Filter implicit parameter instances from data-dependencies #7320
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Occasionally GHC likes to float implicit parameters to the top-level making them look like any other typeclass instances. Implicit parameters are by design not unique so if we try to reconstruct them we get an error about a duplicate instance. I haven’t figured out when exactly GHC floats things to the top-level but I did verify that this testcase breaks without the fix. changelog_begin changelog_end
sofiafaro-da
approved these changes
Sep 3, 2020
| -- how to reliably stop it from doing this so be careful when changing this. | ||
| ] | ||
| withCurrentDirectory (tmpDir </> "dep") $ | ||
| callProcessSilent damlc ["build", "-o", "dep.dar"] |
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I wonder if we can make a test that reliably generates an implicit parameter instance?
Or perhaps, add some a check to this test that an implicit parameter instance is in the DAR.
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Ah, I wonder if we can slip it in to the "Simple DALF" test or something. (It's not so simple...)
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I’ve added an assertion that we have 2 implicit instances. Simple DALF would work as well I guess but this has the advantage that it tests what our users actually hit which seems useful. I still don’t understand how to generate it reliably. Even in this example you can remove the type signature, the inferred type stays the same but now it’s inlined 🤷
changelog_begin changelog_end
hurryabit
reviewed
Sep 4, 2020
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Occasionally GHC likes to float implicit parameters to the top-level
making them look like any other typeclass instances. Implicit
parameters are by design not unique so if we try to reconstruct them
we get an error about a duplicate instance.
I haven’t figured out when exactly GHC floats things to the top-level
but I did verify that this testcase breaks without the fix.
changelog_begin
changelog_end
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