Scrap_Your_Boilerplate_1

syb1.hs

{-# LANGUAGE DeriveDataTypeable #-}
module Main where
import Data.Generics
import Data.Data
import Data.Typeable

data Exp = Add Exp Exp | Sub Exp Exp | Val Int
  deriving (Show, Eq, Typeable, Data)

eval :: Exp -> Exp
eval = everywhere (mkT eval')
  where
    eval' (Add (Val n1) (Val n2)) = Val (n1 + n2)
    eval' (Sub (Val n1) (Val n2)) = Val (n1 - n2)
    eval' e = e


main :: IO ()
main = do
  putStrLn $ show (eval (Add (Val 1) (Val 2)))
  putStrLn $ show (eval (Add (Val 1) (Sub (Val 2) (Val 3))))