2016 Update

Task/Parsing-RPN-calculator-algorithm/Haskell/parsing-rpn-calculator-algorithm-4.hs

@@ -0,0 +1,10 @@
+class Monad m => Logger m where
+  write :: String -> m ()
+
+instance Logger IO where write = putStr
+instance a ~ String => Logger (Writer a) where write = tell
+
+verbose2 f x y = write (show x ++ " " ++
+                        show y ++ " ==> " ++
+                        show res ++ "\n") >> return res
+  where res = f x y