TypedCompose.hs

-- Direct translation of https://gist.github.com/rjmk/d28a706f9b1fa2ac30a5fcd7d767b334

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeOperators #-}
module TypedCompose where

import GHC.TypeLits

infixr 4 :.:

data Compose (n :: Nat) a b where
    Nil :: Compose 0 a a
    (:.:) :: (a -> b) -> Compose n c a -> Compose (n + 1) c b

typedCompose :: Compose n a b -> a -> b
typedCompose Nil = id
typedCompose (f :.: compose) = f . typedCompose compose

foo :: Int -> Int
foo = typedCompose ((+ 3) :.: (+ 4) :.: (* 6) :.: Nil)

main :: IO ()
main = putStrLn . show $ foo 5

TypedComposeSimpler.hs

-- Slightly simpler translation of https://gist.github.com/rjmk/d28a706f9b1fa2ac30a5fcd7d767b334

{-# LANGUAGE GADTs #-}
module TypedComposeSimpler where

infixr 4 :.:

data Compose a b where
    Nil :: Compose a a
    (:.:) :: (a -> b) -> Compose c a -> Compose c b

typedCompose :: Compose a b -> a -> b
typedCompose Nil = id
typedCompose (f :.: compose) = f . typedCompose compose

foo :: Int -> Int
foo = typedCompose ((+ 3) :.: (+ 4) :.: (* 6) :.: Nil)

bar :: Int -> String
bar = typedCompose (show :.: (+ 3) :.: (+ 4) :.: (* 6) :.: Nil)

main :: IO ()
main = do
    print $ foo 5
    print $ bar 5