Here are complete working snippets with Functor, Applicative and Monad instances. This is my first attempt to write own instances rather than use ready ones. Here we go: module Part.Monad where import Control.Applicative {-# ANN module ("HLint: ignore Use let"::String) #-} {- Val' { val = b1 } can also be written as Val' b1 -} data Val a = Val' { val::a } deriving Show data Weather = Cold | Warm deriving Show instance Functor Val where -- (a -> b) -> f a -> f b fmap ab fa = let a1 = val fa b1 = ab a1 in Val' { val = b1 } instance Applicative Val where -- a -> f a pure a = Val' { val = a } -- f (a -> b) -> f a -> f b (<*>) fab fa = let ab1 = val fab a1 = val fa b1 = ab1 a1 in Val' { val = b1 } instance Monad Val where -- a -> m a return a = Val' { val = a } -- m a -> (a -> m b) -> m b (>>=) ma amb = let a1 = val ma in amb a1 -- pure and return in this example are interchangeable main::Int -> IO() main i = do -- do: Val as monad v1 <- pure Val' { val = i } -- pure: applicative v2 <- return $ over20 <$> v1 -- <$> : functor print v2 v3 <- return $ Val' weather <*> v2 -- <*> : applicative print v3 over20::Int-> Bool over20 i | i > 20 = True | otherwise = False weather::Bool-> Weather weather False = Cold weather True = Warm