| >>Here's one concern I have with the deprecation of | >>-XOverlappingInstances: I don't like overlapping instances, I find | >>them confusing and weird and prefer to use code that doesn't | >>include them, because they violate my expectations about how type | >>classes work. When there is a single LANGUAGE pragma, that's a | >>simple, easily-checkable signpost of "this code uses techniques | >>that Ben doesn't understand". When it is all controlled by pragmas | >>I basically have to check every instance declaration individually. I see your point. Though you could just grep for OVERLAP! I suppose that -XOverlappingInstances could mean "silently honour OVERLAPPABLE/OVERLAPPING pragmas", while lacking it would mean "honour OVERLAPPABLE/OVERLAPPING pragmas, but emit noisy warnings" or even "don't honour them and warn". But that is different to the behaviour today, so we'd need a new LANGUAGE pragma. Perhaps -XHonourOverlappingInstances or something. My sense is that the extra faff is not worth it. | >>On a largely unrelated note, here's another thing I don't | >>understand: when is OVERLAPPABLE at one instance declaration | >>preferable to using only OVERLAPPING at the instance declarations | >>that overlap it? It's a user decision. GHC allows - OVERLAPPABLE at the instance that is being overlapped, or - OVERLAPPING at the instance that is doing the overlapping, or - both Another possible choice would be to require both. One or t'other wouldn't do. But the current choice (with the LANGUAGE pragmas -XOverlappingInstances) is the either/or choice, and I had no user pressure to change that. There *is* user pressure for the either/or semantics, so that you can *later* add an un-anticipated OVERLAPPING instance. | > {-# LANGUAGE FlexibleInstances #-} | > module M where | > class C a where f :: a -> a | > instance C a where f x = x | > instance C Int where f x = x + 1 | > | >I suspect many people have the intuition that NoOverlappingInstances | >should forbid the above, but in fact OverlappingInstances or no only | >controls instance *resolution*. I imagine you all already knew this | >but I did not until I carefully reread things. It's pretty clearly stated in the manual, but I'd be delighted to add a paragraph or two, or an example, if you can draft something and say where a good place for it would be (ie where you'd have looked). Thanks Simon