Closed Type Families: separate instance groups?

Richard Eisenberg eir at
Thu Jun 4 00:30:09 UTC 2015

On Jun 3, 2015, at 7:09 PM, AntC <anthony_clayden at> 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.


More information about the Glasgow-haskell-users mailing list