I want to write generic traversal which prints everything on the way: uglyPrint :: (Term t, Show t) => t -> [(String)] uglyPrint = (map snd) . runIdentity . applyTU (full_tdTU uglyPrintStep) uglyPrintStep :: (Show t, Term t) => TU [(t, String)] Identity uglyPrintStep = constTU [] `adhocTU` (return . uglyPrintAny) uglyPrintAny x = [(x,show x)] ugliestPrintEver :: (Term t, Show t) => t -> IO () ugliestPrintEver x = do { putStrLn $ show x } Compiler (GHC 6.0) gives me: Ambiguous type variable `t' in these top-level constraints: `Term t' arising from use of `uglyPrintStep' at ... `Show t' arising from use of `uglyPrintStep' at ... All data types which are instances of Term are instances of Show as well - I know it. Question is - how to persuade GHC? I there any want to use typeclass restrictions with traversal, or there is no luck for me? -- Dmitry Astapov //ADEpt GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D