An implementation of the Either monad in Haskell

Either.hs

module Either where

import Prelude hiding (Either, Left, Right)

data Either failure success = Left failure | Right success
    deriving (Show, Read, Eq)

instance Functor (Either failure) where
    fmap _ (Left x)  = Left x
    fmap f (Right x) = Right (f x)

instance Applicative (Either failure) where
    pure                = Right
    Right f <*> Right x = Right (f x)
    _ <*> Left x        = Left x

instance Monad (Either failure) where
    return        = Right
    Right x >>= f = f x
    Left x  >>= _ = Left x

-- Example usage: validating posts

data Post = Post
    { userID   :: Int
    , title    :: String
    , contents :: String
    } deriving (Show, Read, Eq)

hasTitle :: Post -> Either String Post
hasTitle post
    | null (title post) = Left "post must have a title"
    | otherwise         = Right post

hasContents :: Post -> Either String Post
hasContents post
    | length (contents post) <= 160 = Right post
    | otherwise                     = Left "post must be less than 160 characters"

validatePost :: Post -> Either String Post
validatePost post = hasTitle post >>= hasContents

main :: IO ()
main = do
    let posts = [ Post 1 "First post!" "Hi everyone, this is my first post!"
                , Post 2 "" "Whoops, I forgot a title"
                , Post 3 "Here's a title" "A really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, long post."
                ]
    mapM_ (print . validatePost) posts