[Haskell-cafe] IORef memory leak

[Jim Snow]

Don Stewart wrote:
> dvde:
>   
>> Don Stewart schrieb:
>>     
>>> It is not possible to write a modifyIORef that *doesn't* leak memory!
>>>   
>>>       
>> Why? Or can one read about it somewhere?
>>     
>
>
> Try writing a version of this program, using modifyIORef only, 
> such that it doesn't exhaust the heap:
>
>     import Data.IORef
>     import Control.Monad
>     import System.IO.Unsafe
>
>     ref :: IORef Int
>     ref = unsafePerformIO $ newIORef 0
>     {-# NOINLINE ref #-}
>
>     main = do
>         modifyIORef ref (\a -> a + 1)
>         main
>
> Run it in a constrained environment, so you don't thrash:
>
>     $ ./A +RTS -M100M
>     Heap exhausted;
>     Current maximum heap size is 99999744 bytes (95 MB);
>     use `+RTS -M<size>' to increase it.
>
> The goal is to run in constant space.
>
> -- Don
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>   
Thanks, that's good to know. 

do x <- readIORef ior
              writeIORef ior $! (x+1)

Works for me.  The laziness of modifyIORef and workarounds would be a 
good thing to have documented in the modifyIORef docs [1], since it's 
probably a common source of memory leaks.  I'd also be in favor of a 
strict version of modifyIORef.

[1] 
http://www.haskell.org/ghc/dist/current/docs/libraries/base/Data-IORef.html#v%3AmodifyIORef

-jim