al.hs

import Data.List
solveRPN :: (Num a, Read a) => String -> a
solveRPN = head . foldl foldingFunction [] . words
        where foldingFunction (x:y:ys) "*" = (x * y):ys
              foldingFunction (x:y:ys) "+" = (x + y):ys
              foldingFunction (x:y:ys) "-" = (y - x):ys
              foldingFunction xs numberString = read numberString:xs