FiveMinuteFP.hs

module FiveMinuteFP where

data Option a = Nope | Yup a deriving (Eq, Show)

-- try to make a number odd by adding 1 to it
oddify1 :: Int -> Option Int
oddify1 x =
  case (x `mod` 2) of
    0 -> Yup (x + 1)
    _ -> Nope

-- try to make a number odd by adding 2 to it
oddify2 :: Int -> Option Int
oddify2 x =
  case (oddify1 x) of
    Nope -> Yup (x + 2)
    (Yup _) -> Nope

instance Functor Option where
  fmap f Nope = Nope
  fmap f (Yup a) = Yup (f a)

instance Applicative Option where
  pure a = Yup a
  (<*>) Nope _ = Nope
  (<*>) (Yup f) Nope = Nope
  (<*>) (Yup f) (Yup a) = Yup (f a)

instance Monad Option where
  return = pure
  (>>=) Nope f = Nope
  (>>=) (Yup a) f = f a

-- fmap examples

(+1) <$> Yup 3

-- fails since the function ends up in the context; need the applicative
(+) <$> Yup 3 <$> Yup 4

-- chaining with more structure

oddify1 <$> oddify1 2

oddify1 2 >>= oddify2

oddify1 2 >>= oddify2 >>= oddify2 >>= oddify2 >>= oddify2 >>= oddify2 >>= oddify2

-- applicative examples

:t (+) <$> Yup 3

(+) <$> Yup 3 <*> Yup 4