[Haskell-cafe] Re: Variants of a recursive data structure

Klaus Ostermann schrieb:

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.

How about the following simple parameterization? data Exp label = LNum Int label | LAdd (Exp label) (Exp label) label type SimpleExp = Exp () mkNum i = LNum i () type LabelledExp = Exp String Cheers Christian