Re: [Haskell-cafe] Is there already an abstraction for this?

Hi Jeremy, There are some approaches that support such generic transformations. The simplest is probably Uniplate by Neil Mitchell: http://www-users.cs.york.ac.uk/~ndm/uniplate/ The function 'rewrite' is what you are looking for. If you change the definition of 'identity' to:

identity (Sum (Lit 0) a) = Just a identity (Sum a (Lit 0)) = Just a identity (Difference a (Lit 0)) = Just a identity (Product a (Lit 1)) = Just a identity (Product (Lit 1) a) = Just a identity (Quotient a (Lit 1)) = Just a identity _ = Nothing

then the function 'rewrite identity :: Expr -> Expr' does what you want. Cheers, Sebastian