data Expr = Const Int | Var String | Add Expr Expr
optimize (Add x (Const 0)) = x
You would now want this to be this generic, so the function should be recursive for all other constructors *and* other data types. For example, suppose that Expr is included in other datatype:
data Stm = Assign String Expr | Seq Stm Stm
I now want the "optimize" transformation to work on Stm, like this:
x = optimize (Seq (Assign (Add (Var "x") (Const 0))) blah blah)
For sure, I don't want to write specific code for type Stm. The thing I want is to generically walk through values of those types into Expr where it can apply the pattern.
A one minute solution using the "Scrap your boilerplate" approach is: {-# OPTIONS -fglasgow-exts #-} import Data.Generics data Expr = Const Int | Var String | Add Expr Expr deriving ( Data , Typeable , Show ) optimize :: Expr -> Expr optimize (Add x (Const 0)) = x optimize (Add (Const 0) x) = x optimize x = x data Stmt = Assign String Expr | Seq Stmt Stmt deriving ( Data , Typeable , Show ) stmt :: Stmt stmt = Seq (Assign "y" (Add (Var "x") (Const 0))) (Assign "z" (Add (Const 0) (Var "y"))) Now: *Main> everywhere (mkT optimize) stmt Seq (Assign "y" (Var "x")) (Assign "z" (Var "y")) as wanted. The work to do is to just import the Data.Genrics module and to add the deriving (Data,Typeable) clausel to the data definitions. I think this is easy. The SYB3 paper also describes how to implement functions that are extensible with new type-specific cases. Have fun, -- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ ---