polish.hs

module Main( main ) where

data PolishElement = PolishPlus | PolishNumber Integer deriving (Show)

polish :: [PolishElement] -> (Integer, [PolishElement])
polish (PolishNumber x : tail) = (x, tail)
polish (PolishPlus : tail) = (x + y, tail'')
  where
    (x, tail') = polish tail
    (y, tail'') = polish tail'

reversePolish :: [PolishElement] -> [Integer] -> [Integer]
reversePolish [] result = result -- when there's no more input, the result is the stack
reversePolish (PolishNumber x : tail) stack = reversePolish tail (x : stack)
reversePolish (PolishPlus : tail) (x: (y: stack)) = reversePolish tail (x + y : stack)

main :: IO ()
main = do
  let instructions = [PolishPlus, PolishNumber 10, PolishPlus, PolishNumber 3, PolishNumber 4]
  print $ polish instructions
  print $ reversePolish (reverse instructions) []