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