On Tue, Mar 23, 2010 at 10:20 PM, Simon Marlow
The leak is caused by the Data.Unique library, and coincidentally it was fixed recently. 6.12.2 will have the fix.
Oh yes of course, I've reported that bug myself but didn't realize it was the problem here :-) David, to clarify the problem: newUnqiue is currently implemented as: newUnique :: IO Unique newUnique = do val <- takeMVar uniqSource let next = val+1 putMVar uniqSource next return (Unique next) You can see that the 'next' value is lazily written to the uniqSource MVar. When you repeatedly call newUnique (like in your example) a big thunk is build up: 1+1+1+...+1 which causes the space-leak. In the recent fix, 'next' is strictly evaluated before it is written to the MVar which prevents a big thunk to build up. regards, Bas