stack machine

eval.hs

data Expr = Val Int | Add Expr Expr
data Op = EVAL Expr | ADD Int

value :: Expr -> Int
value e = eval e []

eval :: Expr -> [Op] -> Int
eval (Val n) stack           = exec stack n
eval (Add lexpr rexpr) stack = eval lexpr ((EVAL rexpr):stack)

exec :: [Op] -> Int -> Int
exec [] n             = n
exec (EVAL y:stail) n = eval y (ADD n:stail)
exec (ADD n:stail) m  = exec stail (n+m)

main :: IO ()
main = print $ value (Add ( Add (Val 2) (Val 3)) (Val 4))