rpn.hs

-- Reverse Polish Notation parser
-- from: http://learnyouahaskell.com/functionally-solving-problems

rpn :: String -> Float
rpn expr = head $ foldl foldFunc [] (words expr)
    where foldFunc acc op
            | head op `elem` ['1'..'9'] = (read op :: Float) : acc
            | otherwise = applyOp op (acc !! 1) (head acc) : drop 2 acc
          applyOp op a b = case op of "+" -> a + b
                                      "-" -> a - b
                                      "*" -> a * b
                                      "/" -> a / b

-- original
rpn' :: String -> Float
rpn' = head . foldl parse [] . words
    where parse (x : y : acc) "+" = y + x : acc
          parse (x : y : acc) "-" = y - x : acc
          parse (x : y : acc) "*" = y * x : acc
          parse (x : y : acc) "/" = y / x : acc
          parse (x : y : acc) "^" = y ** x : acc
          parse (x : acc) "ln" = log x : acc
          parse acc "sum" = [sum acc]
          parse acc n = (read n :: Float) : acc