Changeset 6672 for src/Pugs/Monads.hs

Timestamp:

09/03/05 20:41:00 (3 years ago)

Author:

luqui

Message:

r548@feather: fibonaci | 2005-09-03 20:40:28 +0200
Rewrote findSyms to do real shadowing instead of returning all possible symbols. That is, if
there is a matching name in the lexical, it returns no global names. This solves the lexical
operator bug.

Files:

• src/Pugs/Monads.hs (modified) (2 diffs)

src/Pugs/Monads.hs

@@ -18 +18 @@
     enterBlock, enterSub,
     evalVal, tempVar,
+    
+    MaybeT, runMaybeT,
 
     module Control.Monad.RWS
@@ -26 +28 @@
 import Pugs.Types
 import Control.Monad.RWS
+
+
+newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }
+
+instance (Monad m) => Monad (MaybeT m) where
+    (MaybeT mon) >>= f =
+        MaybeT (mon >>= maybe (return Nothing) (runMaybeT . f))
+    return              = MaybeT . return . Just
+
+instance MonadTrans MaybeT where
+    lift mon = MaybeT (mon >>= return . Just)
+
+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
+
 
 {-|