IORefs and weak pointers
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IORefs and weak pointers
by Patai Gergely
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Hello all,
I wanted to create a weak pointer with an IORef as the key and something else as the value, but I saw no way to do it through the API provided. After some experimentation I came up with the following abomination for a solution: myWeakRef (IORef (STRef r)) v f = IO $ \s -> case mkWeak# r v f s of (# s', w #) -> (# s', Weak w #) This works perfectly when the code is compiled both with and without optimisations, but ghci chokes on it with an internal error. So my question is if I can expect this to work at least this much in the long run, or is it a hopelessly fragile hack? Gergely -- http://www.fastmail.fm - Same, same, but different... _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@... http://www.haskell.org/mailman/listinfo/haskell-cafe |
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Re: IORefs and weak pointers
by John Van Enk
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Is the IORef or the value in the IORef your key?
2009/11/2 Patai Gergely <patai_gergely@...> Hello all, _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@... http://www.haskell.org/mailman/listinfo/haskell-cafe |
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Re: IORefs and weak pointers
by Patai Gergely
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> Is the IORef or the value in the IORef your key?
I want the IORef itself to be the key. However, that doesn't work with optimisations turned on (the pointers get wiped out at the first gc), I guess because they remove the box around the MutVar#. Extracting that MutVar# seems to solve the problem. Gergely -- http://www.fastmail.fm - mmm... Fastmail... _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@... http://www.haskell.org/mailman/listinfo/haskell-cafe |
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Re: IORefs and weak pointers
by Job Vranish
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Could mkWeakPair do what you want?
http://haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html#v:mkWeakPair Or are you trying to do something else? - Job 2009/11/2 Patai Gergely <patai_gergely@...> Hello all, _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@... http://www.haskell.org/mailman/listinfo/haskell-cafe |
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Re: IORefs and weak pointers
by Patai Gergely
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> 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 -- http://www.fastmail.fm - A fast, anti-spam email service. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@... http://www.haskell.org/mailman/listinfo/haskell-cafe |
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Re: IORefs and weak pointers
by Job Vranish
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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@...>
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