On Fri, May 25, 2012 at 7:06 AM, AntC
But it looks like the work SPJ pointed to is using closed style. If all they're trying to do is support HList and similar, I guess that's good enough.
I tried to explain all this the best part of a year ago. (Admittedly my explanation was a bit turgid, re-reading it today. And not that I was saying anything that hadn't been said by others -- it's resurfaced several times.) Funny how GHC-central just barrels ahead and ignores all those ideas, apparently without explaining why.
If you're referring to the NewAxioms work Simon linked to in the other thread, I don't see it explicitly stated that all instances have to be within a single module. Especially section 3.3 (Translation) of the pdf[1] seems to suggest otherwise. Though it also doesn't seem to be the same as what you're asking for. As far as I can tell, with NewAxioms, wherever you could currently have a type instance, you could instead have a type instance group. Within a group you could have unrestricted overlap with the first matching instance being selected, while between groups overlap would continue to be forbidden. Relative to explicit disequality guards it seems both more and less powerful: you couldn't have overlap between modules (but could still split instances among modules as long as they *don't* overlap), but overlap within a module would be more powerful (or at least more convenient). It seems vaguely similar to a paper on instance chains[2] I saw once. (Apologies in advance if any of this is inaccurate, I'm just going by what I can see.) [1] https://docs.google.com/open?id=0B1pOVvPp4fVdOTdjZjU0YWYtYTA5Yy00NmFkLTkxMWU... [2] http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.170.9113&rep=rep1&type=pdf