Ability to rewrite imports

[AriFordsham]

This is motivated by the GHC API changes which, for a large part, change module names whilst leaving definitions unchanged.

It would be extremely useful to have the ability to change every import A to import B.

This issue involves a question:

• Is this feasible with the current Retrie library? Can I have some guidance on what it would look like? I see a function getImports, but it's undocumented.

And a feature request:

• Can we have this integrated in the interface/CLI?

[xich]

Hmmm, this isn't currently supported, but wouldn't be hard to add.

I think we'd want to nail down exactly what we'd want to support, because it will inform the implementation. Some thoughts:

1. We could have something like --change-import "Foo.Bar" "Baz.Quux" which is effectively a rewrite on the ModuleName type.
2. We could support rewriting the whole import statement: --change-import "import Foo.Bar" "import Baz.Quux as Baz"
3. Do we want to support rewriting explicit imports? (Maybe in v2 of this feature?) --change-import "import Foo.Bar (bar)" "import Baz.Quux (bar)"
4. If we did support explicit imports, do we want quantifiers? --change-import "forall x y. import Foo.Bar (x, bar, y) = import Baz.Quux (y, bar, x)"

What do you think?

[AriFordsham]

Regarding addressing the GHC use case, there are many ways of approaching the problem. Just implementing (1) if that is quicker/easier would definitely be already helpful.

In an ideal world, I'm not sure I understand (4) completely If it's just to reorder things, I don't see the point. It seems odd to index imported symbols by position/variable, as they're typically not thought of that way and the forall looks a bit confusing. However (3) seems great. What would be the drawback of providing extra power (besides development effort, which I can't really comment on?)

Some background on my GHC migration ideas: https://mail.haskell.org/pipermail/ghc-devs/2021-August/020099.html, although I don't have time to commit to this right now.