mainbak01.hs

module Mainbak01 where
import Stack

comb ::       (Stack a -> (a, Stack a)) 
     -> (a -> (Stack a -> (a, Stack a)))
     ->        Stack a -> (a, Stack a)
comb m n = \stack0 ->
           let (x1, stack1) = m stack0
               (x2, stack2) = n x1 stack1
           in  (x2, stack2)

ret :: a -> (Stack a -> (a, Stack a))
ret x = \stack -> (x, stack)

comb_ :: (Stack a -> (a, Stack a)) 
      -> (Stack a -> (a, Stack a))
      ->  Stack a -> (a, Stack a)
comb_ m n = m `comb` (\_ -> n)

main = do 
  print $ pop `comb` (\x1 -> pop) $ s

  print $ pop `comb` (\x1 -> pop `comb` \x2 -> ret(x1 + x2)) $ s

  print $ pop `comb` (\x1 -> 
          pop `comb`  \x2 -> 
          pop `comb`  \x3 -> 
          ret $ (x1 + x3) * x2) $ s