dynamic)
Parser.many = Monoid => Parser.min(Monoid) (0);


// parse all occurrences of the given pattern but only return the last one

// 1
Parser.last = tx => Parser(ix => {
  let o = Parsed.Valid(null, ix), p = null;

  while (true) {
    o = tx.parse(o.parsed.val[1]);

    if (o.parsed.tag === "valid") p = o;
    else break;
  }

  if (p === null) return Parsed.Invalid(
    new Ex("pattern not once received"), ix);

  return Parsed.Valid(p.parsed.val[0], p.parsed.val[1]);
});


// parse all occurrences of the given pattern but only return the nth one

// 1
Parser.nth = n => tx => Parser(ix => {
  const acc = [];
  let o = Parsed.Valid(null, ix);

  while (true) {
    o = tx.parse(o.parsed.val[1]);

    if (o.parsed.tag === "valid") {
      acc.push(o.parsed.val[0]);
      if (n === acc.length) break;
    }
    
    else return Parsed.Invalid(
      new Ex(`pattern less than ${n} times received`), ix);
  }

  return Parsed.Valid(acc[n].parsed.val[0], acc[n].parsed.val[1]);
});


/* parse the pattern satisfying a predicate as long as the accumulated parsed
result satisfies another predicate. */

// 0..n (dynamic)
Parser.satisfyWhile = Monoid => p => q => tx => Parser(ix => {
  let o = Parsed.Valid(null, ix)
    acc = Monoid.empty;

  while (true) {
    o = tx.parse(o.parsed.val[1]);

    if (o.parsed.tag === "invalid") break;
    else if (!p(o.parsed.val[0])) {o = prev; break}

    else {
      const acc2 = Monoid.append(acc) (o.parsed.val[0]);
      if (q(acc)) acc = acc2;
      else break;
    }
  }

  return Parsed.Valid(acc, o.parsed.val[1].prev);
});


/*
█████ Boolean Based ███████████████████████████████████████████████████████████*/


// beginning of input

Parser.isBoi = Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) return Parsed.Valid(true, ix);
  else return Parsed.Invalid(new Ex("beginning of input expected"), ix);
});


// beginning of line (detects nl, crnl or beginning of input)

Parser.isBol = Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) return Parsed.Valid(true, ix);
  else if (iy.value === "\n") return Parsed.Valid(true, ix);
  else return Parsed.Invalid(new Ex("beginning of line expected"), ix);
});


// end of input

Parser.isEoi = Parser(ix => {
  const iy = ix.next();
  if (iy.done) return Parsed.Valid(true, ix);
  else return Parsed.Invalid(new Ex("end of input expected"), ix);
});


// end of line (detects nl, crnl or end of input)

Parser.isEol = Parser(ix => {
  const iy = ix.next();

  if (iy.done) return Parsed.Valid(true, ix);
  else if (iy.value === "\n") return Parsed.Valid(true, ix);
  
  else if (iy.value === "\r") {
    const iz = iy.next();

    if (iz.value === "\n") return Parsed.Valid(true, ix);
    else return Parsed.Invalid(new Ex("end of line expected"), ix);
  }

  else return Parsed.Invalid(new Ex("end of line expected"), ix);
});


/*
█████ Look Behind/Ahead ███████████████████████████████████████████████████████*/


// look ahead or behind depending on the supplied parser

Parser.look = tx => Parser(ix => {
  return tx.parse(ix).parsed.run({
    Valid: (v, iy) => Parsed.Valid(v, ix),
    Invalid: (e, iz) => Parsed.Invalid(e, iz)
  })
});


/*
█████ No Result ███████████████████████████████████████████████████████████████*/


// beginning of input

Parser.boi = Monoid => Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) return Parsed.Valid(Monoid.empty, ix);
  else return Parsed.Invalid(new Ex("beginning of input expected"), ix);
});


// beginning of line (detects nl, crnl or beginning of input)

Parser.bol = Monoid => Parser(ix => {
  const iy = ix.prev;
  if (iy === undefined) return Parsed.Valid(Monoid.empty, ix);
  else if (iy.value === "\n") return Parsed.Valid(Monoid.empty, ix);
  else return Parsed.Invalid(new Ex("beginning of line expected"), ix);
});


// end of input

Parser.eoi = Monoid => Parser(ix => {
  const iy = ix.next();
  if (iy.done) return Parsed.Valid(Monoid.empty, ix);
  else return Parsed.Invalid(new Ex("end of input expected"), ix);
});


// end of line (detects nl, crnl or end of input)

Parser.eol = Monoid => Parser(ix => {
  const iy = ix.next();

  if (iy.done) return Parsed.Valid(Monoid.empty, ix);
  else if (iy.value === "\n") return Parsed.Valid(Monoid.empty, ix);
  
  else if (iy.value === "\r") {
    const iz = iy.next();

    if (iz.value === "\n") return Parsed.Valid(true, ix);
    else return Parsed.Invalid(new Ex("end of line expected"), ix);
  }

  else return Parsed.Invalid(new Ex("end of line expected"), ix);
});


/*
█████ Result Modification █████████████████████████████████████████████████████*/


/* Replace the next input with a default value, provided the parser yields a
valid result. */

Parser.ignore = x => tx => Parser(ix => {
  return tx.parse(ix).parsed.run({
    Valid: (v, iy) => Parsed.Valid(x, iy),
    Invalid: (e, iz) => Parsed.Invalid(e, iz)
  })
});


// take the input or a default value on failure

Parser.optional = x => tx => Parser(ix => {
  return tx.parse(ix).parsed.run({
    Valid: (v, iy) => Parsed.Valid(v, iy),
    Invalid: (e, iz) => Parsed.Valid(x, iz)
  })
});


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████ PARSER :: TYPE CLASSES ████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


// lift a function into the parser context

Parser.map = f => tx => Parser(ix => {
  return tx.parse(ix).parsed.run({
    Valid: (v, iy) => Parsed.Valid(f(v), iy),
    Invalid: (e, iz) => Parsed.Invalid(e, iz)
  })
});


Parser.Functor = {map: Parser.map};


/*
█████ Functor :: Alt ██████████████████████████████████████████████████████████*/


Parser.alt = Parser.or;


Parser.Alt = ({
  ...Parser.Functor,
  alt: Parser.alt
});


/*
█████ Functor :: Alt :: Plus ██████████████████████████████████████████████████*/


Parser.zero = Parser.reject("zero reject");


Parser.Plus = {
  ...Parser.Alt,
  zero: Parser.zero
};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


// lift an n-ary function into another parser context

Parser.ap = tf => tx => Parser(iw => {
  return tf.parse(iw).parsed.run({
    Valid: (f, ix) => tx.parse(ix).parsed.run({
      Valid: (v, iy) => Parsed.Valid(f(v), iy),
      Invalid: (e, iz) => Parsed.Invalid(e, iw)
    }),

    Invalid: (e, ix) => Parsed.Invalid(e, ix)
  })
});


Parser.Apply = {
  ...Parser.Functor,
  ap: Parser.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


// put a pure value in a parser context

Parser.of = x => Parser(ix => Parsed.Valid(x, ix));


Parser.Applicative = {
  ...Parser.Apply,
  of: Parser.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


Parser.Chain = {
  ...Parser.Apply,
  chain: Parser.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


/* Conditionally sequence two parsers so that the second parser depends on the
result value of the first one. */

Parser.chain = Semigroup => tx => fm => Parser(iw => {
  return tx.parse(iw).parsed.run({
    Valid: (v, ix) => fm(v).parse(ix).parsed.run({
      Valid: (v2, iy) => Parsed.Valid(Semigroup.append(v) (v2), iy),
      Invalid: (e, iz) => Parsed.Invalid(e, iw)
    }),

    Invalid: (e, ix) => Parsed.Invalid(e, ix)
  });
});


Parser.Monad = {
  ...Parser.Applicative,
  chain: Parser.chain
};


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


Parser.append = Parser.and;


Parser.Semigroup = {append: Parser.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


Parser.empty = Parser.reject("empty rejection");


Parser.Monoid = {
  ...Parser.Semigroup,
  empty: Parser.empty
};


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████ PARSER :: COMPOSITION ████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*
█████ CSV █████████████████████████████████████████████████████████████████████*/


Parser.csv = settings => Parser(ix => {
  let o = Parsed.Valid(null, ix);

  if (settings.header) {
    o = Parser.line.parse(ix),
      line = o.parsed.val[0];

    if (o.parsed.tag === "invalid") return o;

    else {

      // replace quotation marks escaped for the csv context with a placeholder

      line = line.replace(new RegExp(settings.quote, "g"), "${qt}");

      /* Strip quotation marks and escape any interleaved separators during the
      process. */

      const sepBy = Parser.sepBy(Str.Monoid) (({acc, c}) => c === settings.sep ? "${sp}" : c);

      o = Parser.all(A.Monoid) ([
        sepBy(Parser.boi, settings.sep),
        Parser.many(A.Monoid) (sepBy(settings.sep)),
        sepBy(settings.sep, Parser.eol)
      ]) (It.from(line));

      // replace placeholder with original characters

      // register number of cols

      // validate number of cols per row
    }    
  }
});


/*
█████ Date ████████████████████████████████████████████████████████████████████*/


/*
█████ Enumeration █████████████████████████████████████████████████████████████*/


/*
█████ Financial ███████████████████████████████████████████████████████████████*/


// Parser.bic


// Parser.iban


/*
█████ Line ████████████████████████████████████████████████████████████████████*/


