> module Data.Cofunctor ( > Cofunctor(..) > ) where > > {- | > /DEPRECATED: please use the "contravariant" package/ > > The Confunctor class is useful for \"sink-like\" types, especially when > combined with GADTs. For instance: > > > {-# LANGUAGE GADTs #-} > > data StringStack a where > > StringStack :: (a -> String) -> [String] -> StringStack a > > > > emptyStack :: StringStack String > > emptyStack = StringStack id [] > > > > push :: a -> StringStack a -> StringStack a > > push a (StringStack f ss) = StringStack f (f s : ss) > > > > instance Confunctor StringStack where > > cofmap f (StringStack g ss) = StringStack (g . f) ss > > See the "split-chan" package for another example. > -}Hopefully we'll have some more interesting supporting functions, classes and instances here that will make this more useful and perhaps express some relations with Functors.

> -- | A class for contravariant functors > class Cofunctor f where > cofmap :: (b -> a) -> f a -> f bTo Consider: - lift product type from Data.Functor.Product to sum type (Either) - make instance for wrapped Category with flipped vars - ...