[Haskell-cafe] rewrite rules to specialize function according to type class?

[Max Bolingbroke]

On 14 February 2011 21:43, Patrick Bahr <pa-ba at arcor.de> wrote:
> Am I doing something wrong or is it not possible for GHC to dispatch a rule
> according to type class constraints?

As you have discovered this is not possible. You can write the rule
for as many *particular* types as you like, but you can't write it in
a way that abstracts over the exact type class instance you mean. This
is a well known and somewhat tiresome issue.

I think the reason that this is not implemented is because it would
require the rule matcher to call back into the type checking machinery
to do instance lookup.

Cheers,
Max