Parser.line = Parser(ix => {
  let o = Parsed.Valid(null, ix), acc = "";

  if (Parser.isBol.parse(ix).parsed.val[0] === false)
    return Parsed.Invalid("beginning of line expected", ix);

  for (let i = 0; i < s.length; i++) {
    o = Parser.take.parse(o.parsed.val[1]);
    
    if (o.parsed.val[0] === "\r") {
      o = Parser.char("\n").parse(o.parsed.val[1]);
      if (o.parsed.tag === "valid") return Parsed.Valid(acc, o.parsed.val[1]);
      else throw new Err(`invalid use of "\r"`);
    }
    
    if (o.parsed.val[0] === "\n") return Parsed.Valid(acc, o.parsed.val[1]);
    else return acc += o.parsed.val[0];
  }

  return Parsed.Valid(acc, o.parsed.val[1]);
});


/*
█████ Natural Language ████████████████████████████████████████████████████████*/


// Parser.abbrevation


// Parser.acronym


// Parser.humanName


// parse any word composed of characters of the supplied codeset

Parser.word = codeset => Parser(ix => {
  let tx, acc = "";

  switch (codeset) {
    case "ascii": {tx = Parser.asciiLetter; break}
    case "latin1": {tx = Parser.latin1Letter; break}
    case "utf8": {tx = Parser.utf8Letter; break}
    default: throw new Err(`unknown codeset "${codeset}"`);
  }

  let o = Parsed.Valid(null, ix);

  for (let i = 0; i < s.length; i++) {
    o = tx.parse(o.parsed.val[1]);
    if (o.parsed.tag === "valid") acc += o.parsed.val[0];
    else break;
  }

  return Parsed.Valid(acc, o.parsed.val[1]);
});


Parser.wordBehind = codeset => Parser(ix => {
  let tx, acc = "";

  switch (codeset) {
    case "ascii": {tx = Parser.asciiLetterBehind; break}
    case "latin1": {tx = Parser.latin1LetterBehind; break}
    case "utf8": {tx = Parser.utf8LetterBehind; break}
    default: throw new Err(`unknown codeset "${codeset}"`);
  }

  let o = Parsed.Valid(null, ix);

  for (let i = s.length - 1; i >= 0; i--) {
    o = tx.parse(o.parsed.val[1]);
    if (o.parsed.tag === "valid") o.parsed.val[0] += acc;
    else break;
  }

  return Parsed.Valid(acc, o.parsed.val[1]);
});


/*
█████ Number ██████████████████████████████████████████████████████████████████*/


// Parser.natural


// Parser.integer


// Parser.float


// Parser.number


// Parser.zipcode


// Parser.phone


/*
█████ Paragraph ███████████████████████████████████████████████████████████████*/


// parse lines until two newlines in a row occur


/*
█████ Range ███████████████████████████████████████████████████████████████████*/


/*
█████ Section █████████████████████████████████████████████████████████████████*/


/* Take all characters between an open and close pattern, where open and close
can be the same or different parsers. */


/*
█████ String ██████████████████████████████████████████████████████████████████*/


// parse a desired string

Parser.string = s => Parser(ix => {
  let o = Parsed.Valid(null, ix), acc = "";

  for (let i = 0; i < s.length; i++) {
    o = Parser.char(s[i]).parse(o.parsed.val[1]);
    if (o.parsed.tag === "valid") acc += o.parsed.val[0];
    else return Parsed.Invalid(new Ex(`"${s}" expected`), ix);
  }

  return Parsed.Valid(acc, o.parsed.val[1]);
});


Parser.stringBehind = s => Parser(ix => {
  let o = Parsed.Valid(null, ix), acc = "";

  for (let i = s.length - 1; i >= 0; i--) {
    o = Parser.charBehind(s[i]).parse(o.parsed.val[1]);
    if (o.parsed.tag === "valid") o.parsed.val[0] += acc;
    else return Parsed.Invalid(new Ex(`"${s}" expected`), ix);
  }

  return Parsed.Valid(acc, o.parsed.val[1]);
});


// parse a desired string case insensitive

Parser.stringCi = s => Parser(ix => {
  let o = Parsed.Valid(null, ix), acc = "";

  for (let i = 0; i < s.length; i++) {
    o = Parser.charCi(s[i]).parse(o.parsed.val[1]);
    if (o.parsed.tag === "valid") acc += o.parsed.val[0];
    else return Parsed.Invalid(new Ex(`"${s}" expected`), ix);
  }

  return Parsed.Valid(acc, o.parsed.val[1]);
});


Parser.stringCiBehind = s => Parser(ix => {
  let o = Parsed.Valid(null, ix), acc = "";

  for (let i = s.length - 1; i >= 0; i--) {
    o = Parser.charCiBehind(s[i]).parse(o.parsed.val[1]);
    if (o.parsed.tag === "valid") o.parsed.val[0] += acc;
    else return Parsed.Invalid(new Ex(`"${s}" expected`), ix);
  }

  return Parsed.Valid(acc, o.parsed.val[1]);
});


/*
█████ Stateful ████████████████████████████████████████████████████████████████*/


/* Parse a nested pattern up to max level of nestings and either capture or
discard the open/close patterns themselves. Use stop characters to further
limit nestings so that they can't span lines, for instance. Apply a trans-
formation to the input depending on the accumulator and the level of nesting. */

Parser.nested = Monoid => ({maxLevel, captureNesting = false, stopChars = new Set(), transform = id}) => (open, close) => Parser(ix => {
  let o = Parsed.Valid(null, ix), acc = Monoid.empty, level = 0;
  
  do {
    o = open.parse(o.parsed.val[1]);

    if (o.parsed.tag === "valid") {
      if (level + 1 > maxLevel)
        return Parsed.Invalid(new Ex("unexpected nesting received"), ix);
      
      else {
        level++;

        if (captureNesting) acc = Monoid.append(acc) (transform({
          acc,
          value: o.parsed.val[0],
          level
        }));
      }
    }

    else {
      o = close.parse(o.parsed.val[1]);

      if (o.parsed.tag === "valid") {
        if (level - 1 < 0) return Parsed.Invalid(new Ex("malformed nesting received"), ix);

        else {
          if (captureNesting) acc = Monoid.append(acc) (transform({
            acc,
            value: o.parsed.val[0],
            level
          }));

          level--;
        }
      }

      else {
        o = Parser.take.parse(o.parsed.val[1]);
        
        if (stopChars.has(o.parsed.val[0]))
          return Parsed.Invalid(new Ex("incomplete nesting received"), ix);

        else acc = Monoid.append(acc) (transform({
          acc,
          value: o.parsed.val[0],
          level
        }));
      }
    }
  } while (level > 0);

  return Parsed.Valid(acc, o.parsed.val[1]);
});


/* Variant that takes several open/close pairs, where open patterns can only be closed
with the associated close pattner. */

Parser.nestedPairs = Monoid => ({maxLevel, captureNesting = false, stopChars = new Set(), transform = id}) => pairs => Parser(ix => {
  const stack = [];
  let o = Parsed.Valid(null, ix), acc = Monoid.empty, i = 0;
  
  do {
    for (const [open, close] of pairs) {
      o = open.parse(o.parsed.val[1]);
      
      if (o.parsed.tag === "valid") {
        if (stack.length + 1 > maxLevel)
          return Parsed.Invalid(new Ex("unexpected nesting received"), ix);
        
        else {
          stack.push(close);
          
          if (captureNesting) acc = Monoid.append(acc) (transform({
            acc,
            value: o.parsed.val[0],
            level: stack.length
          }));

          break;
        }
      }
    }

    if (stack.length === 0) {
      if (i === 0) return Parsed.Invalid(new Ex("no nesting received"), ix);
      else return Parsed.Valid(acc, o.parsed.val[1]);
    }

    else {
      const close = stack[stack.length - 1];
      o = close.parse(o.parsed.val[1]);

      if (o.parsed.tag === "valid") {
        if (captureNesting) acc = Monoid.append(acc) (transform({
          acc,
          value: o.parsed.val[0],
          level: stack.length
        }));

        stack.pop();
      }

      else {
        o = Parser.take.parse(o.parsed.val[1]);
        
        if (stopChars.has(o.parsed.val[0]))
          return Parsed.Invalid(new Ex("incomplete nesting received"), ix);

        else acc = Monoid.append(acc) (transform({
          acc,
          value: o.parsed.val[0],
          level: stack.length
        }));
      }
    }
  } while (i++);
});


/* Parse the input separated by a left and right separators, which can be
characters or strings. BOI, EOI, BOL and EOL are considered in the following
varaints:

* BOL/right
* BOI/right
* left/EOL
* left/EOI

Apply a transformation to the input depending on the accumulator. */

Parser.sepBy = Monoid => transform => (left, right = left) => Parser(ix => {
  let o = Parsed.Valid(null, ix), acc = Monoid.empty;

  if (Parser.isBol.parse(o.parsed.val[1])) NOOP;

  else {
    o = Parser.string(left).parse(o.parsed.val[1]);

    if (o.parsed.tag === "invalid")
      return Parsed.Invalid(new Ex(`separator "${left}" expected`), ix);
  } 

  while (true) {
    if (Parser.isEol.parse(o.parsed.val[1])) break;

    // don't allow nested separators

    else if (left !== right) {
      o = Parser.string(left).parse(o.parsed.val[1]);
  
      if (o.parsed.tag === "valid")
        return Parsed.Invalid(new Ex("nested separators received"), ix);
    }

    o = Parser.string(right).parse(o.parsed.val[1]);

    if (o.parsed.tag === "valid") break;
    else acc = Monoid.append(acc) (transform({acc, value: o.parsed.val[0]}));
  }

  return Parsed.Valid(acc, o.parsed.val[1]);
});


