Hey everyone, Concurrent channels are very opaque right now: all that can be done is checking they're empty. The upcoming release will, thanks to Merijn, contain a way of checking whether a TBQueue is full. However, there may be scenarios where it is desirable to retrieve the current size of a TBQueue. The reason I mention TBQueue specifically is because it already contains said information, but does not export corresponding accessor functions, so the implementation would be fairly easy. Probably the most compelling use case I can think of is debugging. Sometimes "full" or "empty" just isn't enough: it may be useful at which point in time the queue starts being filled disproportionately. (I just had a case where I could have saved many hours.) Another one inspired by someone on #haskell a couple of minutes ago is when a "size moderated" queue is desirable, i.e. one that is usually somewhat filled but still allows being written to in important cases. The specific example given was a device sending data through a channel where the input can be throttled based on how fast the output ends process the data. The code would be simply the following:

lengthTBQueue :: TBQueue a -> STM Int lengthTBQueue (TBQueue rsize _read wsize _write) = readTVar rsize

Another possible addition would be retrieving a TBQueue's maximum size. Although this has to be known in order to create the queue in the first place, when the queue is used deep down in other functions passing the size explicitly as a parameter seems a little unnecessary. David/quchen