acme-cofunctor

A Cofunctor is a structure from category theory dual to Functor. A Functor is defined by the operation fmap: This means that its dual must be defined by the following operation: Since beginning his investigations, the author of this package has discovered that this pattern is at least as commonly used as Functor. In fact, many ubiquitous Haskell types (e.g. [], Maybe, ((->) a) turn out to have a Cofunctor instance.