Code from LambdaConf 2015 Type Kwan Do workshop

TKD.hs

{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE ScopedTypeVariables #-}

module TKD where

import Control.Applicative

befunge :: (x -> y) -> (y -> (w, z)) -> x -> w
befunge f g =
    fst . g . f

newtype Compose f g a =
    Compose { getCompose :: f (g a) }

instance (Functor f, Functor g) => Functor (Compose f g) where
    fmap :: (a -> b) -> Compose f g a -> Compose f g b
    fmap f (Compose fga) =
        Compose $ (fmap . fmap) f fga

instance (Applicative f, Applicative g) => Applicative (Compose f g) where
    pure :: a -> Compose f g a
    pure = Compose . pure . pure
    (<*>) :: Compose f g (a -> b) -> Compose f g a -> Compose f g b
    (<*>) (Compose fgab) (Compose fga) =
        Compose $ (fmap (<*>) fgab) <*> fga