Add Compositor class as superclass of Arrow, in Control.Compositor

[guest]

The Compositor class has two members:

class Compositor comp where
  identity :: comp a a
  (>>>) :: comp a b -> comp b c -> comp a c

with the obvious monoid. A number of useful types are Compositors but not Arrows.

[guest]

darcs patch to base

[guest]

Consensus was to add this under the name Category:

class Category cat where
  id :: cat a a
  (.) :: cat b c -> cat a b -> cat a c

id and (.) would not replace those in the Prelude. Category would become a superclass of the existing Arrow class.

[guest]

updated patch

[desegnis]

Is the question allowed how packages relying on the old Arrow interface should handle the changing situation? Will they be #ifdef'ing on the version of base?