Changeset 16602

Timestamp:

06/01/07 21:29:00 (18 months ago)

Author:

audreyt

Message:

* Pugs.Monads: Add MonadIO instance to the MaybeT monad.

Also simplify the mplus operation so the second action
is only run when the first operation returns Nothing.
(It used to run both and then needlessly discards the second.)

Files:

• src/Pugs/Monads.hs (modified) (1 diff)

src/Pugs/Monads.hs

@@ -43 +43 @@
     lift mon = MaybeT (mon >>= return . Just)
 
+instance (MonadIO m) => MonadIO (MaybeT m) where
+    liftIO ma = MaybeT $ do
+        a <- liftIO ma
+        return (Just a)
+
 instance (Monad m) => MonadPlus (MaybeT m) where
     mzero                       = MaybeT (return Nothing)
     mplus (MaybeT a) (MaybeT b) = MaybeT $ do
         ma <- a
-        mb <- b
-        return $ ma `mplus` mb
+        case ma of
+            Nothing -> b
+            _       -> return ma 
 
 {-|