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.
I'd be interested in what others think. I've started https://ghc.haskell.org/trac/ghc/ticket/10526 to collect these points.
Meanwhile, Sergei, it's clear how to fix docon, so you are no longer stuck.
I hope DoCon is not stuck -- because it relies (so far) on ghc-7.8.3. But I need to understand the subject of overlapping instances (OI). ---------- I am writing this _here_ because to clicking at https://ghc.haskell.org/trac/ghc/ticket/10526 my browser responds Problem occurred while loading the URL https://ghc.haskell.org/trac/ghc/ticket/10526 SSL handshake failed: A TLS fatal alert has been received. Then, I start searching from https://ghc.haskell.org/trac/ghc/ and cannot find #10526 Also I had earlier a certain difficulty with registering to this bug tracker, and suspect that registration was not fully successful, so that I doubt that I can write there. May I, please, discuss the subject here? ------------------- The report of 7.10.1-errReport-may23-2015.zip shows that 7.8.3 and 7.10.1 treat OI differently, But this report is rather complex. Instead consider the following simple example. -------------------------------------------- module A where class Size a where size :: a -> Int instance {-# OVERLAPPING #-} Size [a] where size = length f :: Size a => a -> Int f = size g :: [Maybe Int] -> Int g = size ---------------------------------------- module Main where import A instance {-# OVERLAPPING #-} Size [Maybe a] where size _ = -1 mbs = [] :: [Maybe Int] main = putStr (shows (f mbs) "\n") -- I -1 -- putStr (shows (g mbs) "\n") -- II 0 -------------------------------------------- Making and running in ghc-7.10.1 : ghc --make -XFlexibleInstances Main ./Main It prints "-1" for the line (I) for main, and "0" for the line (II). To compare to ghc-7.8.3, comment out the pragma and add -XOverlappingInstances to the ghc call. And this yields the same results -1 and -0 respectively. Please, how to change (a bit) the above simple example in order to see the difference between 7.8.3 and 7.10.1 ? Thanks, ------ Sergei