[Haskell-cafe] Need help with scrap-your-boilerplate

I'm a casual Haskell user, and am trying to use scrap-your-boilerplate to write a transformation - and failing. The rub is that the base function is polymorphic, boiling down to this: data Foo a = Foo a bar :: Foo a -> Foo a bar x = x Now, I'm trying to use SYB like so: foo :: Typeable a => a -> a foo = mkT bar ... but I get: Could not deduce (Typeable a0) arising from a use of ‘mkT’ from the context (Typeable a) bound by the type signature for foo :: Typeable a => a -> a at foo.hs:... The type variable ‘a0’ is ambiguous Note: there are several potential instances: Is there any way I could make this work? Any help would be much appreciated! -- Regards, Mike