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
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
-- http://www.fastmail.fm - A fast, anti-spam email service.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe