I admit I don't know exactly how MVars are implemented, but given that they can be aliased and have indefinite extent, I would think that they look something vaguely like a cdatatype ** var, basically a pointer to an MVar (which is itself a pointer, modulo some other things such as a thread queue.)

And, I would think that "unpacking" such an structure would basically be eliminating one layer of indirection, so it would then look vague like a cdatatype * var. But again, given aliasing and indefinite extent, this would seem to be a difficult thing to do.

Actually this isn't too difficult if an MVar only exists in a single unpacked structure: other references to the MVar can simply be pointers into the structure. But the case where an MVar is unpacked into two different structures suggests that, at least in some cases, an "unpacked" MVar is still a cdatatype ** var;

So, is my understanding more or less correct? Does anybody have a good, succinct explanation of how MVars are implemented, and how they are unpacked?

One final question, assuming that unpacking an MVar really does eliminate a layer of indirection, and that other references to that MVar are simply pointers into a larger structure, what happens to that larger structure when there are no more references to it (but still some references to the MVar?) Given the complications that must arise out of a doubly "unpacked" MVar, I'm going to guess that the larger structure does get garbage collected in this case, and that the MVar becomes dislodged from this structure. Would that MVar then be placed directly inside another unpacked reference, if one is available?