On 06 July 2004 11:29, Conor T McBride wrote:
OK, here's what I want, what I do with it, and my attempt to deliver it. But I'm not an expert, so please let me know if there's some disastrous flaw...
The signature/spec:
type Hole x
hole :: IO (Hole x) -- returns a fresh empty hole for an x askHole :: Hole x -> IO (Maybe x) -- inspects the current contents (or none) of the hole; mustn't -- block tellHole :: Hole x -> x -> IO (Maybe x) -- tries to write the hole, returns what was previously there; -- if the hole was empty, the value supplied is installed -- if the hole was full, it's unchanged (too late, pal!); -- mustn't block readHole :: Hole x -> IO x -- blocks until the hole has been filled, then returns its -- value instance Eq (Hole x) -- a kind of higher-level `pointer equality'
The following should do the trick (but I wouldn't rule out any lurking race conditions, and perhaps there's an even simpler way to achieve this): data Hole x = Hole (MVar ()) -- wait on this for readHole (MVar (Maybe x)) -- always full, Just x <=> hole contains x instance Eq (Hole x) where (Hole m1 _) == (Hole m2 _) = m1 == m2 hole = do wait <- newEmptyMVar val <- newMVar Nothing return (Hole wait val) askHole (Hole wait val) = readMVar val tellHole (Hole wait val) x = do modifyMVar val $ \mb -> case mb of Nothing -> do putMVar wait (); return (Just x, Nothing) Just y -> return (Just y, Just y) readHole (Hole wait val) = do readMVar wait mb <- readMVar val return (fromJust mb) Cheers, Simon