state.hs

combine1 :: (a -> a) -> State [a] ()
combine1 op = do
    a <- pop
    push $ op a

combine2 :: (a -> a -> a) -> State [a] ()
combine2 op = do
    a <- pop
    b <- pop
    push op a b

combine3 :: (a -> a -> a -> a) -> State [a] ()
combine3 op = do
    a <- pop
    b <- pop
    c <- pop
    push $ op a b c

combine4 :: (a -> a -> a -> a -> a) -> State [a] ()
combine4 op = do
    a <- pop
    b <- pop
    c <- pop
    d <- pop
    push $ op a b c d