lang.mjs

/* Type */

export function isNil(val) {return val == null}
export function reqNil(val) {return isNil(val) ? val : throwErrFun(val, isNil)}
export function optNil(val) {return isNil(val) ? val : reqNil(val)}
export function onlyNil(val) {return isNil(val) ? val : undefined}

export function isSome(val) {return val != null}
export function reqSome(val) {return isSome(val) ? val : throwErrFun(val, isSome)}
export function optSome(val) {return isNil(val) ? val : reqSome(val)}
export function onlySome(val) {return isSome(val) ? val : undefined}

export function isBool(val) {return typeof val === `boolean`}
export function reqBool(val) {return isBool(val) ? val : throwErrFun(val, isBool)}
export function optBool(val) {return isNil(val) ? val : reqBool(val)}
export function onlyBool(val) {return isBool(val) ? val : undefined}
export function laxBool(val) {return isNil(val) ? false : reqBool(val)}

export function isNum(val) {return typeof val === `number`}
export function reqNum(val) {return isNum(val) ? val : throwErrFun(val, isNum)}
export function optNum(val) {return isNil(val) ? val : reqNum(val)}
export function onlyNum(val) {return isNum(val) ? val : undefined}
export function laxNum(val) {return isNil(val) ? 0 : reqNum(val)}

export function isFin(val) {return Number.isFinite(val)}
export function reqFin(val) {return isFin(val) ? val : throwErrFun(val, isFin)}
export function optFin(val) {return isNil(val) ? val : reqFin(val)}
export function onlyFin(val) {return isFin(val) ? val : undefined}
export function laxFin(val) {return isNil(val) ? 0 : reqFin(val)}

export function isFinNeg(val) {return isNum(val) && val < 0 && val > -Infinity}
export function reqFinNeg(val) {return isFinNeg(val) ? val : throwErrFun(val, isFinNeg)}
export function optFinNeg(val) {return isNil(val) ? val : reqFinNeg(val)}
export function onlyFinNeg(val) {return isFinNeg(val) ? val : undefined}

export function isFinPos(val) {return isNum(val) && val > 0 && val < Infinity}
export function reqFinPos(val) {return isFinPos(val) ? val : throwErrFun(val, isFinPos)}
export function optFinPos(val) {return isNil(val) ? val : reqFinPos(val)}
export function onlyFinPos(val) {return isFinPos(val) ? val : undefined}

// TODO: simply use `Number.isSafeInteger`.
export function isInt(val) {return isNum(val) && ((val % 1) === 0)}
export function reqInt(val) {return isInt(val) ? val : throwErrFun(val, isInt)}
export function optInt(val) {return isNil(val) ? val : reqInt(val)}
export function onlyInt(val) {return isInt(val) ? val : undefined}
export function laxInt(val) {return isNil(val) ? 0 : reqInt(val)}

export function isNat(val) {return isInt(val) && val >= 0}
export function reqNat(val) {return isNat(val) ? val : throwErrFun(val, isNat)}
export function optNat(val) {return isNil(val) ? val : reqNat(val)}
export function onlyNat(val) {return isNat(val) ? val : undefined}
export function laxNat(val) {return isNil(val) ? 0 : reqNat(val)}

export function isIntNeg(val) {return isInt(val) && val < 0}
export function reqIntNeg(val) {return isIntNeg(val) ? val : throwErrFun(val, isIntNeg)}
export function optIntNeg(val) {return isNil(val) ? val : reqIntNeg(val)}
export function onlyIntNeg(val) {return isIntNeg(val) ? val : undefined}

export function isIntPos(val) {return isInt(val) && val > 0}
export function reqIntPos(val) {return isIntPos(val) ? val : throwErrFun(val, isIntPos)}
export function optIntPos(val) {return isNil(val) ? val : reqIntPos(val)}
export function onlyIntPos(val) {return isIntPos(val) ? val : undefined}

