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

[Niklas Broberg]

 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 just want to point out that IncoherentInstances is NOT a solution.
It would probably make the files compile, but not at all with the
semantics we want.

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

[Neil Brown]

On 09/11/10 11:53, Neil Brown wrote:

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

I think I should expand slightly more on this bit, especially since I 
chopped off the useful line The choice depends on the instantiation of 
`m'.  The problem is that depending on the choice of m in the instance 
(the EmbedAsChild (IdentityT IO) (XMLGenT m (XML m)) part), you may or 
may not match the second overlapping instance.  If you choose 
m=IdentityT IO then it matches the second instance, if you choose 
anything else then it won't, so GHC can't tell at this point which 
instance to choose.  Which makes them not just overlapping, but also 
leaves no clear choice (which is why it prompts you to use 
IncoherentInstances).  When you typed the genElement foo part 
explicitly and gave a type to the monad m, that fixed the ambiguity.  So 
I think it's not even that you needed incoherent instances, you have an 
unresolvable ambiguity if the type of m is left to be inferred.  So I 
still think the question is why the old instance with the ~ worked fine.


(Does this help, or am I telling you what you've already worked out for 
yourself?)


Thanks,

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

[Neil Brown]

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 Shawjer...@n-heptane.com  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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Re: [Haskell-cafe] Re: change in overlapping instance behavior between GHC 6.12 and GHC 7 causes compilation failure

[Claus Reinke]

 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?  


These two instances are not equivalent: 


- the first matches even if m and m1 differ, causing a type-error.
- the second matches only if m~m1

Claus

{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE MultiParamTypeClasses #-}

class C a bwhere c :: a - b - Bool
instance C a a where c _ _ = True
instance C a b where c _ _ = False

class D a b where d :: a - b - Bool
instance a~b=D a b where d _ _ = True
-- instance  D a b where d _ _ = False -- would be a duplicate instance

{-
*Main c () ()
True
*Main c () True
False
*Main d () ()
True
*Main d () True

interactive:1:0:
   Couldn't match expected type `Bool' against inferred type `()'
   When generalising the type(s) for `it'
-} 
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[Haskell-cafe] Re: change in overlapping instance behavior between GHC 6.12 and GHC 7 causes compilation failure

[Jeremy Shaw]

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 jer...@n-heptane.com 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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