On 5/20/06, Bas van Dijk <basvandijk@home.nl> wrote:
How can I make this work?
As far as I know, you can't. To see the problem, rewrite it using dictionaries: data Simplify a = Simplify { simplify :: a -> a } simplify_HsExp (HsInfixApp e1 op e2) = HsApp (HsApp (opToExp op) e1) e2 simplify_HsExp (HsLeftSection e op)= HsApp (opToExp op) e simplify_HsExp (HsRightSection op e) = HsApp (opToExp op) e simplify_HsExp (HsParen e) = e simplify_HsExp e = e Simplify_HsExp = Simplify simplify_HsExp etc. You will end up with a bunch of Simplify_* objects. Now in the application: preProcess = everywhere (mkT simplify) how do you convert this? If you use (mkT (simplify ...)), you're fixing simplify to only one type. If you use (mkT simplify), you get a type error. GHC is telling you that you need to tell it which instance of simplify to use. In your example, you can use: everywhere (mkT (simplify :: HsExp -> HsExp)) . everywhere (mkT (simplify :: HsPat -> HsPat)) . everywhere (mkT (simplify :: HsRhs -> HsRhs)) Of course, that's not any simpler. -- Taral <taralx@gmail.com> "You can't prove anything." -- Gödel's Incompetence Theorem