Reboot! This has long sat idle, but I propose to now formally start the committee discussion period: mandatory 4 weeks, closing at end of day March 10th, or earlier if consensus occurs. Let's use this email thread for that discussion. In this mail I summarize public discussion and argue for "accept".
In short, the proposal adds a way to have multiple mutable fields within a data-constructor, without the indirection of using IORef. Second to
"linear types",
this proposal generated the most total comments during public discussion (107). This level of discussion was good -- given that accepted GHC proposals so far are mostly syntactic (or API tweaks), this would be the first with major compiler backend & runtime consequences.
Ed Kmett and Ryan Yates have demonstrated the applicability of this concept to data-structure implementation. (Indeed, I think there's a good reason that almost all languages mutation with mutation are implemented so as to allow a single heap object to have multiple mutable fields within it.) During the public discussion, questions were raised about interactions with other features and implementation strategy -- in particularly changes to core. But I believe that all major concerns were eventually answered.
P.S. Iavor, Trevor, and Ryan Yates were all working on implementation of this feature at various points. Not sure what the current status of implementation efforts are.