gistfile1.hs

{-# LANGUAGE DeriveDataTypeable #-}

module Main where

import Data.Generics

data Expr =
    Const Int
  | Var String
  | Add Expr Expr
  deriving (Show, Eq, Typeable, Data)

data Stmt =
    Let Expr Expr
  | If Expr Expr Expr
  deriving (Show, Eq, Typeable, Data)

replaceConst :: Expr -> Expr
replaceConst (Const i)      = Var (show i)
replaceConst (Add lhs rhs)  = Add (replaceConst lhs) (replaceConst rhs)
replaceConst expr = expr

trans :: Stmt -> Stmt
trans = everywhere (mkT replaceConst)

prog = Let (Var "x") (Add (Const 3) (Var "y"))

main = do putStrLn $ show prog
          putStrLn $ show (trans prog)

$ runhaskell Main.hs
Let (Var "x") (Add (Const 3) (Var "y"))
Let (Var "x") (Add (Var "3") (Var "y"))