nice Haskell Type - Riddle

Riddle.hs

{-# LANGUAGE RankNTypes, DeriveFunctor #-}

module Riddle where

type T a b c = forall f . Functor f => (a -> f b) -> f c

to :: (a, b -> c) -> T a b c
to (a, b_c) f = fmap b_c (f a)


from :: T a b c -> (a, b -> c)
from f = (a, b_c)
    where
        a   = unconst $ f Const
        -- ignore `a` and use the functor `b -> _`
        -- to tease out a `b -> c` from (\_ b -> b)
        b_c = f (const id)


newtype Const c a = Const { unconst :: c }
    deriving Functor

straight from this tweet: https://twitter.com/Iceland_jack/status/1014745561642172417