lab11-reader.hs

---Monada Reader

newtype Reader env a = Reader {runReader :: env -> a}

instance Monad (Reader env) where
  return x = Reader (\_ -> x)
  ma >>= k = Reader f
    where
      f env =
        let a = runReader ma env
         in runReader (k a) env

instance Applicative (Reader env) where
  pure = return
  mf <*> ma = do
    f <- mf
    a <- ma
    return (f a)

instance Functor (Reader env) where
  fmap f ma = pure f <*> ma

ask :: Reader env env
ask = Reader id

local :: (r -> r) -> Reader r a -> Reader r a
local f ma = Reader $ (\r -> (runReader ma) (f r))

-- Reader Person String

data Person = Person {name :: String, age :: Int}

-- Câteva persoane date ca exemplu
persoana1 :: Person
persoana1 = Person "ada" 20
persoana2 :: Person
persoana2 = Person "ion" 19

showPersonN :: Person -> String
showPersonN p = "NAME:" ++ name p

showPersonA :: Person -> String
showPersonA p = "AGE:" ++ show (age p)

showPerson :: Person -> String
showPerson p = "(" ++ showPersonN p ++ "," ++ showPersonA p ++ ")"

mshowPersonN :: Reader Person String
mshowPersonN = Reader showPersonN

mshowPersonA :: Reader Person String
mshowPersonA = Reader showPersonA

mshowPerson :: Reader Person String
mshowPerson = Reader showPerson