I would love to know the answer to this. On Tue, Nov 2, 2010 at 3:36 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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