[Haskell-cafe] RFC: concurrent-extra

Neil Brown nccb2 at kent.ac.uk
Tue Feb 16 11:18:25 EST 2010


I had a look at the code for Event (both versions) and Lock (but not the 
others just yet) and it seemed fine.  If you do lots of calls to 
waitTimeout before a set you will accumulate old locks in the list, but 
that won't cause any error that I can see, so it would only be a problem 
in pathological cases.

I'm not sure there is a good way to test MVar algorithms.  One way to do 
it (which the Concurrent Haskell Debugger did online, IIRC: 
http://www.informatik.uni-kiel.de/~fhu/chd/) is to reimplement IO to 
explore different interleavings of the various MVar calls to look for 
deadlocks.  If you're testing STM, generally you can look to capture 
invariants of your transactions and check that they hold after each 
transaction (e.g. that if the state is Set, there shouldn't be any locks 
waiting with an event).



Roel van Dijk wrote:
> Hello,
> We wrote a small library (1) which offers a few extra synchronization
> primitives. These primitives are found in the standard libraries of
> languages like Java and Python, but not in Haskell.
> Before releasing concurrent-extra on hackage, we would like some
> comments on its name, design, implementation, documentation (2) and
> general usefulness.
> All primitives are implemented with MVars and IORefs, except for
> Control.Concurrent.STM.Event.
> There is a small test suite which tests some properties:
>   cabal configure -ftest
>   cabal build
>   cabal test
> The test suite still needs to be expanded a great deal. We found that
> testing concurrent algorithms is really hard. Now we test things like
> "this shouldn't deadlock" by checking if it doesn't take too long. But
> that doesn't feel very formal. Is there a more principled approach to
> testing concurrent algorithms in Haskell?
> After writing our version of a read-write lock we noticed the package
> rwlock (3) on hackage. But there is a difference: rwlock is
> implemented with STM while our version is implemented entirely with
> MVars.
> Regards,
> Roel & Bas van Dijk
> 1 - http://code.haskell.org/~roelvandijk/code/concurrent-extra/
> 2 - http://code.haskell.org/~roelvandijk/code/concurrent-extra/doc/html/concurrent-extra/index.html
> 3 - http://hackage.haskell.org/package/rwlock
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