Reader, Haskell - rextester

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--ghc 7.10
import Data.Char

main = do
  print $ composed "hello"
  print $ fmapped "world"
  print $ tupled "venice"
  print $ tupled' "florence"
  print $ tupled'' "berlin"

cap :: [Char] -> [Char]
cap xs = map toUpper xs

rev :: [Char] -> [Char]
rev xs = reverse xs

composed = cap . rev

fmapped = fmap cap rev

tupled = (,) <$> cap <*> rev

tupled' = do
  x <- cap
  y <- rev
  return (x, y)

tupled'' =
  cap >>= \x -> rev >>= \y -> return (x, y)

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

instance Functor (Reader r) where
  fmap f (Reader ra) =
    Reader $ f . ra

instance Applicative (Reader r) where
  pure x =
    Reader $ const x

(Reader f) <*> (Reader ra) =
    Reader $ \r -> (f r) (ra r)

myLiftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
myLiftA2 f fa fb =
  f <$> fa <*> fb

asks :: (r -> a) -> Reader r a
asks f =
  Reader f

instance Monad (Reader r) where
  return = pure
  (Reader ra) >>= arb =
    Reader $ \r -> runReader (arb (ra r)) r

-- getDogRM = liftM2 Dog dogName address

Show compiler warnings [ + ] Compiler args [ + ] Show input
Compilation time: 0.52 sec, absolute running time: 0.14 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 0,67 sec