Generate names directly?

[tfausak]

Currently Imp works by inserting import statements. However in theory it is possible for it to modify identifiers in place, changing them from Qual to Exact: https://hackage.haskell.org/package/ghc-9.8.2/docs/GHC-Types-Name-Reader.html#t:RdrName

I'm not sure how difficult this would be to pull off. An Exact is just a wrapper around a Name, and generating a Name requires a Unique. That should be possible with the name cache in the HSC env: https://hackage.haskell.org/package/ghc-9.8.2/docs/GHC-Driver-Env-Types.html#t:HscEnv

I'm also not sure if this would be "better" in any meaningful way or just different.

[tfausak]

I forgot to mention that I made this issue because of #11 (comment) and more specifically https://gitlab.haskell.org/ghc/ghc/-/issues/21730#note_436908.

[tfausak]

I know how to do the scaffolding/refactoring part of this, but I'm not sure how to generate an Exact from a Qual. Summarizing my current thoughts:

• A Qual is a ModuleName and an OccName together. Those are both essentially string-based. Nothing fancy going on.
• An Exact is just a wrapper around a Name.
• I think mkExternalName is the right constructor for making a Name in this situation. It requires four things: Unique, Module (not ModuleName), OccName, and SrcSpan.
• Getting a Unique is easy. It's available through the HscEnv. Although it might be better to use one of the name cache helper functions instead of dealing with Unique directly.
• Module is a tough one. We have a ModuleName, and it feels like the HscEnv should have enough context available to turn that into a Module. However I'm not familiar with this stuff and there's not a clear path forward for me. Conceptually I think I need to look up the module ID in some kind of table to find its associated package, then get the unit for that package. Or perhaps you can go directly from module name to unit.
• OccName we already have from the Qual.
• It's unclear if the SrcSpan is actually necessary or if it can be faked with something like generatedSrcSpan.

So really I just need something of type ModuleName -> Hsc Module and I would be able to try this out.

[tfausak]

These will probably come in handy:

• https://hackage.haskell.org/package/ghc-9.8.2/docs/GHC-Driver-Env.html#v:hsc_units
• https://hackage.haskell.org/package/ghc-9.8.2/docs/GHC-Unit-State.html#v:lookupModuleWithSuggestions

[tfausak]

I'm closing this as "not planned" for the reasons I outlined in #14. However if someone comes along and thinks this sounds interesting, I'm open to a PR.