pandora-0.4.0: Pandora/Paradigm/Primary/Functor/Endo.hs

module Pandora.Paradigm.Primary.Functor.Endo where

import Pandora.Pattern.Category (identity, (.), (#))
import Pandora.Pattern.Functor.Invariant (Invariant ((>-<)))
import Pandora.Pattern.Object.Semigroup (Semigroup ((+)))
import Pandora.Pattern.Object.Monoid (Monoid (zero))
import Pandora.Paradigm.Primary.Functor.Function ()

newtype Endo a = Endo { endo :: a -> a }

instance Invariant Endo where
	f >-< g = \(Endo x) -> Endo (f . x . g)

instance Semigroup (Endo a) where
	Endo f + Endo g = Endo # g . f

instance Monoid (Endo a) where
	zero = Endo identity