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

On Tue, Feb 15, 2011 at 12:48 AM, Max Bolingbroke wrote:

On 14 February 2011 21:43, Patrick Bahr 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.

This is a semi-related question I've been meaning to ask at some point: I suppose this also means it's not possible to write a class, write some rules for the class, and then have the rules be applied to every instance? (I.e. you'd have to write them separately for each?)

Cheers, Max

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