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