Support Nix in cabal-install

[ttuegel]

Stack has been integrated with Nix for some time. We are actually in a better position in terms of integration because we keep persistent configuration data. Full integration would take a modest amount of additional work:

• add command-line flags and configuration options to enable or disable integration
• if Nix is enabled and the project has a shell.nix file, instantiate it to a shell.drv during cabal configure
• if Nix is enabled, wrap all calls to cabal ... in nix-shell: nix-shell --run "cabal ..."

This is slightly different than Stack's approach because it requires a shell.nix file, but the cabal2nix utility can generate those automatically from any .cabal file (and most Nix+Haskell users would be familiar with this tool).

[ezyang]

Not a Nix-user, but if a Nixer (I guess you?) maintains the code I don't see any problem with putting this into cabal-install.

[ttuegel]

Not a Nix-user, but if a Nixer (I guess you?) maintains the code I don't see any problem with putting this into cabal-install.

That's the idea, to wean me away from Stack! 😉

For posterity, here is the zsh function I've been using to do the same thing:

nix-cabal () {
    CABAL=$(whence -p cabal)  # always use same cabal as current shell
    if [[ -a "$PWD/.shell.drv" ]]; then
        nix-shell --add-root "$PWD/.result" --indirect \
            "$PWD/.shell.drv" --run "$CABAL $*"
    elif [[ -a "$PWD/shell.nix" ]]; then
        nix-instantiate --add-root "$PWD/.shell.drv" --indirect \
            "$PWD/shell.nix" -A env
        nix-shell --add-root "$PWD/.result" --indirect \
            "$PWD/.shell.drv" --run "$CABAL $*" || rm "$PWD/.shell.drv"
    else
        nix-shell --add-root "$PWD/.result" --indirect --run "$CABAL $*"
    fi
}

I realize that probably means next to nothing to non-Nix-users; I just wanted to illustrate that the maintenance burden can be small because the logic doesn't need to be as complicated as Stack makes it.

[ryantrinkle]

@ttuegel That looks really nice! I hope you don't mind me stealing some of that logic for my own scripts :)

[guaraqe]

This would be extremely welcome!

[ezyang]

Not a priority for 2.0, but @ttuegel if you think you can figure something out in time for 2.0 we can remilestone

[ttuegel]

@ezyang I agree it's not a priority for 2.0. I've discussed this with several Nix people, and I think it will work better with new-build anyway. Maybe we could make a milestone for 3.0? I usually milestone things as _|_ only when nobody has any intention of working on them.

[ttuegel]

I am re-milestoning this because #3728 is blocked on it; however, I don't think either should be a release blocker.

[ttuegel]

After some conversation with @ezyang, I think that #3728 is not really blocked on this. I am going to leave it on the 2.0 milestone at low priority, though, because I have already implemented most of it 😃

[23Skidoo]

@ttuegel Can this ticket be closed as fixed?

[ttuegel]

Indeed, I pushed the fix for this but completely forgot about the issue!

[ysangkok]

Seems like this was fixed in #4193