Add tryReadChan to Chan

[mitar]

Now that isEmptyChan is deprecated (#4154) it would be useful to add tryReadChan. It seems it is possible to define it so that it does not block like isEmptyChan. This is because semantics is a bit different: tryReadChan is allowed to return Nothing also when Chan is non-empty, but it would block. Contrary, isEmptyChan is expected that it would return False only if Chan is really empty.

I have defined also two new MVar functions which helped me implementing tryReadChan: tryReadMVar and tryModifyMVar. Probably it would be useful to make them public.

Please comment. I propose three weeks for discussion.

[mitar]

I am sorry that patch is not in darcs format.

[simonmar]

tryReadChan itself looks ok to me.

If tryModifyMVar were to be public, I'd expect its type to be

tryModifyMVar :: MVar a -> (a -> IO (a, b)) -> IO (Maybe b)

but I can see why you used instead

tryModifyMVar :: MVar a -> (a -> IO (a, Maybe b)) -> IO (Maybe b)

because the former would require an extra Control.Monad.join, whereas the latter has it "built-in".

tryReadMVar looks like the right thing, I wouldn't object to that going in Control.Concurrent.MVar.

What is the purpose of the INLINE pragma?

[mitar]

Replying to simonmar:

What is the purpose of the INLINE pragma?

I just copied it from modifyMVar. I am not sure if it is needed.

[igloo]

Proposal tickets are no longer needed as part of the library submissions process. Instead, a normal ticket should be created once consensus has been achieved. Please see the  process description for details.