Pattern.hs

{-# LANGUAGE StrictData #-}

-- | Common pattern functors (and instances for them).
module Yaya.Pattern where

import Data.Bifunctor

-- | Isomorphic to 'Maybe (a, b)', it’s also the pattern functor for lists.
data XNor a b = Neither | Both ~a b deriving (Functor, Foldable, Traversable)

instance Bifunctor XNor where
  bimap f g = \case
    Neither  -> Neither
    Both a b -> Both (f a) (g b)

-- | Isomorphic to `(a, Maybe b)`, it’s also the pattern functor for non-empty
--   lists.
data AndMaybe a b = Only a | Indeed ~a b
  deriving (Functor, Foldable, Traversable)

instance Bifunctor AndMaybe where
  bimap f g = \case
    Only a -> Only (f a)
    Indeed a b -> Indeed (f a) (g b)