Reaktor dev day Monads workshop exercise

gistfile1.hs

-- #!/usr/bin/env runhaskell
import Control.Applicative
import Control.Monad

plus3 x = x + 3

data List a = Nil | Cons a (List a)
	deriving (Show)

instance Functor List where 
	fmap _ Nil = Nil
	fmap f (Cons x xs) = (Cons (f x) (fmap f xs))

--instance Monad Maybe where
--	Nothing >>= f = Nothing
--	Just a >>= f = f a

data Try a = Ok a | Error String
	deriving (Show)

half x = if even x
	then Ok (x `div` 2)
	else Error "impossiburu"

instance Monad Try where
	Error x >>= f = Error x
	Ok x >>= f = f x
	return x = Ok x

instance Functor Try where
	fmap = liftM

instance Applicative Try where
	(<*>) = ap
	pure = return

-- try these out:
-- fmap (plus3) (Ok 2)
-- (+) <$> Ok 3 <*> Ok 5