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
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