export function isNaN(val) {return val !== val}
export function reqNaN(val) {return isNaN(val) ? val : throwErrFun(val, isNaN)}
export function optNaN(val) {return isNil(val) ? val : reqNaN(val)}
export function onlyNaN(val) {return isNaN(val) ? val : undefined}

export function isInf(val) {return val === Infinity || val === -Infinity}
export function reqInf(val) {return isInf(val) ? val : throwErrFun(val, isInf)}
export function optInf(val) {return isNil(val) ? val : reqInf(val)}
export function onlyInf(val) {return isInf(val) ? val : undefined}

export function isBigInt(val) {return typeof val === `bigint`}
export function reqBigInt(val) {return isBigInt(val) ? val : throwErrFun(val, isBigInt)}
export function optBigInt(val) {return isNil(val) ? val : reqBigInt(val)}
export function onlyBigInt(val) {return isBigInt(val) ? val : undefined}
export function laxBigInt(val) {return isNil(val) ? BigInt(0) : reqBigInt(val)}

export function isStr(val) {return typeof val === `string`}
export function reqStr(val) {return isStr(val) ? val : throwErrFun(val, isStr)}
export function optStr(val) {return isNil(val) ? val : reqStr(val)}
export function onlyStr(val) {return isStr(val) ? val : undefined}
export function laxStr(val) {return isNil(val) ? `` : reqStr(val)}

// TODO shorter name.
export function isValidStr(val) {return isStr(val) && !!val}
export function reqValidStr(val) {return isValidStr(val) ? val : throwErrFun(val, isValidStr)}
export function optValidStr(val) {return isNil(val) ? val : reqValidStr(val)}
export function onlyValidStr(val) {return isValidStr(val) ? val : undefined}

export function isSym(val) {return typeof val === `symbol`}
export function reqSym(val) {return isSym(val) ? val : throwErrFun(val, isSym)}
export function optSym(val) {return isNil(val) ? val : reqSym(val)}
export function onlySym(val) {return isSym(val) ? val : undefined}

// TODO tune perf.
export function isKey(val) {return isPrim(val) && !isJunk(val)}
export function reqKey(val) {return isKey(val) ? val : throwErrFun(val, isKey)}
export function optKey(val) {return isNil(val) ? val : reqKey(val)}
export function onlyKey(val) {return isKey(val) ? val : undefined}

export function isStructKey(val) {return isStr(val) || isSym(val)}
export function reqStructKey(val) {return isStructKey(val) ? val : throwErrFun(val, isKey)}
export function optStructKey(val) {return isNil(val) ? val : reqStructKey(val)}
export function onlyStructKey(val) {return isStructKey(val) ? val : undefined}

export function isPk(val) {return isValidStr(val) || isIntPos(val)}
export function reqPk(val) {return isPk(val) ? val : throwErrFun(val, isPk)}
export function optPk(val) {return isNil(val) ? val : reqPk(val)}
export function onlyPk(val) {return isPk(val) ? val : undefined}

export function isJunk(val) {return isNil(val) || isNaN(val) || isInf(val)}
export function reqJunk(val) {return isJunk(val) ? val : throwErrFun(val, isJunk)}
export function optJunk(val) {return isNil(val) ? val : reqJunk(val)}
export function onlyJunk(val) {return isJunk(val) ? val : undefined}

export function isComp(val) {return isObj(val) || isFun(val)}
export function reqComp(val) {return isComp(val) ? val : throwErrFun(val, isComp)}
export function optComp(val) {return isNil(val) ? val : reqComp(val)}
export function onlyComp(val) {return isComp(val) ? val : undefined}

export function isPrim(val) {return !isComp(val)}
export function reqPrim(val) {return isPrim(val) ? val : throwErrFun(val, isPrim)}
export function optPrim(val) {return isNil(val) ? val : reqPrim(val)}
export function onlyPrim(val) {return isPrim(val) ? val : undefined}

