solveRPN.hs

solveRPN :: String -> Double
-- solveRPN expression = head (foldl foldingFunction [] (words expression))
solveRPN = head . foldl foldingFunction [] . words
    -- where foldingFunction stack item = ...
    where foldingFunction (x:y:ys) "*" = (y * x):ys
          foldingFunction (x:y:ys) "+" = (y + x):ys
          foldingFunction (x:y:ys) "-" = (y - x):ys
          foldingFunction (x:y:ys) "/" = (y / x):ys
          foldingFunction (x:y:ys) "^" = (y ** x):ys
          foldingFunction (x:xs) "ln" = log x:xs
          foldingFunction xs "sum" = [sum xs]
          foldingFunction xs numberString = read numberString:xs

main = do
    print $ solveRPN "3 2 +"
    print $ solveRPN "3 3.5 +"
    print $ solveRPN "10 4 3 + 2 * -"
    print $ solveRPN "90 34 12 33 55 66 + * - +"
    print $ solveRPN "3 3 4 - +"
    print $ solveRPN "10 10 10 10 sum 4 /"
    print $ solveRPN "10 10 10 10 10 sum 4 /"
    print $ solveRPN "10 4 ^"
    print $ solveRPN "10000 2 ln"