On Jun 3, 2015, at 7:09 PM, AntC <anthony_clay...@clear.net.nz> wrote:

> Is this separate instance group idea still a gleam in someone's eye? 
> If not, is there some deep theoretical reason against?

Not to my knowledge (to both questions). But I don't believe we've lost any 
expressiveness over the earlier version. You can always define a helper closed 
type family and have an open type family instance just call a closed type 
family. Of course, it would be nice to have *local* type families (as if, say, 
there were a `where` clause allowed), but this should work for you. Or does 
this not work in your use case?

Having closed type families, as opposed to branched instances, just seemed like 
a cleaner way to package the new functionality. There really wasn't much to it 
other than aesthetics, if I recall the conversations correctly.

Richard
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