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.

  -Ryan

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.