Is Nix integration broken?

[yvan-sraka]

Describe the bug
I can't manage, following the Nix Integration official tutorial, to make it works. Since I use Nix as my daily driver for years, I wonder, it is the documentation that is really misleading? Or is the feature broken, and no one reported it before because there is no user of it?

To Reproduce
I wanted to give Cabal Nix integration a try, so I created a dummy project with a shell.nix that set an environment variable that a Setup.hs will try to read.

• shell.nix:

{ pkgs ? import <nixpkgs> {} }:
  pkgs.mkShell {
    shellHook = "touch NIX_SHELL_HOOK_CALLED";
    VARIABLE_SET_BY_NIX_SHELL = "Hello from Nix shell!";
}

• Setup.hs:

import Distribution.Simple
import System.Environment

main :: IO ()
main = do
    x <- getEnv "VARIABLE_SET_BY_NIX_SHELL"
    putStrLn x
    defaultMain

So, on a machine with Cabal and GHC installed (or in a developer shell spawned with nix-shell -p cabal-install ghc), you can build the project with nix-shell --run "cabal build" or the new experimental command nix develop -c cabal build. It works, and the shell.nix does 2 others side effect for debugging: it prints Hello from Nix shell! (it's Setup.hs that reads and displays the ENV variable VARIABLE_SET_BY_NIX_SHELL) on standard output and touch the file NIX_SHELL_HOOK_CALLED.

So, then if I understand well the Nix integration documentation, I could replace the previous commands by just cabal --enable-nix build or cabal --nix=True build, right? Both of that commands will not trigger any of the side effects defined in the shell.nix and so the build will fail:

$ cabal --enable-nix build
Resolving dependencies...
Build profile: -w ghc-9.2.8 -O1
In order, the following will be built (use -v for more details):
 - test-0.1.0.0 (exe:test) (first run)
[1 of 1] Compiling Main             ( /home/yvan/is-cabal-nix-integration-broken/dist-newstyle/build/x86_64-linux/ghc-9.2.8/test-0.1.0.0/setup/setup.hs, /home/yvan/is-cabal-nix-integration-broken/dist-newstyle/build/x86_64-linux/ghc-9.2.8/test-0.1.0.0/setup/Main.o )
Linking /home/yvan/is-cabal-nix-integration-broken/dist-newstyle/build/x86_64-linux/ghc-9.2.8/test-0.1.0.0/setup/setup ...
setup: VARIABLE_SET_BY_NIX_SHELL: getEnv: does not exist (no environment variable)
Error: cabal: Failed to build test-0.1.0.0. The failure occurred during the
configure step.

Expected behavior
cabal --enable-nix build and cabal --nix=True build should have the same behavior as nix-shell --run "cabal build" or nix develop -c cabal build.

System information

$ uname -a
Linux X230 6.1.42 #1-NixOS SMP PREEMPT_DYNAMIC Thu Jul 27 06:50:53 UTC 2023 x86_64 GNU/Linux
$ cabal --version
cabal-install version 3.10.1.0
compiled using version 3.10.1.0 of the Cabal library 
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 9.2.8

Additional context
This issue was mentioned in the last “cabal devs fortnightly chat”, and the consensus was to depreciated progressively the feature. I will open a PR with a plan for that! :)

[ulysses4ever]

Hey! It has never worked with v2 commands (which is what you you get when you say cabal build these days). Probably a dup of #6444.