added lecture3

exampleCode/lecture3/MaybeIO.hs

@@ -0,0 +1,24 @@
+module MaybeIO where
+
+import Control.Applicative
+
+data MaybeIO a = MaybeIO { runMaybeIO :: IO (Maybe a) }
+
+instance Functor MaybeIO where
+  fmap f = MaybeIO . fmap (fmap f) . runMaybeIO
+
+instance Applicative MaybeIO where
+  pure    = MaybeIO . pure . pure
+  f <*> x = MaybeIO $ (<*>) <$> f' <*> x'
+            where
+              f' = runMaybeIO f
+              x' = runMaybeIO x
+
+instance Monad MaybeIO where
+  return  = pure
+  x >>= f = MaybeIO $ x' >>= runMaybeIO . mb . fmap f
+            where
+              x' = runMaybeIO x
+              mb :: Maybe (MaybeIO a) -> MaybeIO a
+              mb (Just a) = a
+              mb Nothing  = MaybeIO $ return Nothing