I'd like to create a Template Haskell function such that:
$(isInstanceOf ''Read ''SomeType)
will result in either True if SomeType is an instance of Read, and False otherwise.
I tried to look at the result of reify and I think I'm looking for the contents of the ClassI constructor, but the documentation is somewhat lacking and I'm having trouble deciphering what I need. Can someone provide guidance on where to look to find the data needed to create the above function?
Template Haskell already provides a function that does almost what you want. It's there as of version 2.5, and prior to that I'm not aware of any means to look up instances at all.
The difference is that the existing isClassInstance function takes a Name for the class--which is what you get from something like ''Read--but a Type to look for instances with. This probably makes more sense, because with a Name there's no obvious way to check for instances that require type parameters. For example, you wouldn't be able to check directly whether [Int] has a Show instance, or whether Either String is a Monad instance.
Note that a Type can be constructed almost as easily as a Name using a quotation, e.g. you could write something like $(isInstanceOf ''Monad [t| Either String |]).
Given the above, all you'd have to do is a bit of juggling to return a useful value from the splice, whatever you want that to be.
