-- Hutton's Razor in CPS
import Control.Monad.Cont

data Expr = Val Int | Add Expr Expr | TVal Bool | Ite Expr Expr Expr

type Stack = [Int]
type Con = Cont Stack Stack

add :: Stack -> Stack
add (m : n : s) = (m + n) : s
add _ = error "oops"

ite :: Stack -> Stack
ite (0 : _ : e : s) = e : s
ite (1 : e : _ : s) = e : s
ite _ = error "gee whiz"

eval :: Expr -> Con -> Con
eval (Val n) c      = (n :) <$> c
eval (TVal True) c  = (1 :) <$> c
eval (TVal False) c = (0 :) <$> c
eval (Add x y) c    = add <$> eval x (eval y c)
eval (Ite b x y) c  = ite <$> eval b (eval x (eval y c))