Wow, this looks like a bug to me. If it's not a bug, then it's horribly unintuitive. I extended your example in an effort to figure out what was going on. Apparently weak pointers loath live IORefs: import Data.IORef import Data.Maybe import System.Mem import System.Mem.Weak import Control.Monad data A = A String data B = B String (IORef Int) showA (A s) = s showB (B s _) = s main = do -- works as expected: ref <- return $ A "A" ptr <- mkWeak ref 21 Nothing performGC print . isNothing =<< deRefWeak ptr print (showA ref) -- why doesn't this work? ref <- liftM (B "B") $ newIORef 42 ptr <- mkWeak ref 21 Nothing performGC print . isNothing =<< deRefWeak ptr print (showB ref) -- this works, wtf??? ref <- liftM (B "B") $ return undefined ptr <- mkWeak ref 21 Nothing performGC print . isNothing =<< deRefWeak ptr print (showB ref) I don't think this is the expected behavior. The docs on Weak pointers don't mention anything like this. I suspect something in the GC is getting confused by the IORef somehow. - Job 2009/11/2 Patai Gergely <patai_gergely@fastmail.fm>
Could mkWeakPair do what you want?
http://haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html#... No, it's just a convenience function that doesn't help much, because the value already refers to the IORef anyway.
Here's a minimal example to illustrate the problem:
import Data.IORef import Data.Maybe import System.Mem import System.Mem.Weak
main = do ref <- newIORef 42 ptr <- mkWeak ref 21 Nothing performGC print . isNothing =<< deRefWeak ptr print =<< readIORef ref
Depending on whether you compile with optimisations, the weak reference might be reported dead, even though the IORef is alive and kicking. Switching to mkWeakPair (or just mentioning ref in the value somehow) doesn't affect that.
Or are you trying to do something else? The goal is to create mutable objects whose update codes are tracked by the main program, but they can be thrown out when all the other references to the objects are lost. Creating weak pointers with MutVar#s seems to do the trick, but I'm not confident if it is a solution I can trust...
Gergely
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