Clean a bit the code

RPN.hs

@@ -2,24 +2,35 @@
 
 import Data.List
 import Text.Read
+import Text.Printf
 import Control.Monad
 
 main :: IO ()
-main = io (show . rpn)
+main = io (result . rpn)
 
+
+-- Interact line-by-line
 io :: (String -> String) -> IO ()
 io f = interact (unlines . map f . filter (not . null) . lines)
 
-rpn :: String -> Either String Double
-rpn = foldM parse [] . words >=> return . head
 
-parse :: [Double] -> String -> Either String [Double]  
+-- Pretty print RPN result/errors
+result :: Either String Double -> String
+result (Left err) = "Ꞥ∘ " ++ err
+result (Right x)  = printf ("ꟼ∘ " ++ format) x where
+  format = if ceiling n == floor n then "%.0f" else "%.10f"
+
+
+-- Solve a RPN expression
+rpn :: String -> Either String Double
+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"
+    Nothing -> Left "syntax error"
+
 
 -- dyadic functions
 dyad = [ ("+", (+))