Reverse Polish Notation calculator

RPN.hs

module RPN where
  import Data.List
  
  calc :: (Read a, Fractional a) => String -> a
  calc = head . foldl processToken [] . words
    where
      processToken stack@(x:x':xs) token = case token of "/" -> (x' / x):xs
                                                         "*" -> (x' * x):xs
                                                         "+" -> (x' + x):xs
                                                         "-" -> (x' - x):xs
                                                         _   -> read token : stack
      processToken stack token = read token : stack
  
  main :: IO ()
  main = print $ calc "10 4 3 + 2 * -"