On Tue, 2015-06-16 at 00:03 +0400, Sergei Meshveliani wrote:
[..] 2) At least ghc-7.8.3 and ghc-7.10.1 do the same in this example. May be, you can change this example a bit to make ghc-7.8.3 and ghc-7.10.1 diverse, so that my example bug becomes visible? (they diverse on 7.10.1-errReport-may23-2015.zip but this bunch of modules is too complex).
(this is in the end of my last letter). I am trying now to reduce the report of 7.10.1-errReport-may23-2015.zip into something simple. And discover that Main yields a different result for -Onot. Probably, DoCon-2.12.1 will run correct under -Onot. For me, this is not important, because -O is desirable anyway. But for the GHC developers this effect may present some information. Regards, ------ Sergei
On Mon, 2015-06-15 at 09:29 +0000, Simon Peyton Jones wrote:
| This is why I think that ghc-7.8.3 treats the OI notion in a more | natural way than ghc-7.10.1 does. | May be, ghc-7.10.1 has a better technical tool for this, but ghc- | 7.8.3 corresponds to a natural notion of OI. | | Can GHC return to a natural OI notion? | Or am I missing something?
Well it all depends what you mean by "natural". To me it is profoundly un-natural to deliberately have the same type-class constraint solved in two different ways in the same program!
To require this would prevent cross-module specialisation. If I have f :: C a => a -> a in one module, and I specialise it to f_spec :: [Int] -> [Int] in one module, I want to be free to re-use that specialisation in other modules. But under your "natural" story, I cannot do that, because (C [Int]) might be resolved differently there.
Now, I give a simple (and a very contrived) example illustrating of how overlapping instances (OI) are used in DoCon. Also this example is made after the sample that Simon has given in his recent letter:
[..]
2) At least ghc-7.8.3 and ghc-7.10.1 do the same in this example. May be, you can change this example a bit to make ghc-7.8.3 and ghc-7.10.1 diverse, so that my example bug becomes visible? (they diverse on 7.10.1-errReport-may23-2015.zip but this bunch of modules is too complex). [..]