export function isFun(val) {return typeof val === `function`}
export function reqFun(val) {return isFun(val) ? val : throwErrFun(val, isFun)}
export function optFun(val) {return isNil(val) ? val : reqFun(val)}
export function onlyFun(val) {return isFun(val) ? val : undefined}

export function isFunSync(val) {return isFunType(val, `Function`)}
export function reqFunSync(val) {return isFunSync(val) ? val : throwErrFun(val, isFunSync)}
export function optFunSync(val) {return isNil(val) ? val : reqFunSync(val)}
export function onlyFunSync(val) {return isFunSync(val) ? val : undefined}

export function isFunGen(val) {return isFunType(val, `GeneratorFunction`)}
export function reqFunGen(val) {return isFunGen(val) ? val : throwErrFun(val, isFunGen)}
export function optFunGen(val) {return isNil(val) ? val : reqFunGen(val)}
export function onlyFunGen(val) {return isFunGen(val) ? val : undefined}

export function isFunAsync(val) {return isFunType(val, `AsyncFunction`)}
export function reqFunAsync(val) {return isFunAsync(val) ? val : throwErrFun(val, isFunAsync)}
export function optFunAsync(val) {return isNil(val) ? val : reqFunAsync(val)}
export function onlyFunAsync(val) {return isFunAsync(val) ? val : undefined}

export function isFunAsyncGen(val) {return isFunType(val, `AsyncGeneratorFunction`)}
export function reqFunAsyncGen(val) {return isFunAsyncGen(val) ? val : throwErrFun(val, isFunAsyncGen)}
export function optFunAsyncGen(val) {return isNil(val) ? val : reqFunAsyncGen(val)}
export function onlyFunAsyncGen(val) {return isFunAsyncGen(val) ? val : undefined}

export function isObj(val) {return isSome(val) && typeof val === `object`}
export function reqObj(val) {return isObj(val) ? val : throwErrFun(val, isObj)}
export function optObj(val) {return isNil(val) ? val : reqObj(val)}
export function onlyObj(val) {return isObj(val) ? val : undefined}

export function isNpo(val) {return isObj(val) && isNil(Object.getPrototypeOf(val))}
export function reqNpo(val) {return isNpo(val) ? val : throwErrFun(val, isNpo)}
export function optNpo(val) {return isNil(val) ? val : reqNpo(val)}
export function onlyNpo(val) {return isNpo(val) ? val : undefined}
export function laxNpo(val) {return isNil(val) ? Emp() : reqNpo(val)}

export function isDict(val) {
  if (!isObj(val)) return false
  const pro = Object.getPrototypeOf(val)
  return isNil(pro) || (
    pro === Object.prototype
    && (!own.call(val, `constructor`) || enu.call(val, `constructor`))
  )
}
export function reqDict(val) {return isDict(val) ? val : throwErrFun(val, isDict)}
export function optDict(val) {return isNil(val) ? val : reqDict(val)}
export function onlyDict(val) {return isDict(val) ? val : undefined}
export function laxDict(val) {return isNil(val) ? Emp() : reqDict(val)}

export function isStruct(val) {return isObj(val) && !(Symbol.iterator in val)}
export function reqStruct(val) {return isStruct(val) ? val : throwErrFun(val, isStruct)}
export function optStruct(val) {return isNil(val) ? val : reqStruct(val)}
export function onlyStruct(val) {return isStruct(val) ? val : undefined}
export function laxStruct(val) {return isNil(val) ? Emp() : reqStruct(val)}

export function isArr(val) {return Array.isArray(val)}
export function reqArr(val) {return isArr(val) ? val : throwErrFun(val, isArr)}
export function optArr(val) {return isNil(val) ? val : reqArr(val)}
export function onlyArr(val) {return isArr(val) ? val : undefined}
export function laxArr(val) {return isNil(val) ? [] : reqArr(val)}

