Implementing the Trivial Monad in haskell, including Applicative and Functor instances (required by recent GHC versions)

TrivialMonad.hs

module TrivialMonad where

-- See also http://hackage.haskell.org/package/transformers-0.4.1.0/docs/src/Data-Functor-Identity.html#Identity

import Control.Applicative (Applicative(..)) -- import type and all exported constructors using '(..)'
-- How is that different from "import Control.Applicative"
import Control.Monad       (liftM, ap)

data Trivial a = Trivial a deriving (Show) -- wrapper type

instance Monad Trivial where  
    return = Trivial
    Trivial x >>= f = f x

-- To avoid compiler warnings about GHC 7.10 onwards,
-- derive functor and applicative from monad as per
-- http://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal

instance Functor Trivial where
    fmap = liftM
 
instance Applicative Trivial where
    pure  = return
    (<*>) = ap

-- Various articles stress the fact that there's no universal function to extract values from 
-- a monad; yet many of the individual instances have just such a function (runWriter etc)
runTrivial :: Trivial a -> a
runTrivial (Trivial x) = x

main :: IO ()
main = putStrLn . runTrivial $ Trivial "Hello" >>= Trivial . reverse

-- See also http://www.haskell.org/haskellwiki/All_About_Monads#No_way_out

-- But in this case we can just pattern match to extract the value:
main2 :: IO ()
main2 = do 
    let Trivial t = Trivial "Hello" >>= Trivial . reverse
    putStrLn t