Context aliases, also known as class aliases, are a long-requested feature of Haskell. This feature would allow class hierarchies to be restructured without breaking compatibility to a certain degree. Also, it would make fine-grained class hierarchies usable.
1 The proposal
1.1 The original class alias proposal
The original proposal can be found on a page on John Meachem’s website.
A “class alias” actually doesn’t stand for a class but for a context (or a part of a context). So it might be better to choose a slightly different syntax:
context Foobar a = (Foo a, Bar a)
John Meacham proposes the following syntax for class aliases (context aliases) with superclass constraints:
class alias Num a = Eq a => (Additive a, Multiplicative a)
This is not consistent with the superclass syntax of class declarations. I think, we should use this syntax:
class alias Eq a => Num a = (Additive a, Multiplicative a)
context Eq a => Num a = (Additive a, Multiplicative a)
Starting an implementation of context aliases is planned for the 5th Haskell Hackathon.