Yup, that's a bug. Not clear if it's fixable.
That too. A very similar bug in fact, if there is a fix it will probably fix both of them. The problem is that readChan holds a lock on the read end of the Chan, so neither isEmptyChan nor unGetChan can work when a reader is blocked.
I wrote my Chan around the abstraction: data Chan a = Chan (MVar (Either [a] [MVar a])) The Chan either has elements in it (Left), or has readers waiting for elements (Right). To get the fairness properties on Chan you might want to make these two lists Queue's, but I think the basic principle still works. By using this abstraction my Chan was a lot simpler. With this scheme implementing isEmpyChan or unGetChan would both work nicely. My Chan was not designed for performance. (In truth I replaced the Left with IntMap a, and inserted elements with a randomly chosen key, but the basic idea is the same.)
own Chan implementation worked. My Chan had different properties (it queues items randomly) and a subset of the Chan functions, so it still doesn't prove any issue with Chan - but I am now sceptical.
It's surprising how difficult it is to get these MVar-based abstractions right. Some thorough testing of Chan is probably in order.
Agreed! In this project I wrote 8 different concurrency abstractions. I had bugs in most. MVar is a great building block on which to put higher layered abstractions, but using it correctly is tricky. I found that I used MVar's in four ways: 1) MVar's which are always full, and are just locks around data for consistency. Created with newMVar, used with modifyMVar. 2) MVar's which contain unit and are used for locking something other than data (i.e. a file on disk). Created with newMVar, used with withMVar. 3) MVar's which are used to signal computation can begin, created with newMVarEmpty, given to someone who calls putMVar (), and waited on by the person who created them. 4) MVar's which go in a higher-level concurrency operation - CountVars (variables which wait until they have been signaled N times), RandChan (Chan but with randomness), Pool (thread pool) etc. Thanks, Neil