2011/6/21 Simon Peyton-Jones <simonpj@microsoft.com>

| One thing you could do to help in this specific case would be to use a
| different M1 tag--e.g., M1 S ... for selectors and M1 NS ... for
| fields without selectors (or K1 NS). I presume you've already
| considered this and/or it's too late to make such a change. (Or to
| move the distinction up to the constructor with two different
| constructor tags, CR and CN for record and no-record.)

I don't think it's too late to make a change. The stuff has only just gone in, so it's still very malleable. There may be other considerations, but legacy code isn't one of them!

I suppose that could be changed, yes, but what exactly are we trying to solve here? One can already specify different behavior for constructors with/without named fields. Are we trying to avoid OverlappingInstances? Then yes, this might help, but I'm not sure this change alone would make all generic programming possible without OverlappingInstances.

(Also, I always thought UndecidableInstances were "more evil", in some sense, and this change does nothing to remove the use of UndecidableInstances for generic programming.)

Cheers,
Pedro