haskell monad porland

porland.hs

import Control.Monad
import Data.List
import Test.HUnit

readMaybe :: (Read a) => String -> Maybe a
readMaybe st = case reads st of [(x, "")] -> Just x
                                _ -> Nothing

foldingFunction :: [Double] -> String -> Maybe [Double]
foldingFunction (x:y:ys) "*" = return $ (y * x) : ys
foldingFunction (x:y:ys) "+" = return $ (y + x) : ys
foldingFunction (x:y:ys) "-" = return $ (y - x) : ys
foldingFunction xs numberString = liftM (:xs) (readMaybe numberString)


tests1 :: Test
tests1 =
  let stack = [1.0, 2.0]
      t = foldingFunction
  in
   "foldinfFunction" ~:
   ["test1" ~: (t stack "*") ~?= Just [2.0],
   "test2" ~: (t stack "+") ~?= Just [3.0],
   "test3" ~: (t stack "-") ~?= Just [1.0],
   "test4" ~: (t stack "3") ~?= Just [3.0, 1.0, 2.0],
   "test5" ~: (t stack "aawaw") ~?= Nothing
   ]


solveRPN :: String -> Maybe Double
solveRPN st = do
  [result] <- foldM foldingFunction [] $ words st
  return result

tests2 :: Test
tests2 =
  let t = solveRPN in
  "solveRPN" ~:
  ["test1" ~: (t "aaa") ~?= Nothing,
   "test2" ~: (t "1 2 +") ~?= Just 3.0,
   "test3" ~: (t "1 2 + 3 *") ~?= Just 9.0
   ]

main :: IO ()
main = do
  runTestTT $ TestList [tests1, tests2]
  return ()