RFC remove allow-older and allow-newer support from Setup.hs

[phadej]

Shortly: revert changes made in #3163 & #3165, as well as followed --allow-older changes.

Motivation: Such functionality isn't necessary (not used by anyone) to be provided by Cabal/Setup.hs^†, and results into duplicate code (starting from argument parsing etc). --exact-configuration should be enough for everyone to force "I know what I'm doing". For the situations were "I almost know what I do, help me with rest", one could/should use external tools (e.g. cabal-install could have cabal solver command to produce configuration file).

Optional:

• Add --exact-configuration to cabal-install
• Add cabal solver (we now use cabal configure -v2 --dry-run, but it feels semantically wrong).

Alternative: leave --allow-newer in Cabal, only remove followed --allow-older changes. IMHO this would lead to confusion. --allow-older hasn't hit Hackage AFAIK.

Note: this is proposal to remove them only from Cabal-the-library, cabal-install will continue to have them.

^† AFAICS stack opted to use --exact-configuration commercialhaskell/stack#1579 (comment)

ping @hvr, @ezyang, @23Skidoo, @dcoutts

[ezyang]

OK, fine. But we need a test which demonstrates that --exact-configuration is sufficient to override the package's build-depends.

[kosmikus]

Seems I'm missing a whole lot. In particular, I have not the slightest idea what --exact-configuration is or does. Where can I read up on this?

[ezyang]

@kosmikus The situation is that Cabal the library has a miniature dependency solver, which picks dependencies when a user runs ./Setup configure. This solver is run even when you say --dependency="foo=foo-0.5-blahblah" because you may not have specified a complete set of dependencies and flags. Of course, if you're running cabal-install, by the time you get to the setup script you know exactly what dependencies you want. So you can then say ./Setup configure --exact-configuration --dependency="foo=foo-0.5-blahblah" and then no solver is run; we just take the command line arguments on faith.

exact-configuration is completely undocumented and I know about it from reading the configure source code.

[23Skidoo]

Motivation: Such functionality isn't necessary (not used by anyone)

Well, the reason it was added was because a user requested it. If @snoyberg no longer needs it, we can reconsider, but I don't think removing it buys us that much. #3165 added ~100 lines total, and more than half of those were comments and tests.

[snoyberg]

CCing @mgsloan, who will know more about this than I do.

[mgsloan]

We don't need such a flag. I recall a discussion that likely caused this to be implemented.

stack build --allow-newer is more permissive than cabal-install's allow-newer. It causes all version constraints to instead be warnings. As far as I know, cabal-install does not have this capability. I guess it would mean installing all the latest versions, maybe?

[acertain]

Nix currently runs Setup.hs, and not cabal configure (or any cabal command), so this is needed to avoid modifying cabal files afaik (which on nix is fairly involved). I was using this.

EDIT: Nevermind, nix has doJailbreak