Hi all, I have a problem which is probably not a problem at all for Haskell experts, but I am struggling with it nevertheless. I want to model the following situation. I have ASTs for a language in two variatns: A "simple" form and a "labelled" form, e.g. data SimpleExp = Num Int | Add SimpleExp SimpleExp data LabelledExp = LNum Int String | LAdd LabelledExp LabelledExp String I wonder what would be the best way to model this situation without repeating the structure of the AST. I tried it using a fixed point operator for types like this: data Exp e = Num Int | Add e e data Labelled a = L String a newtype Mu f = Mu (f (Mu f)) type SimpleExp = Mu Exp type LabelledExp = Mu Labelled Exp The "SimpleExp" definition works fine, but the LabeledExp definition doesn't because I would need something like "Mu (\a -> Labeled (Exp a))" where "\" is a type-level lambda. However, I don't know how to do this in Haskell. I'd need something like the "." operator on the type-level. I also wonder whether it is possible to curry type constructors. The icing on the cake would be if it would also be possible to have a function unlabel :: LabeledExp -> Exp that does *not* need to know about the full structure of expressions. So, what options do I have to address this problem in Haskell? Klaus