Where (else) do I need to register instances from loaded modules?

[Matthew Pickering]

Hi Gergo,

Some things I noticed:

* The HPT only contains modules from the current home unit. It seems
that registerModule ignores this so you break this invariant.
* It seems that `Instance` contains an orphan, but you don't fill in
the `deps` field when making a `ModIface`, which should contain all
the orphan modules below the module you are compiling.

I think that if you fill in `deps` correctly then things may work out.
`Dependencies` eventually becomes `ImportAvail`, which gets consulted
by `tcVisibleOrphanMods` which is called in `tcGetInstEnvs` which is
called by `matchInstEnv`.

Matt

On Tue, Aug 24, 2021 at 3:20 AM Erdi, Gergo via ghc-devs
<ghc-devs at haskell.org> wrote:
>
> PUBLIC
>
>
> Hi,
>
>
>
> I am trying to typecheck & load three modules using the GHC API. The first one defines a class, the second one defines an instance of said class, and the third one uses the instance (attached as Class.src, Instance.src and Use.src, respectively). The problem is that when I typecheck Use.src, it complains about the missing instance. It should be noted that the class is found, only the instance isn’t.
>
>
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> My code is bypassing the normal GHC driver to feed it modules instead of letting GHC itself come up with dependencies and then loading them from disk; that is because in my real use case, some dependencies will have interfaces generated on the fly (there is no underlying Haskell source code), or loaded over HTTP, etc. So I am using parseModule and typeCheckModule on each module, make a ModIface from each, and then I try to register them with GHC, by extending the HPT, the moduleNameProvidersMap, and the EPS. Evidently, this is not enough, and the instances should be registered somewhere else as well – where is that place?
>
>
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> I have attached a simplified version of my code as Main.hs; it is targeting the GHC 9.0 API. It hardcodes the GHC 9.0 package.conf.d due to laziness on my part; please change line 96 to your environment before running it. As written, it fails when typechecing Use.src with:
>
>
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> input/Use.src:7:7: error:
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>     • No instance for (C X) arising from a use of ‘method’
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>     • In the expression: method
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>       In an equation for ‘foo’: foo = method
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>   |
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> 7 | foo = method
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>   |       ^^^^^^
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>
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> Thanks,
>
>             Gergo
>
>
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