Re: Proposal: replace readMVar with atomicReadMVar, breaking BC

-----Original message----- From: "Edward Z. Yang" Sent: 10 Jul 2013, 02:20

GHC HEAD recently got a new primitive: atomicReadMVar#, which allows you to read MVars without first taking and then putting back (removing a nasty race where readMVar only works properly when there are no waiting putters).

That is great, I have been missing such a primitive.

The general feeling Simon and I have is that everyone really wanted to make believe readMVar was atomicReadMVar, and so maybe we should break BC and make readMVar do the right thing. But it is probably worth some discussion, and I entreat you to think about the second point more carefully.

+1 from me. Even though it is a BC and a change in semantics, it is a change to the behaviour people expect or hope for when using readMVar. Cheers, Milan