Re: [Haskell-cafe] Re: change in overlapping instance behavior between GHC 6.12 and GHC 7 causes compilation failure

I'm not sure whether to reply to the list(s) or the ticket; maybe if you think my comments are valid they can be copied to the ticket. From looking, it seems to me that you do have overlapping instances, and I wonder if it's actually a 6.12 bug for accepting the code, not a 7 bug for rejecting it. I took your XMLGenerator.lhs file from the ticket and loaded it in GHC 6.12; it loaded fine. Then I looked through and squinted at this instance:

instance (EmbedAsChild m c, m1 ~ m) => EmbedAsChild m (XMLGenT m1 c)

That looked to me like a long-winded way of saying:

instance (EmbedAsChild m c) => EmbedAsChild m (XMLGenT m c)

Unless I'm missing something? So I made that change, then loaded it again. Suddenly I have overlapping instances in 6.12: XMLGenerator.lhs:64:16: Overlapping instances for EmbedAsChild (IdentityT IO) (XMLGenT m (XML m)) arising from a use of `asChild' at XMLGenerator.lhs:64:16-22 Matching instances: instance [overlap ok] (XML m ~ x, XMLGen m) => EmbedAsChild m x -- Defined at XMLGenerator.lhs:37:11-52 instance [overlap ok] (EmbedAsChild m c) => EmbedAsChild m (XMLGenT m c) -- Defined at XMLGenerator.lhs:31:11-60 It seems to me that the two instances can overlap; consider if I define an instance for XMLGen:

data WhateverM instance XMLGen WhateverM where type XML WhateverM = XMLGenT WhateverM ()

At that point your instances would overlap if I want an instance for EmbedAsChild WhateverM (XMLGenT WhateverM ()). My understanding of the way type-classes are checked is that since that instance could exist somewhere in a different module, we must reject the two instances for EmbedAsChild. (Or rather: ignoring the head of the instance declarations, the instances can overlap.) But I may be wrong! So my guess is you've always had overlapping instances (assuming this code is representative of your original), and my hypothesis is that the use of "~" in one of the instances was somehow stopping GHC 6.12 from spotting this. Thoughts (and corrections!) welcome. Thanks, Neil. On 08/11/10 23:30, Jeremy Shaw wrote:

Hello,

I have narrowed this down further to a single file. And created a trac bug for it:

http://hackage.haskell.org/trac/ghc/ticket/4485

This is (the only thing?) holding up HSP and happstack moving to GHC 7.

- jeremy

On Tue, Nov 2, 2010 at 5:36 PM, Jeremy Shaw wrote:

...

Hello,

I have a module, XMLGenerator, which has some overlapping instances. I have a second module, Test, which imports that module and also adds some more overlapping instances.

Both modules contain {-# LANGUAGE OverlappingInstances #-} at the top.

Under some old version of 6.13 (and probably 6.12), if I put both modules in the same directory and try to load Test.hs, it gets the error:

Test.hs:16:15: Overlapping instances for EmbedAsChild (M IO) (XMLGenT m (XML m)) arising from a use of `asChild' at Test.hs:16:15-21 Matching instances: instance (m1 ~ m, EmbedAsChild m c) => EmbedAsChild m (XMLGenT m1 c) -- Defined at XMLGenerator.hs:16:10-68 instance (XML m ~ x, XMLGen m) => EmbedAsChild m x -- Defined at XMLGenerator.hs:19:10-51 In the first argument of `($)', namely `asChild' In the expression: asChild $ (genElement "foo") In the definition of `asChild': asChild b = asChild $ (genElement "foo")

If I put the XMLGenerator module in a separate package, dummy-hsx, and the Test modules links against it, I still get the error.

*but* if I add:

Extensions: OverlappingInstances

to the dummy-hsx.cabal file, then Test.hs compiles just fine! So, for starters, I do not understand why that happens.

Under GHC 7.0rc1, modifying the .cabal file has no effect. Instead I always get the error:

Test.hs:16:15: Overlapping instances for EmbedAsChild (M IO) (XMLGenT m (XML m)) arising from a use of `asChild' Matching instances: instance [overlap ok] (m1 ~ m, EmbedAsChild m c) => EmbedAsChild m (XMLGenT m1 c) -- Defined in XMLGenerator (The choice depends on the instantiation of `m' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations)

Adding the IncoherentInstances flag does make it compile -- but I have never enabled that flag and not regretted it.

What changed between GHC 6.12 and GHC 7.0? Is there a some solution besides using IncoherentInstances in every module that imports XMLGenerator?

I have attached XMLGenerator.hs, Test.hs, and dummy-hsx.cabal.

thanks! - jeremy

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