Use pattern matching instead of case

hsilop.hs

@@ -27,9 +27,8 @@ rpn = foldM parse [] . words >=> return . head where
   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"
+  parse xs       (readMaybe -> Just x) = Right (x : xs)
+  parse xs       _                     = Left "syntax error"
 
 
 -- dyadic functions