RE: inferred type doesn't type-check (using type families)

| I think (untested) that in this particular case you can get around the | problem using scoped type variables: | | > g :: forall a. Num a => T a -> T a | > g x = f x (42 :: a) | | In fact, this seems to be the general pattern for fixing problems like | this with type families: add extra "witness" arguments which GHC can | use to unify type variables that are "hidden" inside type family | applications. But in this case all you've done is to make g have an ambiguous type. I've drafted a FAQ about this at http://haskell.org/haskellwiki/GHC/Type_families#Frequently_asked_questions Please, everyone, edit and extend. Simon