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

[Jeremy Shaw]

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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: dummy-hsx.cabal
Type: application/octet-stream
Size: 351 bytes
Desc: not available
Url : http://www.haskell.org/pipermail/haskell-cafe/attachments/20101102/75080538/dummy-hsx.obj
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Test.hs
Type: text/x-haskell
Size: 547 bytes
Desc: not available
Url : http://www.haskell.org/pipermail/haskell-cafe/attachments/20101102/75080538/Test.bin
-------------- next part --------------
A non-text attachment was scrubbed...
Name: XMLGenerator.hs
Type: text/x-haskell
Size: 694 bytes
Desc: not available
Url : http://www.haskell.org/pipermail/haskell-cafe/attachments/20101102/75080538/XMLGenerator.bin