Educational Reimplementation of the Maybe Monad

MyMaybe.hs

-- | Re-implementing the Maybe monad from memory, to make sure I
-- understand it.
module MyMaybe where

import Prelude hiding (Maybe, Just, Nothing)

data MyMaybe a = MyNothing | MyJust a
    deriving (Eq, Show)

instance Monad MyMaybe where
    MyNothing >>= f     = MyNothing·
    MyJust v  >>= f     = f v

    return x            = MyJust x

instance Functor MyMaybe where
    fmap f MyNothing  = MyNothing
    fmap f (MyJust x) = MyJust $ f x


{-
*MyMaybe> MyJust 5 >>= \x -> return $ x * 2
MyJust 10
it :: MyMaybe Integer

*MyMaybe> do { x <- MyJust 5; return $ x * 2 }
MyJust 10
*MyMaybe> do { x <- MyNothing; return $ x * 2 }
MyNothing

*MyMaybe> (*2) `fmap` (MyJust 5)
MyJust 10
-}
~