/*
At the time of writing, in V8, array subclasses perform MUCH worse than true
arrays. In some hotspots we enforce true arrays for consistent performance.
*/
export function isTrueArr(val) {return isArr(val) && val.constructor === Array}
export function reqTrueArr(val) {return isTrueArr(val) ? val : throwErrFun(val, isTrueArr)}
export function optTrueArr(val) {return isNil(val) ? val : reqTrueArr(val)}
export function onlyTrueArr(val) {return isTrueArr(val) ? val : undefined}
export function laxTrueArr(val) {return isNil(val) ? [] : reqTrueArr(val)}

export function isReg(val) {return isInst(val, RegExp)}
export function reqReg(val) {return isReg(val) ? val : throwErrFun(val, isReg)}
export function optReg(val) {return isNil(val) ? val : reqReg(val)}
export function onlyReg(val) {return isReg(val) ? val : undefined}

export function isDate(val) {return isInst(val, Date)}
export function reqDate(val) {return isDate(val) ? val : throwErrFun(val, isDate)}
export function optDate(val) {return isNil(val) ? val : reqDate(val)}
export function onlyDate(val) {return isDate(val) ? val : undefined}

export function isValidDate(val) {return isDate(val) && isFin(val.valueOf())}
export function reqValidDate(val) {return isValidDate(val) ? val : throwErrFun(val, isValidDate)}
export function optValidDate(val) {return isNil(val) ? val : reqValidDate(val)}
export function onlyValidDate(val) {return isValidDate(val) ? val : undefined}

export function isInvalidDate(val) {return isDate(val) && !isValidDate(val)}
export function reqInvalidDate(val) {return isInvalidDate(val) ? val : throwErrFun(val, isInvalidDate)}
export function optInvalidDate(val) {return isNil(val) ? val : reqInvalidDate(val)}
export function onlyInvalidDate(val) {return isInvalidDate(val) ? val : undefined}

export function isSet(val) {return isInst(val, Set)}
export function reqSet(val) {return isSet(val) ? val : throwErrFun(val, isSet)}
export function optSet(val) {return isNil(val) ? val : reqSet(val)}
export function onlySet(val) {return isSet(val) ? val : undefined}
export function laxSet(val) {return isNil(val) ? new Set() : reqSet(val)}

export function isMap(val) {return isInst(val, Map)}
export function reqMap(val) {return isMap(val) ? val : throwErrFun(val, isMap)}
export function optMap(val) {return isNil(val) ? val : reqMap(val)}
export function onlyMap(val) {return isMap(val) ? val : undefined}
export function laxMap(val) {return isNil(val) ? new Map() : reqMap(val)}

export function isPromise(val) {return isComp(val) && `then` in val && isFun(val.then)}
export function reqPromise(val) {return isPromise(val) ? val : throwErrFun(val, isPromise)}
export function optPromise(val) {return isNil(val) ? val : reqPromise(val)}
export function onlyPromise(val) {return isPromise(val) ? val : undefined}

export function isIter(val) {return isObj(val) && Symbol.iterator in val}
export function reqIter(val) {return isIter(val) ? val : throwErrFun(val, isIter)}
export function optIter(val) {return isNil(val) ? val : reqIter(val)}
export function onlyIter(val) {return isIter(val) ? val : undefined}

export function isIterAsync(val) {return isObj(val) && Symbol.asyncIterator in val}
export function reqIterAsync(val) {return isIterAsync(val) ? val : throwErrFun(val, isIterAsync)}
export function optIterAsync(val) {return isNil(val) ? val : reqIterAsync(val)}
export function onlyIterAsync(val) {return isIterAsync(val) ? val : undefined}

export function isIterator(val) {return isIter(val) && hasNext(val)}
export function reqIterator(val) {return isIterator(val) ? val : throwErrFun(val, isIterator)}
export function optIterator(val) {return isNil(val) ? val : reqIterator(val)}
export function onlyIterator(val) {return isIterator(val) ? val : undefined}

