Let me answer this myself - brain burnt... Of course it is OK, that is precisely the semantics of MVars - the are empty or full, thus assuring the mutual exclusion between threads. Been a hard week..... Neil ... so the deeper question - why don't you realise these mistakes till five minutes *after* you've pressed the send button.... Neil Davies wrote:

Hi, I was looking over the libraries for bits of GHC (no doubt a standard form of relaxation for readers of this list), and noticed the following statement (in Data.Unique):

-- | Creates a new object of type 'Unique'. The value returned will -- not compare equal to any other value of type 'Unique' returned by -- previous calls to 'newUnique'.

This set me thinking - so I looked at the code

newUnique :: IO Unique newUnique = do val <- takeMVar uniqSource let next = val+1 putMVar uniqSource next return (Unique next)

In the concurrent execution world in which we live - I don't think that the implementation supports the "uniqueness" statement above - or am I not understanding something?

Cheers

Neil