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████ PARSER :: UTILITY ██████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*
█████ Codesets & Char Classes █████████████████████████████████████████████████*/


Parser.asciiCodeset = {
  get letter() {
    delete this.letter;
    this.letter = new RegExp(/[a-z]/, "i");
    return this.letter;
  },

  get ucl() {
    delete this.ucl;
    this.ucl = new RegExp(/[A-Z]/, "");
    return this.ucl;
  },

  get lcl() {
    delete this.lcl;
    this.lcl = new RegExp(/[a-z]/, "");
    return this.lcl;
  },

  get digit() {
    delete this.digit;
    this.digit = new RegExp(/[0-9]/, "");
    return this.digit;
  },

  get alnum() {
    delete this.alnum;
    this.alnum = new RegExp(`${this.digit.source}|${this.letter.source}`, "");
    return this.alnum;
  },

  get control() {
    delete this.control;
    this.control = new RegExp(/[\0\a\b\t\v\f\r\n\cZ]/, "");
    return this.control;
  },

  get punct() {
    delete this.punct;
    this.punct = new RegExp(/[!"#$%&'()*+,-./:;<=>?@\[\]\\^_`{|}~]/, "");
    return this.punct;
  },

  get currency() {
    delete this.currency;
    this.currency = new RegExp(/[$]/, "");
    return this.currency;
  },

  get space() {
    delete this.space;
    this.space = new RegExp(/ /, "");
    return this.space;
  },

  get nonAlnum() {
    delete this.nonAlnum;
    
    this.nonAlnum = new RegExp(
      `${this.control.source}|${this.punct.source}|${this.currency.source}|${this.space.source}`, "");
    
    return this.nonAlnum;
  }
};


/* CP1252 (Windows 1252) has the same characaters as latin1 (ISO-8859-1) but
between 128-159 they are not at the same code points. */

Parser.latin1CodeSet = {
  get letter() {
    delete this.letter;
    this.letter = new RegExp(/[a-zßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ]/, "i");
    return this.letter;
  },

  get ucl() {
    delete this.ucl;
    this.ucl = new RegExp(/[A-ZÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞ]/, "");
    return this.ucl;
  },

  get lcl() {
    delete this.lcl;
    this.lcl = new RegExp(/[a-zßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ]/, "");
    return this.lcl;
  },

  get digit() {
    delete this.digit;
    this.digit = new RegExp(/[0-9]/, "");
    return this.digit;
  },

  get alnum() {
    delete this.alnum;
    this.alnum = new RegExp(`${this.digit.source}|${this.letter.source}`, "");
    return this.alnum;
  },

  get control() {
    delete this.control;
    this.control = new RegExp(/[\0\a\b\t\v\f\r\n\cZ]/, "");
    return this.control;
  },
  
  get punct() {
    delete this.punct;
    this.punct = new RegExp(/[!"#$%&'()*+,-./:;<=>?@\[\]\\^_`{|}~€‚„…†‡ˆ‰‹‘’“”•–­—˜™›¡¢£¤¥¦§¨©ª«¬®¯°±²³´µ¶·¸¹º»¼½¾¿]/, "");
    return this.punct;
  },
  
  get currency() {
    delete this.currency;
    this.currency = new RegExp(/[¤$€£¥¢]/, "");
    return this.currency;
  },
  
  get space() {
    delete this.space;
    this.space = new RegExp(/  /, "");
    return this.space;
  },

  get nonAlnum() {
    delete this.nonAlnum;

    this.nonAlnum = new RegExp(
      `${this.control.source}|${this.punct.source}|${this.currency.source}|${this.space.source}`, "");

    return this.nonAlnum;
  }
};


Parser.utf8Codeset = {
  get letter() {
    delete this.letter;
    this.letter = new RegExp(/\p{L}/, "u");
    return this.letter;
  },

  get ucl() {
    delete this.ucl;
    this.ucl = new RegExp(/\p{Lu}/, "u");
    return this.ucl;
  },

  get lcl() {
    delete this.lcl;
    this.lcl = new RegExp(/\p{Lu}/, "u");
    return this.lcl;
  },

  get digit() {
    delete this.digit;
    this.digit = new RegExp(/\p{N}/, "u");
    return this.digit;
  },

  get alnum() {
    delete this.alnum;
    this.alnum = new RegExp(`${this.digit.source}|${this.letter.source}`, "u");
    return this.alnum;
  },

  get control() {
    delete this.control;
    this.control = new RegExp(/[\p{C}\p{Zl}\p{Zp}]/, "u");
    return this.control;
  },

  get punct() {
    delete this.punct;
    this.punct = new RegExp(/[\p{P}\p{S}\p{F}]/, "u");
    return this.punct;
  },

  get currency() {
    delete this.currency;
    this.currency = new RegExp(/\p{Sc}/, "u");
    return this.currency;
  },

  get space() {
    delete this.space;
    this.space = new RegExp(/\p{Zs}/, "u");
    return this.space;
  },

  get nonAlnum() {
    delete this.nonAlnum;
    
    this.nonAlnum = new RegExp(
      `${this.control.source}|${this.punct.source}|${this.currency.source}|${this.space.source}`, "u");

    return this.nonAlnum;
  }
};


/*
█████ Parsers █████████████████████████████████████████████████████████████████*/


Parser.asciiLetter = Parser.satisfy(
  c => Parser.asciiCodeset.letter.test(c),
  "ASCII letter expected");


Parser.asciiLetterBehind = Parser.satisfyBehind(
  c => Parser.asciiCodeset.letter.test(c),
  "ASCII letter expected");


Parser.asciiLcl = Parser.satisfy(
  c => Parser.asciiCodeset.lcl.test(c),
  "ASCII lower case letter expected");


Parser.asciiLclBehind = Parser.satisfyBehind(
  c => Parser.asciiCodeset.lcl.test(c),
  "ASCII lower case letter expected");


Parser.asciiUcl = Parser.satisfy(
  c => Parser.asciiCodeset.ucl.test(c),
  "ASCII upper case letter expected");


Parser.asciiUclBehind = Parser.satisfyBehind(
  c => Parser.asciiCodeset.ucl.test(c),
  "ASCII upper case letter expected");


Parser.asciiDigit = Parser.satisfy(
  c => Parser.asciiCodeset.digit.test(c),
  "ASCII digit expected");


Parser.asciiDigitBehind = Parser.satisfyBehind(
  c => Parser.asciiCodeset.digit.test(c),
  "ASCII digit expected");


Parser.asciiAlnum = Parser.satisfy(
  c => Parser.asciiCodeset.alnum.test(c),
  "ASCII alphanumeric character expected");


Parser.asciiAlnumBehind = Parser.satisfyBehind(
  c => Parser.asciiCodeset.alnum.test(c),
  "ASCII alphanumeric character expected");


Parser.asciiPunct = Parser.satisfy(
  c => Parser.asciiCodeset.punct.test(c),
  "ASCII punctuation expected");


Parser.asciiPunctBehind = Parser.satisfyBehind(
  c => Parser.asciiCodeset.punct.test(c),
  "ASCII punctuation expected");


Parser.asciiSpace = Parser.satisfy(
  c => Parser.asciiCodeset.space.test(c),
  "ASCII space character expected");


Parser.asciiSpaceBehind = Parser.satisfyBehind(
  c => Parser.asciiCodeset.space.test(c),
  "ASCII space character expected");


Parser.latin1Letter = Parser.satisfy(
  c => Parser.latin1CodeSet.letter.test(c),
  "Latin1 letter expected");


Parser.latin1LetterBehind = Parser.satisfyBehind(
  c => Parser.latin1CodeSet.letter.test(c),
  "Latin1 letter expected");


Parser.latin1Lcl = Parser.satisfy(
  c => Parser.latin1CodeSet.lcl.test(c),
  "Latin1 lower case letter expected");


Parser.latin1LclBehind = Parser.satisfyBehind(
  c => Parser.latin1CodeSet.lcl.test(c),
  "Latin1 lower case letter expected");


Parser.latin1Ucl = Parser.satisfy(
  c => Parser.latin1CodeSet.ucl.test(c),
  "Latin1 upper case letter expected");


Parser.latin1UclBehind = Parser.satisfyBehind(
  c => Parser.latin1CodeSet.ucl.test(c),
  "Latin1 upper case letter expected");


Parser.latin1Digit = Parser.satisfy(
  c => Parser.latin1CodeSet.digit.test(c),
  "Latin1 digit expected");


Parser.latin1DigitBehind = Parser.satisfyBehind(
  c => Parser.latin1CodeSet.digit.test(c),
  "Latin1 digit expected");


Parser.latin1Alnum = Parser.satisfy(
  c => Parser.latin1CodeSet.alnum.test(c),
  "Latin1 alphanumeric character expected");


Parser.latin1AlnumBehind = Parser.satisfyBehind(
  c => Parser.latin1CodeSet.alnum.test(c),
  "Latin1 alphanumeric character expected");


Parser.latin1Punct = Parser.satisfy(
  c => Parser.latin1CodeSet.punct.test(c),
  "Latin1 punctuation expected");


Parser.latin1PunctBehind = Parser.satisfyBehind(
  c => Parser.latin1CodeSet.punct.test(c),
  "Latin1 punctuation expected");


Parser.latin1Space = Parser.satisfy(
  c => Parser.latin1CodeSet.space.test(c),
  "Latin1 space character expected");


Parser.latin1SpaceBehind = Parser.satisfyBehind(
  c => Parser.latin1CodeSet.space.test(c),
  "Latin1 space character expected");


Parser.utf8Letter = Parser.satisfy(
  c => Parser.utf8Codeset.letter.test(c),
  "UTF8 letter expected");


Parser.utf8LetterBehind = Parser.satisfyBehind(
  c => Parser.utf8Codeset.letter.test(c),
  "UTF8 letter expected");


Parser.utf8Lcl = Parser.satisfy(
  c => Parser.utf8Codeset.lcl.test(c),
  "UTF8 lower case letter expected");


Parser.utf8LclBehind = Parser.satisfyBehind(
  c => Parser.utf8Codeset.lcl.test(c),
  "UTF8 lower case letter expected");


Parser.utf8Ucl = Parser.satisfy(
  c => Parser.utf8Codeset.ucl.test(c),
  "UTF8 upper case letter expected");


Parser.utf8UclBehind = Parser.satisfyBehind(
  c => Parser.utf8Codeset.ucl.test(c),
  "UTF8 upper case letter expected");


Parser.utf8Digit = Parser.satisfy(
  c => Parser.utf8Codeset.digit.test(c),
  "UTF8 digit expected");


Parser.utf8DigitBehind = Parser.satisfyBehind(
  c => Parser.utf8Codeset.digit.test(c),
  "UTF8 digit expected");


Parser.utf8Alnum = Parser.satisfy(
  c => Parser.utf8Codeset.alnum.test(c),
  "UTF8 alphanumeric character expected");


Parser.utf8AlnumBehind = Parser.satisfyBehind(
  c => Parser.utf8Codeset.alnum.test(c),
  "UTF8 alphanumeric character expected");


Parser.utf8Punct = Parser.satisfy(
  c => Parser.utf8Codeset.punct.test(c),
  "UTF8 punctuation expected");


Parser.utf8PunctBehind = Parser.satisfyBehind(
  c => Parser.utf8Codeset.punct.test(c),
  "UTF8 punctuation expected");


Parser.utf8Space = Parser.satisfy(
  c => Parser.utf8Codeset.space.test(c),
  "UTF8 space character expected");


Parser.utf8SpaceBehind = Parser.satisfyBehind(
  c => Parser.utf8Codeset.space.test(c),
  "UTF8 space character expected");


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


// map all special letters from latin-based alphabets onto ASCII

Object.defineProperty(Parser, "toAscii", {
  get() {
    const m = new Map([
      ["Æ", "AE"], ["æ", "ae"], ["Ä", "Ae"], ["ä", "ae"], ["Œ", "OE"],
      ["œ", "oe"], ["Ö", "Oe"], ["ö", "oe"], ["ß", "ss"], ["ẞ", "ss"],
      ["Ü", "Ue"], ["ü", "ue"], ["Ⱥ", "A"], ["ⱥ", "a"], ["Ɑ", "A"],
      ["ɑ", "a"], ["ɐ", "a"], ["ɒ", "a"], ["Ƀ", "B"], ["ƀ", "b"],
      ["Ɓ", "B"], ["ɓ", "b"], ["Ƃ", "b"], ["ƃ", "b"], ["ᵬ", "b"],
      ["ᶀ", "b"], ["Ƈ", "C"], ["ƈ", "c"], ["Ȼ", "C"], ["ȼ", "c"],
      ["Ɗ", "D"], ["ɗ", "d"], ["Ƌ", "D"], ["ƌ", "d"], ["ƍ", "d"],
      ["Đ", "D"], ["đ", "d"], ["ɖ", "d"], ["ð", "d"], ["Ɇ", "E"],
      ["ɇ", "e"], ["ɛ", "e"], ["ɜ", "e"], ["ə", "e"], ["Ɠ", "G"],
      ["ɠ", "g"], ["Ǥ", "G"], ["ǥ", "g"], ["ᵹ", "g"], ["Ħ", "H"],
      ["ħ", "h"], ["Ƕ", "H"], ["ƕ", "h"], ["Ⱨ", "H"], ["ⱨ", "h"],
      ["ɥ", "h"], ["ɦ", "h"], ["ı", "i"], ["Ɩ", "I"], ["ɩ", "i"],
      ["Ɨ", "I"], ["ɨ", "i"], ["Ɉ", "J"], ["ɉ", "j"], ["ĸ", "k"],
      ["Ƙ", "K"], ["ƙ", "k"], ["Ⱪ", "K"], ["ⱪ", "k"], ["Ł", "L"],
      ["ł", "l"], ["Ƚ", "L"], ["ƚ", "l"], ["ƛ", "l"], ["ȴ", "l"],
      ["Ⱡ", "L"], ["ⱡ", "l"], ["Ɫ", "L"], ["ɫ", "l"], ["Ľ", "L"],
      ["ľ", "l"], ["Ɯ", "M"], ["ɯ", "m"], ["ɱ", "m"], ["Ŋ", "N"],
      ["ŋ", "n"], ["Ɲ", "N"], ["ɲ", "n"], ["Ƞ", "N"], ["ƞ", "n"],
      ["Ø", "O"], ["ø", "o"], ["Ɔ", "O"], ["ɔ", "o"], ["Ɵ", "O"],
      ["ɵ", "o"], ["Ƥ", "P"], ["ƥ", "p"], ["Ᵽ", "P"], ["ᵽ", "p"],
      ["ĸ", "q"], ["Ɋ", "Q"], ["ɋ", "q"], ["Ƣ", "Q"], ["ƣ", "q"],
      ["Ʀ", "R"], ["ʀ", "r"], ["Ɍ", "R"], ["ɍ", "r"], ["Ɽ", "R"],
      ["ɽ", "r"], ["Ƨ", "S"], ["ƨ", "s"], ["ȿ", "s"], ["ʂ", "s"],
      ["ᵴ", "s"], ["ᶊ", "s"], ["Ŧ", "T"], ["ŧ", "t"], ["ƫ", "t"],
      ["Ƭ", "T"], ["ƭ", "t"], ["Ʈ", "T"], ["ʈ", "t"], ["Ʉ", "U"],
      ["ʉ", "u"], ["Ʋ", "V"], ["ʋ", "v"], ["Ʌ", "V"], ["ʌ", "v"],
      ["ⱴ", "v"], ["ⱱ", "v"], ["Ⱳ", "W"], ["ⱳ", "w"], ["Ƴ", "Y"],
      ["ƴ", "y"], ["Ɏ", "Y"], ["ɏ", "y"], ["ɤ", "Y"], ["Ƶ", "Z"],
      ["ƶ", "z"], ["Ȥ", "Z"], ["ȥ", "z"], ["ɀ", "z"], ["Ⱬ", "Z"],
      ["ⱬ", "z"], ["Ʒ", "Z"], ["ʒ", "z"], ["Ƹ", "Z"], ["ƹ", "z"],
      ["Ʒ", "Z"], ["ʒ", "z"]
    ]);

    delete this.toAscii;
    this.toAscii = m;
    return m;
  }
});


/*█████████████████████████████████████████████████████████████████████████████
██████████████████████████████████ PREDICATE ██████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// dynamic predicates


export const Pred = type("Pred");


/*
█████ Boolean Logic ███████████████████████████████████████████████████████████*/


Pred.iff = tx => ty => Pred(x => {
  if (tx.pred(x) === true && ty.pred(x) === true)
    return true;

  else if (tx.pred(x) === false && ty.pred(x) === false)
    return true;

  else return false;
});


Pred.imply = tx => ty => Pred(x => {
  if (tx.pred(x) === true && ty.pred(x) !== true)
    return false;

  else return true;
});


Pred.not = tx => Pred(x => !tx.pred(x));


/*
█████ Conjunction █████████████████████████████████████████████████████████████*/


Pred.all = xs => Pred(x => {
  for (const tx of xs)
    if (tx.pred(x) === false) return false;

  return true;
});


Pred.and = tx => ty => Pred(x => tx.pred(x) && ty.pred(x));


/*
█████ Constant ████████████████████████████████████████████████████████████████*/


Pred.then = x => true;


Pred.else = x => false;


/*
█████ Disjunction █████████████████████████████████████████████████████████████*/


Pred.any = preds => Pred(x => {
  for (const tx of xs)
    if (tx.pred(x) === true) return true;

  return false;
});


Pred.or = tx => ty => Pred(x => tx.pred(x) || ty.pred(x));


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


Pred.append = Pred.and;


Pred.Semigroup = {append: Pred.append};


/*
█████ Semigroup (Disjunction) █████████████████████████████████████████████████*/


Pred.Disjunct = {};


Pred.Disjunct.append = Pred.or;


Pred.Disjunct.Semigroup = {append: Pred.Disjunct.append};


/*
█████ Monoid ██████████████████████████████████████████████████████████████████*/


Pred.empty = Pred.then;


Pred.Monoid = {
  ...Pred.Semigroup,
  empty: Pred.empty
};


/*
█████ Monoid (Disjunction) ████████████████████████████████████████████████████*/


Pred.Disjunct.empty = Pred.else;


Pred.Disjunct.Monoid = {
  ...Pred.Disjunct.Semigroup,
  empty: Pred.Disjunct.empty
};


/*█████████████████████████████████████████████████████████████████████████████
█████████████████████████████ REGULAR EXPRESSIONS █████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const Rex = {};


/*
█████ Combinators █████████████████████████████████████████████████████████████*/


Rex.escapeRegExp = s => s.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');


Rex.normalizeNewline = s => s.replace(/\r\n/g, "\n");


