Documentation in Control.Monad.STM.TMVar - isEmptyTMVar

Simon Marlow marlowsd at
Tue Nov 22 17:43:35 CET 2011

On 26/10/2011 19:25, Heinrich Apfelmus wrote:
> Heinrich Apfelmus wrote:
>> As of stm-, the documentation for the isEmptyTMVar function [1]
>> reads
>> isEmptyTMVar :: TMVar a -> STM BoolSource
>> Check whether a given TMVar is empty.
>> Notice that the boolean value returned is just a snapshot of the state
>> of the TMVar. By the time you get to react on its result, the TMVar
>> may have been filled (or emptied) - so be extremely careful when using
>> this operation. Use tryTakeTMVar instead if possible.
>> This being software transactional memory, where everything is wrapped
>> into a call to the atomically function, I believe that the second
>> paragraph is now completely obsolete, to the point of being misleading
>> (it sure scared me). I would be relieved if someone could remove it.
>> Most likely, it was originally copied directly from the corresponding
>> documentation of Control.Concurrent.MVar .
>> [1]:
> I forgot to set a discussion period for this, but two weeks have passed
> and it seems to me that there is unanimous support for this
> documentation change (removing the second paragraph).
> I would like to move this towards inclusion in the stm library. Should I
> make a track ticket? Or maybe a kind GHC guru shows mercy and applies
> the change as it is?

I'll do it.  Sorry for the delay, I'm a bit behind with 
libraries at


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