gistfile1.hs

module Main where

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

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

class Main a where

  replaceConst :: a -> a 

instance Main Stmt where

  replaceConst (Let lhs rhs) = Let (replaceConst lhs) (replaceConst rhs)
  replaceConst (If cond thenE elseE) = If (replaceConst cond) (replaceConst thenE) (replaceConst elseE)

instance Main Expr where

  replaceConst (Const i)      = Var (show i)
  replaceConst (Add lhs rhs)  = Add (replaceConst lhs) (replaceConst rhs)
  replaceConst expr           = expr


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

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


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