View source for Context alias

=== Improvements ===

==== “Context alias” instead of “class alias” ====

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)

However if we allow class "aliases" to be extended with new methods then a class "alias" is not just a name for a context. (It is actually a context with a new class)

Maybe we should keep the syntax really light like:

 class Foobar a = (Foo a, Bar a)

==== Superclass constraints ====

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)

Or better:

 context Eq a => Num a = (Additive a, Multiplicative a)

==== Functional dependencies ====

Does the following make sense?

  class alias A a b = (B a b, C a b) | a -> b where ...

==== Associated data types and type synonyms ====

When <tt>{-# LANGUAGE TypeFamilies #-}</tt> is enabled, classes may declare [[GHC/Type_families|associated data types or associated type synonyms]].

If we allow class aliases to be extended with new methods, I think it make sense to also allow them to be extended with associated data types and type synonyms:

  class A a = (B a, C a) where
      type T a
      data D a

==== Equality constraints ====

When <tt>{-# LANGUAGE TypeFamilies #-}</tt> is enabled, type contexts can include [[GHC/Type_families#Equality_constraints|equality constraints]] (<tt>t1 ~ t2</tt>).

It makes sense to also allow them in class aliases (context aliases)
     
==== Things to have in mind ====

In order to get the context alias extension well, we should have an eye on problems we might want to solve with the help of context aliases. Here are some:

* <hask>MonadPlus</hask> should just be a combination of <hask>Alternative</hask> and <hask>Monad</hask> (actually, <hask>Alternative f</hask> should just be a combination of <hask>Applicative f</hask> and <hask>forall a. Monoid (f a)</hask>)

* <hask>Applicative</hask> should be a superclass of <hask>Monad</hask>