I wrote:
We could allow for case 3 by not providing any default instances for Indexable.
Henning Thielemann wrote:
This would obstruct 'start GHCi and play around' sessions, since you cannot create a set of something on the fly. (However, GHCi could still be adapted.)
Yes. I am suggesting also leaving the current versions that use Ord, so we are no worse off with that than we were before, at least.
Still, later in the above thread, Johannes Waldmann throws in Local Instances: http://www.haskell.org/pipermail/libraries/2007-April/007416.html
Ah, yes. That would be great! I would especially like it for GHCI - in general, not just because of the special case you mentioned above. But this has come up before. There are people who are not sure about this feature, because of some ambiguous and/or confusing cases that can come up when you redefine an instance that already exists in an enclosing context. And anyway - this is a serious language change. It will at best take time. Your Indexable class could be used right now to make a lot of the most common cases much easier to handle. -Yitz