Oops, there's a small bug in my last patchbundle. Please ignore it and use the attached instead. regards, Bas

Hello, My proposal[1] of turning the type synonym SampleVar into an abstract newtype is nearing its deadline (1 day to go). Please let me hear your +1s or -1s before we can finalize this proposal. regards, Bas [1] http://hackage.haskell.org/trac/ghc/ticket/3928 On Thu, Mar 18, 2010 at 6:04 PM, Bas van Dijk wrote:

Hello,

I was browsing over the code of Control.Concurrent.SampleVar and noticed the same kind of bugs I also discovered yesterday in Data.Unique:

* Throwing an asynchronous exception at the right time can make the state of a SampleVar inconsistent which can cause a dead-lock.

* Numeric expressions are put in a MVar lazily which can cause a space leak.

The attached patch bundle contains a patch that fixes them both.

The bundle also contains another patch which changes the API of Control.Concurrent.SampleVar. So this patch has to go through the library submission procedure.

I propose making SampleVar an abstract (new)type instead of a type synonym.

Advantages:

* Users can not "screw up" the internal state like they can do now.

* We can define instances for SampleVars without using the TypeSynonymInstances language extension. (This is actually the reason I bumped into the type synonym nature of SampleVars. I'm making a small library that defines some classes and I couldn't make SampleVar instances for them)

Disadvantages:

* Potentially breaks existing code. However if it breaks your code you are probably touching the internals which you shouldn't do anyway.

Discussion period: 1 week (Because I think this is a no-brainer)

Note I didn't derive an instance for Typeable for the SampleVar newtype. This should be done, but I don't know the proper way (manually, LANGUAGE DeriveDataTypeable or INSTANCE_TYPEABLE1(SampleVar,sampleVarTc,"SampleVar") ?).

regards,

Bas