Reader Stringに相当するモナドを自作する例

env.hs

import Control.Applicative

type Env = String
newtype F a = F (Env -> a)

onEnv :: (Env -> a) -> F a
onEnv = F

runOnEnv :: F a -> Env -> a
runOnEnv (F f) = f 

instance Functor F where
    fmap f x = onEnv $ \env -> f (x `runOnEnv` env)

instance Applicative F where
    -- pure :: a -> F a
    pure x = onEnv $ \env -> x
    
    -- (<*>) :: F (a -> b) -> F a -> F b
    f <*> a = onEnv $ \env -> (f `runOnEnv` env) (a `runOnEnv` env)

instance Monad F where
    -- return :: a -> F a
    return = pure
    -- (>>=) :: F a -> (a -> F b) -> F b
    m >>= k = onEnv $ \env -> k (runOnEnv m env) `runOnEnv` env

count char = sum . map (fromEnum . (char==))

main = print $ runOnEnv ex "Hey, I'm an environment!" where
    ex = do
        n <- onEnv $ length
        m <- onEnv $ count 'e'
        p <- onEnv $ count ' '
        return (n + m, n * m, p)