Cabal: Meta-issue: Remove need for fake-package

[phadej]

The internals of cabal-install should allow project planning without writing things to file system.

This is high-level goal, and I don't think that it could be solved with single PR. But internal changes moving us closer to it will improve state of the codebase, which is very welcome!

[fendor]

Where is this fake-package used?
I know of:

• cabal scrips

[phadej]

@fendor, install, run, repl outside of project directories. About everywhere where we "solve", but don't have a local project.

[phadej]

6885 is another (upcoming) example, where planning without relying (selectively!) on "real world" / "filesystem" inputs is desired.

[alexbiehl]

We could modify the Rebuild monad. Currently it's the one that creates all the caches and triggers rebuilding in case any input changed. By providing configuration or a different interpreter we could just make it not cache things and rebuild all the time.

Pair that with allowing an empty, implicit project and we might be golden.

[phadej]

@alexbiehl yes, that what I have tried. Yet Rebuild monad is used very little, here and there. One clear subgoal is to make project planning contained completely in Rebuild monad.

[alexbiehl]

One clear subgoal is to make project planning contained completely in Rebuild monad

Yes. Though a less invasive change might be to pass down the configuration/interpreter to the runRebuild calls. They are not too many. Maybe it even makes sense to tie the runRebuild to DistDirLayout; we are passing that around everywhere and usually the call looks like runRebuild distDirProjectRoot $ do ....

[phadej]

... except the very first runRebuild is runRebuild "".

I also don't understand why we work with relative paths at all. That's another yak to shave. #6667 might help.

EDIT: I emphasize, this shouldn't be fixed in a "smallest possible diff" way, but in the "cleanest end result" way.

[typedrat]

This is very interesting. I never liked the whole fake-package chicanery, and I think I still understand how that part of Cabal works pretty well, even if I haven't been up to my arms in it recently.

I had a few thinks about how this could be done, actually. Let me try to find my old notes!

[fgaz]

6999 is a specific bug caused by fake-package