Hello. I have two types for expression: data Expr = Add Expr Expr | Mul Expr Expr | Const Int data AExpr = AAdd AExpr AExpr | AConst Int The first one supports addition and multiplication and the second only addition. I can write a function to simplify the first expression: simplify :: Expr -> Expr simplify = {- replaces: "a*1" and "1*a" by "a", "a+0" and "0+a" by a -} And I would like to use the function simplify for the second type AExpr. What can I do is to convert AExpr to Expr, simplify it and convert it back. But I don't like this solution because conversions take some time. I would prefer following: I say to the compiler that AAdd is like Add and AConst is like Const and the compiler derives function asimplify for AExpr. Is it possible to do this? In fact I want to have two distinct types where one is "extension" of the second (Expr is extension of AExpr) and I want to define a function for the extended type (Expr) and use it for the original type (AExpr). I assume that the function won't introduce Mul to the expression which had no Mul. Thanks in advance Radek Micek