PreList.hs

module Main where

import Data.Functor.Constant
import Data.Bifunctor

data Riap b a = Riap a b

-- data PreList a b
--   = Nil
--   | PreList a b
newtype PreList a b = PreList (Either (Constant () a) (Riap b a))

instance Functor f => Functor (Riap (f b)) where
  fmap f (Riap a b) = Riap (f a) (fmap f b)

instance Bifunctor PreList where
  bimap f g (PreList (Left c)) = PreList (Left (fmap f c))
  bimap f g (PreList (Right op)) = PreList (Right (fmap g op))

main :: IO ()
main = undefined

更正， PreList a b 不用是個 Functor ：

module Main where

import Data.Functor.Constant
import Data.Bifunctor

-- data PreList a b
--   = Nil
--   | PreList a b
newtype PreList a b = PreList (Either (Constant () a) (a, b))

instance Bifunctor PreList where
  bimap f g (PreList (Left c)) = PreList (Left (fmap f c))
  bimap f g (PreList (Right (a, b))) = PreList (Right ((f a), (g b)))

main :: IO ()
main = undefined