Is there a way, in template haskell, to find out what instance declarations exist? Not at the moment, I'm afraid, but it's the kind of question that TH ought to be able to answer. I could offer guidance if someone wanted to implement it.
I have a concern about how computed class memberships would work. Class membership is global, so creating a class instance is a globally visible side effect of TH code. Once you add the ability for TH code to inspect instances, the side effects mean that the order in which different splices get executed affects the compiler output. This is probably a bad thing. I'm not sure how it could be designed to work nicely; the only approach I can think of at the moment is to give splice evaluation a dataflow execution semantics that causes splices to execute and possibly reexecute at the "appropriate" times. I'd like to hear if anyone else has ideas on how to solve this problem. .....tian: I think that's a good point in general about conditional code generation. I wasn't thinking about conditional generation based on compile-time settings, but rather reusable TH library functions that inspect the class memberships of their arguments. Of course, this can be used for good or evil :) --heatsink