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<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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