contrived.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeOperators #-}

import GHC.Base (Type)

data Nil = Nil

data a :<> b = a :<> b
infixr 8 :<>

class Score (a :: [Type]) where
  type Input a :: Type
  score :: Input a -> Int

instance forall rest a. (Score rest) => Score (a ': rest) where
  type Input (a ': rest) = Int :<> Input rest
  score (s :<> rest) = s + score @rest rest

instance Score '[] where
  type Input '[] = Nil
  score Nil = 0

-- λ  score @'[Int, Bool] (1 :<> 2 :<> Nil)
-- 3