Dear TH users In Template Haskell, it's not currently possible to say "give me all the instances of this class"; it's a long-standing feature request. Sam Anklesaria has come up with a design and implemented it. Before committing to it, I want to check that other Template Haskell users are happy with the design. You can find the details here: http://hackage.haskell.org/trac/ghc/ticket/1835 But a summary of the design appears below. If you have a stake in Template Haskell, pls RSVP by adding a comment to the ticket. Thanks Simon ================ Proposed design * Extending the Quasi monad to have class (Monad m, Functor m) => Quasi m where ... qReify :: Name -> m Info qIsClassInstance :: Name -> [Type] -> m Bool isClassInstance :: Name -> [Type] -> Q Bool isClassInstance n ts = Q (qIsClassInstance n ts) * Extending the info you get back when reifying a Class, by adding its instances: data Dec = ... | ClassI Dec [([TyVarBndr],Cxt,[Type])] Some random thoughts * In principle, I suppose that isClassInstance isn't really necessary: you could reify the class and search the list yourself. But I can see that it's convenient. * Currently isClassInstance returns True if any instances match, even if they overlap etc. Might you want to know more?