export function isIteratorAsync(val) {return isIterAsync(val) && hasNext(val)}
export function reqIteratorAsync(val) {return isIteratorAsync(val) ? val : throwErrFun(val, isIteratorAsync)}
export function optIteratorAsync(val) {return isNil(val) ? val : reqIteratorAsync(val)}
export function onlyIteratorAsync(val) {return isIteratorAsync(val) ? val : undefined}

export function isGen(val) {return isIterator(val) && hasMeth(val, `return`) && hasMeth(val, `throw`)}
export function reqGen(val) {return isGen(val) ? val : throwErrFun(val, isGen)}
export function optGen(val) {return isNil(val) ? val : reqGen(val)}
export function onlyGen(val) {return isGen(val) ? val : undefined}

// TODO add `isErrCls`.
export function isCls(val) {return isFun(val) && !!val.prototype}
export function reqCls(val) {return isCls(val) ? val : throwErrFun(val, isCls)}
export function optCls(val) {return isNil(val) ? val : reqCls(val)}
export function onlyCls(val) {return isCls(val) ? val : undefined}

// TODO tests.
export function isSubCls(sub, sup) {return isCls(sub) && (sub === sup || isInst(sub.prototype, sup))}

// TODO tests.
export function reqSubCls(sub, sup) {
  if (isSubCls(sub, sup)) return sub
  throw TypeError(`expected subclass of ${show(sup)}, found ${show(sub)}`)
}

export function isList(val) {return isSome(val) && (isArr(val) || (isIter(val) && isNat(getLength(val))))}
export function reqList(val) {return isList(val) ? val : throwErrFun(val, isList)}
export function optList(val) {return isNil(val) ? val : reqList(val)}
export function onlyList(val) {return isList(val) ? val : undefined}
export function laxList(val) {return isNil(val) ? [] : reqList(val)}

export function isSeq(val) {return isArr(val) || isSet(val) || isList(val) || isIterator(val)}
export function reqSeq(val) {return isSeq(val) ? val : throwErrFun(val, isSeq)}
export function optSeq(val) {return isNil(val) ? val : reqSeq(val)}
export function onlySeq(val) {return isSeq(val) ? val : undefined}

export function isVac(val) {return !val || (isArr(val) && (!val.length || val.every(isVac)))}
export function reqVac(val) {return isVac(val) ? val : throwErrFun(val, isVac)}
export function optVac(val) {return isNil(val) ? val : reqVac(val)}
export function onlyVac(val) {return isVac(val) ? val : undefined}

export function isScalar(val) {
  if (isObj(val)) {
    const fun = get(val, `toString`)
    return isFun(fun) && fun !== Object.prototype.toString && fun !== Array.prototype.toString
  }
  return !(isNil(val) || isSym(val) || isFun(val))
}
export function reqScalar(val) {return isScalar(val) ? val : throwErrFun(val, isScalar)}
export function optScalar(val) {return isNil(val) ? val : reqScalar(val)}
export function onlyScalar(val) {return isScalar(val) ? val : undefined}

export function isScalarOpt(val) {return isNil(val) || isScalar(val)}
export function reqScalarOpt(val) {return isScalarOpt(val) ? val : throwErrFun(val, isScalarOpt)}
export function optScalarOpt(val) {return isNil(val) ? val : reqScalarOpt(val)}
export function onlyScalarOpt(val) {return isScalarOpt(val) ? val : undefined}

export function isArrble(val) {return isIter(val) && `toArray` in val && isFun(val.toArray)}
export function reqArrble(val) {return isArrble(val) ? val : throwErrFun(val, isArrble)}
export function optArrble(val) {return isNil(val) ? val : reqArrble(val)}
export function onlyArrble(val) {return isArrble(val) ? val : undefined}

