Define MVar "This function is atomic only if there are no other producers for this MVar."

The sentence:

This function is atomic only if there are no other producers for this MVar.

Is repeated lots of times in the Control.Concurrent.MVar documentation.

However, there is no definition of what "producers for an MVar" means.

There needs to be a definition of producer (and consumer I guess).

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
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changed weight to 5

added Tbug Trac import core libraries labels

The sentence might need to link to the section:

"In particular, the "bigger" functions in this module (readMVar, swapMVar, withMVar, modifyMVar_ and modifyMVar) are simply the composition of a takeMVar followed by a putMVar with exception safety. These only have atomicity guarantees if all other threads perform a takeMVar before a putMVar as well; otherwise, they may block."

or that section needs to define "producer".

Agreed that the documentation should be improved. I was also confused about this some time ago.

Producer simply means "putMVar". For example, swapMVar takes the current value and puts a new value. If another thread puts between the swapMVars take and put, swapMVar blocks and other threads observe the intermediate step, so it won't be atomic in this case.

Similarly consumer means "takeMVar".

Would you like to open a pull request? I think we accept documentation patches via Github PRs these days.

added documentation label

added Pnormal label

Is this issue still relevant with base-4.14.0.0? Should I add Control.Concurrent.MVar to #17929?

@Kleidukos FWIW I think enumerating specific modules in #17929 isn't useful, since ideally any module in base could potentially do with some improvements. I think instead it might be useful as a place to refer to MRs that aim to improve documentation in base.

@alexanderkjeldaas would you like to put up a PR?

@osa1 I am willing to do this PR on GitHub, but that would be my first PR to the Haskell docs. Could you please me show against which repo/branch I need to do it?

@why-not-try-calmer I can show you. And if you want, we can even make a call so that I guide you step by step.

That's a very kind offer! Let me write this up and I'll get back to you soon with my material so we can see to it.

@Kleidukos I just lodged a Merge Request there: !6139 (closed). I hope it satisfies the formal requirements.