Add error handling

RPN.hs

@@ -1,6 +1,8 @@
 {-# LANGUAGE ViewPatterns #-}
 
 import Data.List
+import Text.Read
+import Control.Monad
 
 main :: IO ()
 main = io (show . rpn)
@@ -8,12 +10,16 @@ main = io (show . rpn)
 io :: (String -> String) -> IO ()
 io f = interact (unlines . map f . lines)
 
-rpn :: String -> Double
-rpn = head . foldl parse [] . words where
-  parse (y:x:xs) (flip lookup dyad  -> Just f) = f x y : xs
-  parse (x:xs)   (flip lookup monad -> Just f) = f x : xs
-  parse xs       (flip lookup nilad -> Just k) = k : xs
-  parse xs x = read x : xs
+rpn :: String -> Either String Double
+rpn = return . head <=< foldM parse [] . words
+
+parse :: [Double] -> String -> Either String [Double]  
+parse (y:x:xs) (flip lookup dyad  -> Just f) = Right (f x y : xs)
+parse (x:xs)   (flip lookup monad -> Just f) = Right (f x : xs)
+parse xs       (flip lookup nilad -> Just k) = Right (k : xs)
+parse xs x = case readMaybe x of
+  Just x  -> Right (x : xs)
+  Nothing -> Left "Syntax error"
 
 -- dyadic functions
 dyad = [ ("+", (+))