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/ ---
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