export function isEqable(val) {return isObj(val) && `eq` in val && isFun(val.eq)}
export function reqEqable(val) {return isEqable(val) ? val : throwErrFun(val, isEqable)}
export function optEqable(val) {return isNil(val) ? val : reqEqable(val)}
export function onlyEqable(val) {return isEqable(val) ? val : undefined}

export function isClearable(val) {return isObj(val) && `clear` in val && isFun(val.clear)}
export function reqClearable(val) {return isClearable(val) ? val : throwErrFun(val, isClearable)}
export function optClearable(val) {return isNil(val) ? val : reqClearable(val)}
export function onlyClearable(val) {return isClearable(val) ? val : undefined}

export function isErr(val) {return isInst(val, Error)}
export function reqErr(val) {return isErr(val) ? val : throwErrFun(val, isErr)}
export function optErr(val) {return isNil(val) ? val : reqErr(val)}
export function onlyErr(val) {return isErr(val) ? val : undefined}

export function isArrOf(val, fun) {
  reqFun(fun)
  return isArr(val) && val.every(fun)
}

// TODO improve error message.
export function reqArrOf(val, fun) {
  for (const elem of reqArr(val)) req(elem, fun)
  return val
}

export function optArrOf(val, fun) {return isNil(val) ? val : reqArrOf(val, fun)}

// TODO consolidate with `hasLen`.
// The two functions must be exactly inverse.
export function isEmpty(val) {
  if (!isObj(val)) return true
  if (isList(val)) return val.length === 0
  if (isIter(val)) return getSize(val) === 0
  return false
}

export function isInst(val, cls) {return isObj(val) && val instanceof cls}

/* Assert */

export function req(val, fun) {
  if (reqValidator(fun)(val)) return val
  throw errFun(val, fun)
}

export function reqOneOf(val, funs) {
  for (const fun of reqArr(funs)) if (fun(val)) return val
  throw errOneOf(val, funs)
}

export function opt(val, fun) {
  reqValidator(fun)
  return isNil(val) ? val : req(val, fun)
}

export function optOneOf(val, funs) {
  return isNil(val) ? val : reqOneOf(val, funs)
}

export function reqInst(val, cls) {
  if (isInst(val, cls)) return val
  throw errInst(val, cls)
}

export function optInst(val, cls) {
  reqCls(cls)
  return isNil(val) ? val : reqInst(val, cls)
}

export function only(val, fun) {return reqValidator(fun)(val) ? val : undefined}

export function onlyInst(val, cls) {return isInst(val, cls) ? val : undefined}

/* Cast */

export function toInst(val, cls) {return isInst(val, cls) ? val : new cls(val)}
export function toInstOpt(val, cls) {return isNil(val) ? val : toInst(val, cls)}

export function render(src) {
  const out = renderOpt(src)
  if (isSome(out)) return out
  throw errConv(src, `string`)
}

export function renderOpt(val) {
  if (isStr(val)) return val
  if (isDate(val)) return renderDate(val)
  if (isScalar(val)) return String(val)
  return undefined
}

export function renderLax(val) {return isNil(val) ? `` : render(val)}

export function show(val, vis = new Map()) {
  if (isStr(val)) return JSON.stringify(val)
  if (isSym(val)) return val.toString()
  if (isFun(val)) return showFun(val)
  if (isObj(val)) return showObj(val, vis)
  return String(val)
}

export function toTrueArr(val) {
  if (isNil(val)) return []
  if (isTrueArr(val)) return val
  return [...reqIter(val)]
}

