mainbak03.hs

module Mainbak03 where
import Stack

type StackOp a b = Stack a -> (b, Stack a)

-- comb :: StackOp a b -> (b -> StackOp a b) -> StackOp a b -- これは間違い
comb :: StackOp a b -> (b -> StackOp a c) -> StackOp a c
comb m n = \stack0 ->
           let (x1, stack1) = m stack0
               (x2, stack2) = n x1 stack1
           in  (x2, stack2)

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

-- comb_ :: StackOp a b -> StackOp a b -> StackOp a b -- これは間違い
comb_ :: StackOp a b -> StackOp a c -> StackOp a c
comb_ m n = m `comb` (\_ -> n)


topis :: (a -> Bool) -> Stack a -> (Bool, Stack a)
topis p s = let (a, s') = pop s
            in  (p a, s')

main = do
  print $ topis (> 4) `comb` (\x1 -> 
          topis (> 3) `comb`  \x2 ->
          topis (> 2) `comb`  \x3 ->
          ret $ and [x1,x2,x3]) $ s

  print $ pop `comb` push' $ s
  print $ push' 10 `comb_` pop $ s
  print $ push' 10 `comb_` push' 9 `comb_` push' 8 $ s

  print $ empty' $ s
  print $ push' 10 `comb_` empty' `comb_` push' 0 `comb_` push' 1 $ s