I am familiar with the source of Control.Concurrent.MVar, and I do see {-# UNPACK #-}'ed MVars around, for example in GHC's IO manager. What I should have asked is, what does an MVar# look like? This cannot be inferred from Haskell source; though I suppose I could have tried to read the Runtime source.

Now, one would hope that and (MVar# RealWorld footype) would approximately correspond to a "footype * mvar;" variable in C. The problem is this cannot _always_ be the case, because you can alias the (MVar# RealWorld footype) by placing a single MVar into two unpacked columns in two different data structures. So you would need to be able to still sometimes represent an MVar# by a footype ** mvar at runtime, even though one would hope that it would be represented by a "footype * mvar" in one particular data structure.

On Tue, Jul 31, 2012 at 1:04 AM, Ryan Ingram <ryani.spam@gmail.com> wrote:

Because of this, boxed MVars can be garbage collected without necessarily garbage-collecting the MVar# it holds, if a live reference to that MVar# still exists elsewhere.

I was asking the dual question: if the MVar# exists in some data structure, can that data structure still be garbage collected when there is a reference to the MVar#, but not the data structure it is contained within.

Best,

Leon