CI RFC: remove dependency on cabal-plan

[ulysses4ever]

Currently, our CI cabal installs cabal-plan on every validate job. Here're timings of every step in a Validate ubuntu-20.04 ghc-9.4.2 run (only longer-than-minute steps are shown):

Step	Time
Run haskell/actions/setup@v2	2m 19s
Install cabal-plan	5m 42s
Validate build	15m 5s
Validate lib-test	1m 9s
Validate lib-suite	4m 33s
Validate cli-test	4m 31s
Validate cli-suite	11m 29s

My rough calculation is that the cabal-plan step takes more than 10% of time (~13% perhaps). I think it's a non-negligable price. Why do we pay it?

If you look into validate.sh -- the actual validation script, it uses cabal-plan for two use cases:

1. cabal-plan list-bin to find executables built by cabal
2. cabal-plan top to list all packages planned for the build topologically sorted.

The first use case is obsolete since we have cabal list-bin. It should be easy to swap one for another.

The second use case doesn't feel useful to me. Does anyone have a different feeling? Does it make sense to pay 13% of time for it?

If the answer for the latter question is No, I think we should consider removing dependency on cabal-plan.

---

There's one more reason I'd like to remove this dependency: cabal-plan does not update for newer GHCs particularly fast. E.g. GHC 9.6.1 has been released more than one month ago, and cabal-plan still does not build without allow-newer.

---

If people think that we should have cabal-plan in CI, I think we could do it and solve the two problems I mention here via prebuilt binaries: downloading those will both save most of the time and relieve us of compatibility considerations.

[andreabedini]

I’m ok with this. I remember that when I tried replacing cabal plan list-bin with cabal list-bin something wasn’t working as expected but perhaps things have changed now.

[andreabedini]

I have forgotten all the details but in #7952 i have a commit that removes cabal plan 3c8dd20 and another one that puts in back because cabal list-bin didn’t quite work the same 77ae21a

IMHO it’s worth trying again though.

[fgaz]

I think it was because of #7679 (and also it started building stuff according to #7648 (comment) ?!?)

If cabal list-bin works now, i agree on removing cabal-plan

[Mikolaj]

Me too. Mostly for simplicity. One less moving part.

[ulysses4ever]

I just saw that HaskellWeekly uses cabal freeze to get the whole plan. Maybe I should have done it in #8893. But oh well, we have something, and CI doesn't spend time on building cabal-plan anymore (#8893). Closing…