/*
█████ Patterns ████████████████████████████████████████████████████████████████*/


Rex.iso = {
  dates: {
    date6: /^(?<y>\d\d)(?<m>\d\d)(?<d>\d\d)$/,
    date8: /^(?<y>\d\d)-(?<m>\d\d)-(?<d>\d\d)$/,
    date8_: /^(?<y>\d{4})(?<m>\d\d)(?<d>\d\d)$/,
    date10: /^(?<y>\d{4})-(?<m>\d\d)-(?<d>\d\d)$/,
    datetime: /^(?<y>\d{4})-(?<m>\d\d)-(?<d>\d\d)T(?<h>\d\d):(?<min>\d\d):(?<s>\d\d).(?<ms>\d{3})(?<tz>Z|(?:\+|\-)\d\d:\d\d)$/, // all ISO
    datetimeUTC: /^(?<y>\d{4})-(?<m>\d\d)-(?<d>\d\d)T(?<h>\d\d):(?<min>\d\d):(?<s>\d\d).(?<ms>\d{3})(?<tz>Z)$/, // YYYY-MM-DDTHH:MM:SS.MMMZ
    datetimeLoc: /^(?<y>\d{4})-(?<m>\d\d)-(?<d>\d\d)T(?<h>\d\d):(?<min>\d\d):(?<s>\d\d).(?<ms>\d{3})(?<tz>(?:\+|\-)\d\d:\d\d)$/ // YYYY-MM-DDTHH:MM:SS.MMM+/-HH:MM
  },

  times: {
    time5: /^(?<h>\d\d):(?<min>\d\d)$/,
    time8: /^(?<h>\d\d):(?<min>\d\d):(?<s>\d\d)$/
  },

  numbers: {
    nat: /^(?<sign>\+)?(?<int>[1-9]\d*)$/, // natural numbers
    int: /^(?<sign>\+|\-)?(?<int>[1-9]\d*)$/, // integers
    float: /^(?<sign>\+|\-)?(?<int>\d+)\.(?<frac>\d+)$/, // only floating point numbers
    num: /^(?<sign>\+|\-)?(?<int>\d+)(?:\.(?<frac>\d+))?$/ // all numbers
  }
};


Rex.i18n = {
  deDE: {
    dates: {
      date6: /^(?<d>\d{1,2})(?<m>\d{1,2})(?<y>\d\d)$/,
      date8: /^(?<d>\d{1,2})\.(?<m>\d{1,2})\.(?<y>\d\d)$/,
      date8_: /^(?<d>\d\d)(?<m>\d\d)(?<y>\d{4})$/,
      date10: /^(?<d>\d{1-2})\.(?<m>\d{1-2})\.(?<y>\d{4})$/,
      datetime16: /^(?<d>\d\d)\.(?<m>\d\d)\.(?<y>\d{4}) (?<h>\d\d):(?<min>\d\d)$/,
      datetime18: /^(?<d>\d\d)\.(?<m>\d\d)\.(?<y>\d{4}) (?<h>\d\d):(?<min>\d\d):(?<s>\d\d)$/,
      dateLong: /^(?<d>\d{1,2})\.? +(?<m>[A-Z][a-zä]+)\.? +(?<y>\d{2,4})$/
    },

    numbers : {
      num: /^(?<sign>\+|\-)?(?<int>\d+(?:\.\d{3})*),(?<frac>\d+)$/
    },

    months: /(\b(Januar|Februar|März|Maerz|April|Mai|Juni|Juli|August|September|Oktober|November|Dezember)\b)/,
    months_: /(\b(Jan|Feb|Mär|Mar|Apr|Mai|Jun|Jul|Aug|Sep|Okt|Nov|Dez)\b)\.?/
  }
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ SERIAL ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Encode asynchronous I/O computations evaluated in serial. Use..

  * `Parallel` for asynchronous computations in parallel
  * `Cont` for synchronous computations in CPS

The type has the following properties:

  * pure core/impure shell concept
  * asynchronous, serial evaluation
  * deferred nested function calls
  * stack-safe due to asynchronous calls
  * non-reliable return values
  * base monad (no transformer)

`Serial` isn't asynchronous by default, i.e. the next continuation (invocation
of the next `then` handler in `Promise` lingo) may be in the same microtask of
Javascript's event loop. Hence an alternative mechanism is needed to provide
stack-safety in the context of long synchronous chains. `runSafe` intersperses
`Promise` into the chain. This process is completely transparent to the caller
because `Serial` doesn't rely on return values.

`Serial` is based on multi-shot continuations, i.e. its continuation can be
invoked several times and thus the corresponding async computation is evaluated
several times. If you need sharing, provide a function scope in applicative or
monadic style that provides the only once evaluated expressions.

The type doesn't handel exceptions but you need take care of them yourself. */


// smart constructor

export const Serial = k => {
  const o = {
    ser: k,

    // stack-safe

    serSafe: f => {
      if (asyncCounter > 100) {
        asyncCounter = 0;
        return Promise.resolve(null).then(_ => k(f));
      }

      else {
        asyncCounter++;
        return k(f);
      }
    }
  };

  Object.defineProperty(o, TAG, {value: "Serial"});
  return o;
};


export const S = Serial; // shortcut


/*
█████ Category ████████████████████████████████████████████████████████████████*/


S.comp = f => g => S(k => x => g(x).ser(f).ser(k));


S.id = tx => tx.ser(id);


S.Category = {
  comp: S.comp,
  id: S.id
};


/*
█████ Composition █████████████████████████████████████████████████████████████*/


// (r -> r) -> S r t -> S r t
S.mapCont = f => tx => S(k => f(tx.ser(k)));


S.pipe = g => f => S(k => x =>
  g(x).ser(f).ser(k));


// ((s -> r) -> t -> r) -> S r t -> S r s
S.withCont = f => tx => S(k => tx.ser(f(k)));


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


S.fromParallel = tx => S(tx.par);


/*
█████ Conjunction █████████████████████████████████████████████████████████████*/


S.and = tx => ty =>
  S(k =>
    tx.ser(x =>
      ty.ser(y =>
        k(Pair(x, y)))));


S.allArr = () =>
  A.seqA({
    map: S.map,
    ap: S.ap,
    of: S.of});


S.allObj = o => {
  return Object.keys(o).reduce((acc, key) => {
    return S(k =>
      acc.ser(p =>
        o[key].ser(x =>
          k((p[key] = x, p)))));
  }, S.of({}));
};


/*
█████ Debugging ███████████████████████████████████████████████████████████████*/


S.async = f => msecs => x => S(k => setTimeout(comp(k) (f), msecs, x));


/*
█████ Excaption Handling ██████████████████████████████████████████████████████*/


S.tryCatch = k2 => tx => S(k => tx.ser(x => {
  if (x?.constructor?.name === "Exception") return k2(x);
  else return k(x);
}));


S.tryThrow = tx => S(k => tx.ser(x => {
  if (x?.constructor?.name === "Exception") throw x;
  else return k(x);
}));


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


S.map = f => tx =>
  S(k => tx.ser(x => k(f(x))));


S.Functor = {map: S.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


S.ap = tf => tx => S(k =>
  S.and(tf) (tx).ser(([f, x]) =>
    k(f(x))));


S.Apply = {
  ...S.Functor,
  ap: S.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


S.of = x => S(k => k(x));


S.Applicative = {
  ...S.Apply,
  of: S.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


S.chain = mx => fm =>
  S(k => mx.ser(x => fm(x).ser(k)));


S.Chain = {
  ...S.Apply,
  chain: S.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


S.Monad = {
  ...S.Applicative,
  chain: S.chain
};


/*
█████ Profunctor ██████████████████████████████████████████████████████████████*/


S.dimap = h => g => f => S(k => x => h(x).ser(f).ser(g).ser(k));


S.lmap = S.pipe;


S.rmap = S.comp;


S.Profunctor = {
  ...S.Functor,
  dimap: S.dimap,
  lmap: S.lmap,
  rmap: S.rmap
};


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


S.append = Semigroup => tx => ty => S(k =>
  S.and(tx) (ty).ser(([x, y]) =>
    k(Semigroup.append(x) (y))));


S.Semigroup = {append: S.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


S.empty = Monoid => S(k => k(Monoid.empty));


S.Monoid = {
  ...S.Semigroup,
  empty: S.empty
};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


S.reify = k => x => S(_ => k(x));


/*
█████ Resolve Deps ████████████████████████████████████████████████████████████*/


S.allArr = S.allArr();


/*█████████████████████████████████████████████████████████████████████████████
█████████████████████████████████████ SET █████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const _Set = {}; // namespace


/*
█████ Clone ███████████████████████████████████████████████████████████████████*/


_Set.clone = s => new Set(s);


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


_Set.fromIt = ix => {
  const s = new Set();
  for (const k of ix) s.add(k);
  return s;
};


_Set.fromItKeys = ix => {
  const s = new Set();
  for (const [k,] of ix) s.add(k);
  return s;
};


_Set.fromItValues = ix => {
  const s = new Set();
  for (const [, v] of ix) s.add(v);
  return s;
};


_Set.interconvert = f => s => new Set(f(Array.from(s)));


_Set.interconvertBy = f => g => s => new Set(f(Array.from(s).map(g)));


/*
█████ Generators ██████████████████████████████████████████████████████████████*/


_Set.entries = s => s[Symbol.iterator] ();


/*
█████ Getters/Setters █████████████████████████████████████████████████████████*/


_Set.has = k => s => s.has(k);


_Set.set = k => s => s.add(k);


