Arka, as you already mentioned, you want to have a look at the "Scrap your Boilerplate" approach.
import Data.Generics ... data Expr = Const Int | Var String | Add Expr Expr deriving (Typeable, Data)
will derive the necessary Data instance and allow you to define
optimizeDeep :: Data a => a -> a optimizeDeep = everywhere (mkT optimize)
On 7/1/05, Arka al Eel
Hi,
I'm playing with generic programming. At the moment I'm interested in reusable transformations on data types. Provided for example a toy datatype Expr like this:
data Expr = Const Int | Var String | Add Expr Expr
Plus a function "optimize" that optimizes a pattern "x + 0" into "x":
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)
with a sensible solution in an hour, while I can do this in two minutes in Scheme. After all, writing compilers is supposed to be a *strong* point of Haskell. Real world is knocking on your door, guys!
Thomas