This package provides a bounded channel type for STM. TBChan (in stm-chans) and TBQueue (introduced in stm 2.4) are bounded channels that limit the number of items in the channel. SBChan, on the other hand, limits the total size of items in the channel, where "size" is defined by providing an instance of the ItemSize class: data Entry = Entry Int64 ByteString Time -- | Estimated amount of memory an 'Entry' requires, -- including channel overhead instance ItemSize Entry where itemSize (Entry _ s _) = B.length s + 200 Then, "SBChan Entry" is a channel that limits (approximately) the amount of memory the entries take up. SBChan can also be used as a regular count-bounded channel by using the SBCItem newtype wrapper, where itemSize is always 1. http://hackage.haskell.org/package/stm-sbchan Enjoy! -Joey --- Implementation details --- itemSize returns an Int. I originally considered using an associated type, so users could pick their own number type to use. However, this would have made the implementation harder to reason about, if we had to worry about the user picking an ill-behaved number type like Float. Besides, Int is usually adequate for representing in-memory sizes. This library takes a lot of ideas from TChan and TBChan. I decided to use the linked list of TVars approach that TChan uses, rather than the pair of lists approach T(B)Queue uses, to avoid a potential problem with code like this: msg <- readTBQueue case msg of Foo -> ... Bar -> ... If the transaction is repeated a lot due to retries or invalidation, and readTBQueue needs to turn around the list at this point, then we'll end up repeating O(n) work a bunch of times. SBChan uses stm-tlist, a library I wrote that is based on TChan's internal representation. Also, SBChan uses the usual trick for reducing reader-writer contention by having two counters for capacity, one for readers and one for writers.