_Set.del = k => s => s.delete(k);


/*
█████ Ranges ██████████████████████████████████████████████████████████████████*/


Object.defineProperty(_Set, "_0to9", {
  get() {
    const s = new Set(["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"]);
    delete this._0to9;
    this._0to9 = s;
    return s;
  },

  configurable: true,
  enumerable: true
});


Object.defineProperty(_Set, "atoz", {
  get() {
    const s = new Set(["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]);
    delete this.atoz;
    this.atoz = s;
    return s;
  },

  configurable: true,
  enumerable: true
});


Object.defineProperty(_Set, "atoZ", {
  get() {
    const s = new Set(["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"])
    delete this.atoZ;
    this.atoZ = s;
    return s;
  },

  configurable: true,
  enumerable: true
});


/*
█████ Union ███████████████████████████████████████████████████████████████████*/


_Set.union = s => t => {
  new Set(t).forEach(k => s.set(k));
};


// destructive

_Set.union_ = s => t => {
  new t.forEach(k => s.set(k));
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ STREAM ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* An effectful stream is a generalized list utilizing monads, which requires
lazy evaluation. Since Javascript is neither lazy nor has a monad ecosystem,
there is no useful implementation based on a lazy-lists-like data structure.
The closest you can get are an impure implementation using iterators. */


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ STRING ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


export const Str = {}; // namespace


/*
█████ Chars ███████████████████████████████████████████████████████████████████*/


const countChar = chr => s => {
  let n = 0, i = 0;

  while (true) {
    i = s.indexOf(chr, i);
    if (i >= 0) {++n; i++}
    else break;
  }

  return n;
};


/*
█████ Concatenization █████████████████████████████████████████████████████████*/


Str.catWith = s => (...xs) => xs.join(s);


Str.cat = Str.catWith("");


Str.cat_ = Str.catWith(" ");


/*
█████ Normalization ███████████████████████████████████████████████████████████*/


// convert to ISO date fragment/string

Str.normalizeDate = (locale, century = 20) => s => {
  switch (locale) {
    case "de-DE": {
      for (const r of O.values(Rex.i18n.deDE.dates)) {
        if (r.test(s)) {
          const rx = s.match(r);
          let yearPrefix = "";

          if (rx.groups.y.length === 2) yearPrefix = century;

          if (rx.groups.m.length > 2) {
            if (Rex.i18n.deDE.months.test(rx.groups.m))
              rx.groups.m = _Map.monthsFullDe.get(rx.groups.m);

            else if (Rex.i18n.deDE.months_.test(rx.groups.m))
              rx.groups.m = _Map.monthsShortDe.get(rx.groups.m);            
          }

          return Str.cat(
            yearPrefix + rx.groups.y,
            "-" + rx.groups.m.padStart(2, "0"),
            "-" + rx.groups.d.padStart(2, "0"));
        }
      }

      throw new Err(`invalid date string "${s}"`);
    }

    default: throw new Err(`unknown locale "${locale}"`);
  }
};


// convert to ISO number string

Str.normalizeNum = locale => s => {
  switch (locale) {
    case "de-DE": {
      for (const r of O.values(Rex.i18n.deDE.numbers)) {
        if (r.test(s)) {
          const rx = s.match(r);
          let decPoint = "";

          if (rx.groups.sign === undefined) rx.groups.sign = "";

          if (rx.groups.frac === undefined) rx.groups.frac = "";
          else decPoint = ".";

          return rx.groups.sign + rx.groups.int + decPoint + rx.groups.frac;
        }
      }

      throw new Err(`invalid date string "${s}"`);
    }

    default: throw new Err(`unknown locale "${locale}"`);
  }
};


/*
█████ Searching ███████████████████████████████████████████████████████████████*/


/* Find all indices of the given pattern and returns the one referenced by `i`.
`i` can be positive or negative. A positive value denotes the normal index
whereas a negative one denotes the index relative to the end. -1 means the last
index. -2 the penultimate one. */

Str.findIndexNth = (pattern, i) => s => {
  const xs = Array.from(s.matchAll(pattern));
  let rx;

  if (xs.length === 0) return null;
  else if (i < 0) rx = xs.slice(i) [0];
  else rx = xs[i];

  if (rx === undefined) return null;
  else return rx.index;
};


// find the first and last index of a pattern in the passed string

Str.findIndexBounds = pattern => s => {
  const xs = Array.from(s.matchAll(pattern));
  let rx;

  if (xs.length === 0) return null;
  else if (xs.length === 1) return Pair(xs[0].index, xs[0].index);
  else return Pair(xs[0].index, xs[xs.length - 1].index);
};


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


Str.append = s => t => "" + s + t;


Str.Semigroup = {append: Str.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


Str.empty = "";


Str.Monoid = {
  ...Str.Semigroup,
  empty: Str.empty
};


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


Str.capitalize = s => s[0].strToUpper() + s.slice(1).strToLower();


Str.splitChunk = ({from, to}) => s =>
  s.match(new RegExp(`.{${from},${to}}`, "g")) || [];


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ THESE ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// encodes logical (A & B) || A || B


export const These = variant("These") (
  unary("This"),
  unary("That"),
  binary("Both"));


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


These.map = f => tx => tx.these.run({
  this: x => These.This(f(x)),
  that: _ => tx,
  both: x => y => These.Both(f(x)) (y)
});


These.Functor = {map: These.map};


/*
█████ Functor :: Alt ██████████████████████████████████████████████████████████*/


These.alt = tx => ty => tx.these.run({
  this: _ => tx,
  that: _ => ty,
  both: x => _ => These.This(x)
});


These.Alt = {
  ...These.Functor,
  alt: These.alt
};


/*
█████ Functor :: Alt :: Plus ██████████████████████████████████████████████████*/


These.zero = Monoid => These.That(Monoid.empty);


These.Plus = {
  ...These.Alt,
  zero: These.zero
};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


These.ap = Semigroup => tf => tx => tf.these.run({
  this: f => tx.these.run({
    this: x => These.This(f(x)),
    that: y => These.Both(f) (y),
    both: x => y => These.Both(f(x)) (y)
  }),

  that: y => tx.these.run({
    this: x => These.Both(x) (y),
    that: y2 => These.That(Semigroup.append(y) (y2)),
    both: x => y => These.Both(x) (Semigroup.append(y) (y2))
  }),

  both: f => y => tx.these.run({
    this: x => These.Both(f(x)) (y),
    that: y => These.Both(f) (y),
    both: x => y2 => These.Both(f(x)) (Semigroup.append(y) (y2))
  })
});


These.Apply = {
  ...These.Functor,
  ap: These.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


These.of = These.That;


These.Applicative = {
  ...These.Apply,
  of: These.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


These.chain = Semigroup => mx => fm => mx.these.run({
  this: x => fm(x),
  that: _ => mx,

  both: x => y => fm(x).these.run({
    this: x2 => These.Both(x2) (y),
    that: y2 => These.Both(x) (Semigroup.append(y) (y2)),
    both: x2 => y2 => These.Both(x2) (Semigroup.append(y) (y2))
  })
});


These.Chain = {
  ...These.Apply,
  chain: These.chain
};


/*
█████ Functor :: Applicative :: Monad █████████████████████████████████████████*/


These.Monad = {
  ...These.Applicative,
  chain: These.chain
};


/*
█████ Semigroup ███████████████████████████████████████████████████████████████*/


These.append = (Semigroup, Semigroup2) => tx => ty => tx.these.run({
  this: x => ty.these.run({
    this: x2 => These.This(Semigroup.append(x) (x2)),
    that: y => These.Both(x) (y),
    both: x2 => y => These.Both(Semigroup.append(x) (x2)) (y),
  }),

  that: y => ty.these.run({
    this: x => These.Both(x) (y),
    that: y2 => These.That(Semigroup2.append(y) (y2)),
    both: x => y2 => These.Both(x) (Semigroup2.append(y) (y2)),
  }),

  both: x => y => ty.these.run({
    this: x2 => These.Both(Semigroup.append(x) (x2)) (y),
    that: y2 => These.Both(x) (Semigroup2.append(y) (y2)),
    both: x2 => y2 => These.Both(Semigroup.append(x) (x2)) (Semigroup2.append(y) (y2)),
  })
});


These.Semigroup = {append: These.append};


/*
█████ Semigroup :: Monoid █████████████████████████████████████████████████████*/


These.empty = Monoid => These.That(Monoid.empty);


These.Monoid = {
  ...These.Semigroup,
  empty: These.empty
};


/*█████████████████████████████████████████████████████████████████████████████
█████████████████████████████████ TRAMPOLINE ██████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Allow stack-safe monad recursion through the trampoline mechanism. The type
is the less powerful version of the continuation monad. Due to its structure
consisting of an imperative loop and tagged objects the `Trampoline` type
doesn't have a monad transformer but must be used as the outermost base monad
of a transformer stack. */


export const Trampoline = o => {
  while (o.constructor === Trampoline.bounce) o = o.f(o.x);

  if (o.constructor === Trampoline.return) return o.x;
  else return o;
};


export const Tramp = Trampoline;


Tramp.bounce = x => f => ({constructor: Tramp.bounce, f, x});


Tramp.bounce_ = f => x => ({constructor: Tramp.bounce, f, x});


Tramp.return = x => ({constructor: Tramp.return, x});


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ TREE █████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* 2-3 tree binary search tree implementation as the foundation of persistent
data structures. */


const Tree = {};


// smart constructor

Tree.Empty = thisify(o => {
  Object.defineProperty(o, TAG, {value: "Tree"});
  o.tag = "Empty";
  return o;
});