/* Misc */

export function is(one, two) {return one === two || (isNaN(one) && isNaN(two))}
export function truthy(val) {return !!val}
export function falsy(val) {return !val}
export function nop() {}
export function id(val) {return val}
export function val(src) {return function val() {return src}}
export function panic(val) {if (isSome(val)) throw val}
export function True() {return true}
export function False() {return false}
export function vac(val) {return isVac(val) ? undefined : val}
export function bind(fun, ...args) {return reqFun(fun).bind(this, ...args)}

export function not(fun) {
  reqFun(fun)
  return function not() {return !fun.apply(this, arguments)}
}

export function hasOwn(val, key) {return isComp(val) && own.call(val, key)}
export function hasOwnEnum(val, key) {return isComp(val) && enu.call(val, key)}
export function hasInherited(val, key) {return isComp(val) && key in val && !own.call(val, key)}
export function hasMeth(val, key) {return isComp(val) && key in val && isFun(val[key])}

export function eq(one, two) {
  if (is(one, two)) return true
  if (!isEqable(one)) return false
  const con = getCon(two)
  return !!con && (con === getCon(one)) && one.eq(two)
}

/* Cls */

export function setProto(tar, cls) {
  if (Object.getPrototypeOf(tar) !== cls.prototype) {
    Object.setPrototypeOf(tar, cls.prototype)
  }
}

export function Emp() {return new.target && new.target !== Emp ? this : Object.create(null)}
Emp.prototype = null

/* Op */

// TODO add type checking. `+` must require consistent types.
export function add(a, b) {return a + b}
export function sub(a, b) {return a - b}
export function mul(a, b) {return a * b}
export function div(a, b) {return a / b}
export function rem(a, b) {return a % b}
export function lt(a, b) {return a < b}
export function gt(a, b) {return a > b}
export function lte(a, b) {return a <= b}
export function gte(a, b) {return a >= b}
export function neg(val) {return -val}
export function inc(val) {return val + 1}
export function dec(val) {return val - 1}

/* Internal */

const own = Object.prototype.hasOwnProperty
const enu = Object.prototype.propertyIsEnumerable

function isFunType(val, name) {return isFun(val) && val.constructor.name === name}
function instDesc(val) {return isFun(val) ? `instance of ${showFunName(val)} ` : ``}
function hasNext(val) {return `next` in val && isFun(val.next)}

function reqValidator(fun) {
  if (!isFun(fun)) {
    throw TypeError(`expected validator function, got ${show(fun)}`)
  }
  return fun
}

export function errType(val, msg) {return TypeError(msgType(val, msg))}
export function msgType(val, msg) {return `expected variant of ${msg}, got ${show(val)}`}

export function errFun(val, fun) {return TypeError(msgFun(val, fun))}
export function msgFun(val, fun) {return msgType(val, showFunName(fun))}
export function throwErrFun(val, fun) {throw errFun(val, fun)}

export function errConv(val, msg) {return TypeError(msgConv(val, msg))}
export function errSynt(val, msg) {return SyntaxError(msgConv(val, msg))}
export function msgConv(val, msg) {return `unable to convert ${show(val)} to ${msg}`}

export function errConvInst(val, inst) {return TypeError(msgConvInst(val, inst))}
export function msgConvInst(val, inst) {return msgConv(val, inst.constructor.name)}

export function errInst(val, cls) {return TypeError(msgInst(val, cls))}
export function msgInst(val, cls) {return `expected instance of ${showFunName(cls)}, got ${instDesc(getCon(val))}${show(val)}`}

export function errIn(val, key) {return TypeError(msgIn(val, key))}
export function msgIn(val, key) {return `unable to find ${show(key)} in ${show(val)}`}

export function errImpl() {return TypeError(msgImpl())}
export function msgImpl() {return `not implemented`}

export function errTrans(err, cls, msg) {
  err = toInst(err, cls)
  const pre = renderLax(msg)
  const suf = renderLax(err.message)
  err.message = pre && suf ? pre + `: ` + suf : pre || suf
  return err
}

export function errWrap(err, cls, msg) {
  if (isErrorCauseSupported()) return new cls(msg, {cause: err})
  return errTrans(err, cls, msg)
}

export function errCause(val) {
  if (isErrorCauseSupported()) while (isErr(val)) val = val.cause
  return val
}

