https://twitter.com/sjoerd_visscher/status/1686714359135846400

O.hs

{-# LANGUAGE RankNTypes, GADTs #-}
module O where

data Coyoneda g a where
  Coyoneda :: forall g a x. g x -> (x -> a) -> Coyoneda g a

newtype Obj f g = Obj { unObj :: forall a. f a -> Coyoneda g (a, Obj f g) }

compose :: Obj f g -> Obj g h -> Obj f h
compose (Obj a) (Obj b) = Obj (\f ->
  case a f of
    Coyoneda g k -> case b g of
      Coyoneda h l -> Coyoneda h (kcompose l k))

kcompose :: (y -> (x, Obj g h)) -> (x -> (z, Obj f g)) -> (y -> (z, Obj f h))
kcompose l k y =
  let (x, b) = l y
      (z, a) = k x in (z, compose a b)