Tree.Leaf = x => {
  const o = {height: 0, size: 1, min: x, x, tag: "Leaf"};
  Object.defineProperty(o, TAG, {value: "Tree"});
  return o;
};


Tree.Node2 = (height, size, min, left, right) => {
  const o = {height, size, min, left, right, tag: "Node2"};
  Object.defineProperty(o, TAG, {value: "Tree"});
  return o;
};


Tree.Node3 = (height, size, min, left, middle, right) => {
  const o = {height, size, min, left, middle, right, tag: "Node3"};
  Object.defineProperty(o, TAG, {value: "Tree"});
  return o;
};


Tree.node2 = (left, right) =>
  Tree.Node2(left.height + 1, left.size + right.size, left.min, left, right);


Tree.node3 = (left, middle, right) =>
  Tree.Node3(left.height + 1, left.size + middle.size + right.size, left.min, left, middle, right);


/*
█████ Implementation Details ██████████████████████████████████████████████████*/


Tree.levelUp = args => {
  switch(args.length) {
    case 2: return [Tree.node2(...args)];
    case 3: return [Tree.node3(...args)];  
    case 4: return [Tree.node2(args[0], args[1]), Tree.node2(args[2], args[3])];
    default: throw new Err("unexpected number of arguments");
  }
}


Tree.levelHeight = (left, right) => {
  if (left.height < right.height) {
    if (right.tag === "Node2") {
      const xs = Tree.levelHeight(left, right.left)
        .concat(right.right);

      return Tree.levelUp(xs);
    }

    else {
      const xs = Tree.levelHeight(left, right.left)
        .concat([right.middle, right.right]);

      return Tree.levelUp(xs);
    }
  }

  else if (left.height > right.height) {
    if (left.tag === "Node2") {
      const xs = [left.left]
        .concat(Tree.levelHeight(left.right, right));

      return Tree.levelUp(xs);
    }

    else {
      const xs = [left.left, left.middle]
        .concat(Tree.levelHeight(left.right, right));

      return Tree.levelUp(xs);
    }    
  }

  else return [left, right];
};


Tree.merge = (left, right) => {
  if (left.tag === "Empty") return right;
  else if (right.tag === "Empty") return left;

  else {
    const xs = Tree.levelHeight(left, right);

    if (xs.length === 1) return xs[0];
    else return Tree.node2(...xs);
  }
};


Tree.split = (tree, f) => {
  if (tree.tag === "Empty") return [Tree.Empty, Tree.Empty];

  else if (tree.tag === "Leaf") {
    if (f(tree.x)) return [Tree.Empty, Tree.Leaf(tree.x)];
    else return [Tree.Leaf(tree.x), Tree.Empty];
  }

  else if (tree.tag === "Node2") {
    if (f(tree.right.min)) {
      const [left, right] = Tree.split(tree.left, f);
      return [left, Tree.merge(right, tree.right)];
    }

    else {
      const [left, right] = Tree.split(tree.right, f);
      return [Tree.merge(tree.left, left), right];
    }
  }

  else {
    if (f(tree.middle.min)) {
      const [left, right] = Tree.split(tree.left, f);
      return [left, Tree.merge(right, Tree.node2(tree.middle, tree.right))];
    }

    if (f(tree.right.min)) {
      const [left, right] = Tree.split(tree.middle, f);
      return [Tree.merge(tree.left, left), Tree.merge(right, tree.right)];
    }

    else {
      const [left, right] = Tree.split(tree.right, f);
      return [Tree.merge(Tree.node2(tree.left, tree.middle), left), right];
    }
  }
};


/*
█████ Catamorphism ████████████████████████████████████████████████████████████*/


// catamorphism (structural fold)

Tree.cata = ({empty, leaf, node2, node3}) => function go(tree) {
  switch (tree.tag) {
    case "Empty": return empty();
    case "Leaf": return leaf(tree.x);

    case "Node2": return node2(
      tree.height,
      tree.min,
      go(tree.left),
      go(tree.right))

    case "Node3": return node3(
      tree.height,
      tree.min,
      go(tree.left),
      go(tree.middle),
      go(tree.right))
  }
};


/*
█████ Conversion ██████████████████████████████████████████████████████████████*/


Tree.fromArr = xs => xs.reduce((acc, x) => Tree.ins(acc, x), Tree.Empty)


Tree.prepend = (tree, xs) => {
  if (tree.tag === "Empty") return xs;
  else if (tree.tag === "Leaf") return (xs.unshift(tree.x), xs);
  
  else if (tree.tag === "Node2")
    return Tree.prepend(tree.left, Tree.prepend(tree.right, xs));

  else if (tree.tag === "Node3")
    return Tree.prepend(tree.left, Tree.prepend(tree.middle, Tree.prepend(tree.right, xs)));
};


Tree.toArr = tree => Tree.prepend(tree, []);


/*
█████ Foldable ████████████████████████████████████████████████████████████████*/


// left fold in ascending order

Tree.foldl = f => acc => Tree.cata({
  empty: () => acc,
  leaf: id,
  node2: (height, min, left, right) => f(f(acc) (left)) (right),
  node3: (height, min, left, middle, right) => f(f(f(acc) (left)) (middle)) (right)
});


// left fold in descending order

Tree.foldl_ = f => acc => Tree.cata({
  empty: () => acc,
  leaf: id,
  node2: (height, min, right, left) => f(f(acc) (left)) (right),
  node3: (height, min, right, middle, left) => f(f(f(acc) (left)) (middle)) (right)
});


// right fold in ascending order

Tree.foldr = f => function go(acc) {
  return tree => {
    switch (tree.tag) {
      case "Empty": return acc;
      case "Leaf": {return f(tree.x) (acc);}
      case "Node2": return go(go(acc) (tree.right)) (tree.left);
      case "Node3": return go(go(go(acc) (tree.right)) (tree.middle)) (tree.left);
    }
  };
};


// right fold in descending order

Tree.foldr_ = f => function go(acc) {
  return tree => {
    switch (tree.tag) {
      case "Empty": return acc;
      case "Leaf": {return f(tree.x) (acc);}
      case "Node2": return go(go(acc) (tree.left)) (tree.right);
      case "Node3": return go(go(go(acc) (tree.left)) (tree.middle)) (tree.right);
    }
  };
};


Tree.Foldable = {
  foldl: Tree.foldl,
  foldr: Tree.foldr
};


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Tree.map = f => Tree.cata({
  empty: () => Tree.Empty,
  leaf: x => Tree.Leaf(f(x)),

  node2: (height, min, left, right) =>
    Tree.Node2(height, left.size + right.size, f(min), left, right),

  node3: (height, min, left, middle, right) =>
    Tree.Node3(height, left.size + middle.size + right.size, f(min), left, middle, right)
});


Tree.Functor = {map: Tree.map};


/*
█████ Getter/Setter ███████████████████████████████████████████████████████████*/


/* The underscore versions compose an additional function `f`. It must be passed
as an argument and is called before the relational function, so that the actual
data `y` can be transformed for the latter. */


Tree.has = (tree, x) => {
  const [left, right] = Tree.split(tree, y => y >= x);

  if (right.tag === "Empty") return false;
  else return right.min === x;
};


Tree.has_ = (tree, f, x) => {
  const [left, right] = Tree.split(tree, y => (z => z >= x) (f(y)));

  if (right.tag === "Empty") return false;
  else return right.min === x;
};


Tree.ins = (tree, x) => {
  const [left, right] = Tree.split(tree, y => y >= x);
  return Tree.merge(Tree.merge(left, Tree.Leaf(x)), right);
};


Tree.ins_ = (tree, f, x) => {
  const [left, right] = Tree.split(tree, y => (z => z >= x) (f(y)));
  return Tree.merge(Tree.merge(left, Tree.Leaf(x)), right);
};


Tree.del = (tree, x) => {
  const [left, right] = Tree.split(tree, y => y >= x),
    [, right2] = Tree.split(right, y => y > x);

  return Tree.merge(left, right2);
};


Tree.del_ = (tree, f, x) => {
  const [left, right] = Tree.split(tree, y => (z => z >= x) (f(y))),
    [, right2] = Tree.split(right, y => (z => z > x) (f(y)));

  return Tree.merge(left, right2);
};


/*
█████ Meta Information ████████████████████████████████████████████████████████*/


Tree.leafs = Tree.cata({
  empty: () => 0,
  leaf: _ => 1,
  node2: (height, min, left, right) => left + right,
  node3: (height, min, left, middle, right) => left + middle + right
});


Tree.nodes = Tree.cata({
  empty: () => 0,
  leaf: _ => 0,
  node2: (height, min, left, right) => 1 + left + right,
  node3: (height, min, left, middle, right) => 1 + left + middle + right,
});


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████ TUPLE :: PAIR ████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// smart constructor

export const Pair = (x, y) => {
  const o = {
    0: x, 1: y, length: 2,

    [Symbol.iterator]: function*() {
      yield x;
      yield y;
    }
  };

  Object.defineProperty(o, TAG, {value: "Pair"});
  return o;  
};


// smart constructor to define lazy getters

export const Pair_ = o => {
  Object.defineProperty(o, TAG, {value: "Pair"});
  o.length = 2;

  o[Symbol.iterator] = function*() {
    yield o[0];
    yield o[1];
  };

  return o;
};


/*
█████ Extracting ██████████████████████████████████████████████████████████████*/


Pair.fst = tx => tx[0];


Pair.snd = tx => tx[1];


