[Haskell-cafe] IORefs and weak pointers

[Job Vranish]

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 at fastmail.fm>

> > Could mkWeakPair do what you want?
> >
> http://haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html#v:mkWeakPair
> 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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>
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