idmonad.hs

data Id a = Id a
  deriving (Show)

instance Functor Id where
  fmap f (Id a) = Id $ f a

instance Applicative Id where
  pure = Id
  (Id f) <*> (Id a) = Id (f a)

instance Monad Id where
  return = Id
  (Id a) >>= f = f a
  a >> f = f

thirty :: Id Int

thirty = do
  x <- Id 5
  y <- Id 7
  return (x * y)

main = print thirty