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Pair.map = f => ({1: x, 2: y}) => Pair(x, f(y));


Pair.Functor = {map: Pair.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


Pair.ap = Semigroup => ({1: x, 2: f}) => ({1: y, 2: z}) =>
  Pair(Semigroup.append(x) (y), f(z));


Pair.Apply = {
  ...Pair.Functor,
  ap: Pair.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


Pair.of = Monoid => x => Pair(Monoid.empty, x);


Pair.Applicative = {
  ...Pair.Apply,
  of: Pair.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


Pair.chain = Semigroup => fm => ({1: x, 2: y}) => {
  const {1: x2, 2: y2} = fm(y);
  return Pair(Semigroup.append(x) (x2), y2);
};


Pair.Chain = {
  ...Pair.Apply,
  chain: Pair.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


Pair.Monad = {
  ...Pair.Applicative,
  chain: Pair.chain
};


/*
█████ Functor :: Bifunctor ████████████████████████████████████████████████████*/


/* bimap/dimap comparison:

  bimap :: (a -> b) -> (c -> d) -> bifunctor  a c -> bifunctor  b d
            ^^^^^^
  dimap :: (b -> a) -> (c -> d) -> profunctor a c -> profunctor b d
            ^^^^^^                                                  */


Pair.bimap = f => g => tx => Pair(f(tx[0]), g(tx[1]));


Pair.Bifunctor = ({
  ...Pair.Functor,
  bimap: Pair.bimap
});


/*
█████ Functor :: Extend ███████████████████████████████████████████████████████*/


// w a -> w (w a)
Pair.duplicate = wx => Pair(wx[0], wx);


// (w a -> b) -> w a -> w b
Pair.extend = fw => wx => Pair(wx[0], fw(wx));


Pair.Extend = {
  ...Pair.Functor,
  extend: Pair.extend
};


/*
█████ Functor :: Extend :: Comonad ████████████████████████████████████████████*/


// w a -> a
Pair.extract = Pair.snd;


Pair.Comonad = {
  ...Pair.Extend,
  extract: Pair.extract
};


/*
█████ Getters/Setters █████████████████████████████████████████████████████████*/


Pair.setFst = x => tx => Pair(x, tx[1]);


Pair.setSnd = x => tx => Pair(tx[0], x);


/*
█████ Misc. ███████████████████████████████████████████████████████████████████*/


Pair.mapFst = f => tx => Pair(f(tx[0]), tx[1]);


Pair.swap = tx => Pair(tx[1], tx[0]);


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████ TUPLE :: TRIPLE ███████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


// smart constructor

export const Triple = (x, y, z) => {
  const o = {
    0: x, 1: y, 2: z, length: 3,

    [Symbol.iterator]: function*() {
      yield x;
      yield y;
      yield z;
    }
  };

  Object.defineProperty(o, TAG, {value: "Triple"});
  return o;  
};


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████ YONEDA ████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* Encodes dynamic function composition within a functor. Useful in cases when
the composition cannot be defined manually upfront but only at runtime. */


export const Yoneda = type("Yoneda", "yo");


export const Yo = Yoneda; // shortcut


/*
█████ Functor █████████████████████████████████████████████████████████████████*/


Yo.map = f => tx => Yo(g => tx.yo(comp(g) (f)));


Yo.Functor = {map: Yo.map};


/*
█████ Functor :: Apply ████████████████████████████████████████████████████████*/


Yo.ap = Apply => tf => tx => Yo(f => Apply.ap(tf.yo(comp(f))) (tx.yo(id)));


Yo.Apply = {
  ...Yo.Functor,
  ap: Yo.ap
};


/*
█████ Functor :: Apply :: Applicative █████████████████████████████████████████*/


Yo.of = Applicative => x => Yo(f => Applicative.of(f(x)));


Yo.Applicative = {
  ...Yo.Apply,
  of: Yo.of
};


/*
█████ Functor :: Apply :: Chain ███████████████████████████████████████████████*/


Yo.chain = Chain => mx => fm =>
  Yo(f => Chain.chain(mx.yo(id)) (x => fm(x).yo(f)));
    

Yo.Chain = {
  ...Yo.Apply,
  chain: Yo.chain
};


/*
█████ Functor :: Apply :: Applicative :: Monad ████████████████████████████████*/


Yo.Monad = {
  ...Yo.Applicative,
  chain: Yo.chain
};


/*
█████ Lift/Unlift █████████████████████████████████████████████████████████████*/


Yo.lift = Functor => tx => Yo(f => Functor.map(f) (tx));


Yo.lower = tx => tx.yo(id);


/*█████████████████████████████████████████████████████████████████████████████
████████████████████████████████ RESOLVE DEPS █████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


A.fromList = A.fromList();


A.fromQueue = A.fromQueue();


A.unzip = A.unzip();


Eff.A.trampOf = Eff.A.trampOf();


Eff.Fold.array = Eff.Fold.array();


Eff.Fold.list = Eff.Fold.list();


Eff.Trav.array = Eff.Trav.array();


Eff.Trav.arraySeq = Eff.Trav.arraySeq();


Eff.Trav.list = Eff.Trav.list();


Eff.Trav.listSeq = Eff.Trav.listSeq();


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████
█████████████████████████████████████ IO ██████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*█████████████████████████████████████████████████████████████████████████████
█████████████████████████████████ FILE SYSTEM █████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/* The file system implementation is supplied as a formal parameter to avoid a
hard dependency. You can pass two different constructors to determine the
continuation semantics:

  * Parallel
  * Serial

Since both types don't have built-in exception handling, you either immediately
throw or pass on exceptions for later handling. */


export const FileSysHandle = fs => Cons => thisify(o => {
  o.copy = src => dest => Cons(k =>
    fs.copyFile(src, dest, e =>
      e ? k(new Ex(e)) : k(null)));

  o.move = src => dest =>
    Cons.chain(o.copy(src) (dest)) (e =>
      e?.constructor?.name === "Exception" ? Cons.of(e) : o.unlink(src));

  o.read = opt => path => Cons(k =>
    fs.readFile(path, opt, (e, x) =>
      e ? k(new Ex(e)) : k(x)));

  o.readUtf8 = opt => path => Cons(k =>
    fs.readFile(path, Object.assign(opt, {encoding: "utf8"}), (e, x) =>
      e ? k(new Ex(e)) : k(x)));

  o.readLatin1 = opt => path => Cons(k =>
    fs.readFile(path, Object.assign(opt, {encoding: "latin1"}), (e, x) =>
      e ? k(new Ex(e)) : k(x)));

  o.readStream = opt => path => Cons(k => {
    const stream = fs.createReadStream(path, opt);
    let s = "";

    stream.on("close", () => k(s));
    stream.on("error", e => k(new Ex(e)));
    stream.on("data", chunk => {s += chunk});
  });

  o.scanDir = path => Cons(k =>
    fs.readdir(path, (e, xs) =>
      e ? k(new Ex(e)) : k(xs)));

  o.stat = path => Cons(k =>
    fs.stat(path, (e, o) =>
      e ? k(new Ex(e)) : k(o)));

  o.unlink = path => Cons(k =>
    fs.unlink(path, e =>
      e ? k(new Ex(e)) : k(null)));

  o.write = opt => path => s => Cons(k =>
    fs.writeFile(path, s, opt, e =>
      e ? k(new Ex(e)) : k(s)));

  return o;
});


export const FileSysThrow = fs => Cons => thisify(o => {
  o.copy = src => dest => Cons(k =>
    fs.copyFile(src, dest, e =>
      e ? _throw(e) : k(null)));

  o.move = src => dest =>
    Cons.chain(o.copy(src) (dest)) (_ =>
      o.unlink(src));

  o.read = opt => path => Cons(k =>
    fs.readFile(path, opt, (e, x) =>
      e ? _throw(e) : k(x)));

  o.readUtf8 = opt => path => Cons(k =>
    fs.readFile(path, Object.assign(opt, {encoding: "utf8"}), (e, x) =>
      e ? _throw(e) : k(x)));

  o.readLatin1 = opt => path => Cons(k =>
    fs.readFile(path, Object.assign(opt, {encoding: "latin1"}), (e, x) =>
      e ? _throw(e) : k(x)));

  o.readStream = opt => path => Cons(k => {
    const stream = fs.createReadStream(path, opt);
    let s = "";

    stream.on("close", () => k(s));
    stream.on("error", _throw);
    stream.on("data", chunk => {s += chunk});
  });

  o.scanDir = path => Cons(k =>
    fs.readdir(path, (e, xs) =>
      e ? _throw(e) : k(xs)));

  o.stat = path => Cons(k =>
    fs.stat(path, (e, o) =>
      e ? _throw(e) : k(o)));

  o.unlink = path => Cons(k =>
    fs.unlink(path, e =>
      e ? _throw(e) : k(null)));

  o.write = opt => path => s => Cons(k =>
    fs.writeFile(path, s, opt, e =>
      e ? _throw(e) : k(s)));

  return o;
});


/*█████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████
████████████████████████████████████ TODO █████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████
███████████████████████████████████████████████████████████████████████████████*/


/*

  * add child_process support

  * backtacking
    * depth/breadth first strategies
      * BFS is fair but biased for all branches
      * DFS is unfair for all branches
    * disjunctions are encoded by Alt/Plus
    * conjunctions are encoded by Chain
    * pruning: e.g. stop at the first result
    * List implements depth first
    * Logic implements breadth first
    * DFS adds new tasks at the front of the queue
    * BFS adds them at the tail of the queue
    * TODO: implement logict

*/