pandora-0.4.9: Pandora/Pattern/Functor/Invariant.hs

module Pandora.Pattern.Functor.Invariant where

{- |
> When providing a new instance, you should ensure it satisfies:
> Identity morphisms: invmap identity identity = identity
> Interpreted of morphisms: invmap g j . invmap f h = invmap (g . f) (h . j)
-}

infixl 4 <!<

class Invariant (t :: * -> *) where
	{-# MINIMAL (<!<) #-}
	(<!<) :: (a -> b) -> (b -> a) -> t a -> t b
	-- | Prefix version of '<!<'
	invmap :: (a -> b) -> (b -> a) -> t a -> t b
	invmap f g x = (f <!< g) x