Is there a way, in template haskell, to find out what instance declarations exist? I'm not trying to accomplish anything specific right now, but I can see two potential uses for this. One is to emit compile-time error messages from TH code which would be more informative than the typechecker's output. The other is to produce TH functions that conditionally generate code depending on the instances satisfied by their arguments. I offer the following as a motivational example. Suppose you want to write an extensible library to handle files for, e.g., image data: class FileFormat a where formatName :: a -> String read :: Handle -> IO a write :: Handle -> a -> IO () instance FileFormat TheStandardFormat where .... class (FileFormat a, FileFormat b) => Convert a b where convert :: a -> b Now it would be nice to generate instances of (Convert a TheStandardFormat, Convert TheStandardFormat b) => Convert a b _except_ where there is an explicit definition for (Convert a b). AFAIK, there's no way to do this at present, but it seems like something that could be computed in TH. --heatsink