/*
The feature is standard, but at the time of writing, has limited engine support.
Many engines, including Chrome, do not report causes unless explicitly asked
for, and Safari <15 does not support causes at all.
*/
function isErrorCauseSupported() {
  return ERROR_CAUSE ??= `cause` in Error(``, {cause: undefined})
}

let ERROR_CAUSE

function errOneOf(val, funs) {return TypeError(msgType(val, `[` + showFuns(funs) + `]`))}

export function convType(tar, src, msg) {
  if (isSome(tar)) return tar
  throw errConv(src, msg)
}

export function convSynt(tar, src, msg) {
  if (isSome(tar)) return tar
  throw errSynt(src, msg)
}

function showFun(val) {return `[function ${val.name || val}]`}
function showFuns(funs) {return funs.map(showFunName).join(`, `)}
export function showFunName(fun) {return fun.name || showFun(fun)}

function showObj(src, vis) {
  const ind = vis?.get(src)
  if (ind) return `[cyclic ` + ind + `]`
  vis?.set(src, vis.size + 1)

  if (isErr(src)) return String(src)
  if (isArr(src)) return showArr(src, vis)

  const con = getCon(src)
  if (!con || con === Object) return showDictOpt(src, vis) || `{}`

  const name = getName(con) || getTag(src)
  if (isInst(src, Boolean)) return `[${name || `Boolean`}: ${show(src.valueOf())}]`
  if (isInst(src, Number)) return `[${name || `Number`}: ${show(src.valueOf())}]`
  if (isInst(src, BigInt)) return `[${name || `BigInt`}: ${show(src.valueOf())}n]`
  if (isInst(src, String)) return `[${name || `String`}: ${show(src.valueOf())}]`
  if (isInst(src, Symbol)) return `[${name || `Symbol`}: ${show(src.valueOf())}]`

  const dict = showDictOpt(src, vis)
  if (!name) return dict || `{}`
  return `[object ${name}${dict && `: `}${dict}]`
}

function showArr(src, vis) {
  const tar = []
  for (src of src) tar.push(show(src, vis))
  return `[` + tar.join(`, `) + `]`
}

function showDictOpt(src, vis) {
  const buf = []
  for (const key of Object.getOwnPropertySymbols(src)) {
    buf.push(showDictEntry(key, src[key], vis))
  }
  for (const key of Object.getOwnPropertyNames(src)) {
    buf.push(showDictEntry(key, src[key], vis))
  }
  const out = buf.join(`, `)
  return out && (`{` + out + `}`)
}

function showDictEntry(key, val, vis) {
  return showDictKey(key, vis) + `: ` + show(val, vis)
}

function showDictKey(val, vis) {
  if (isSym(val)) return `[` + show(val, vis) + `]`
  if (/^(?:\d+|\d+\.\d+|\d+n|[A-Za-z_$][\w$]*)$/.test(reqStr(val))) {
    return val
  }
  return show(val, vis)
}

/*
Like `val?.[key]` but with sanity checks: works only on composite values and
avoids accessing the property unless it satisfies the `in` check.
*/
export function get(val, key) {
  return isComp(val) && key in val ? val[key] : undefined
}

export function getOwn(val, key) {return hasOwn(val, key) ? val[key] : undefined}

export function reqGet(val, key) {
  if (isComp(val) && key in val) return val[key]
  throw errIn(val, key)
}

function getName(val) {return get(val, `name`)}
function getLength(val) {return get(val, `length`)}
function getSize(val) {return get(val, `size`)}
function getCon(val) {return get(val, `constructor`)}
function getTag(val) {return get(val, Symbol.toStringTag)}

// This is actually faster than default rendering.
function renderDate(val) {
  if (val.toString === Date.prototype.toString) return val.toISOString()
  return val.toString()
}

export function structKeys(val) {
  return isNil(val) ? [] : Object.keys(reqStruct(val))
}