nix develop --build and phases confusion

[thufschmitt]

When buildPhase is defined, nix develop --build does not work due to confusion between the variable and function during stdenv eval.

{
  description = "A flake that uses Hello World";
  # inputs.flake01.url = "/tmp/flake01";
  outputs = { self, nixpkgs }: {
    defaultPackage.x86_64-linux =
      with import nixpkgs { system = "x86_64-linux"; };
      stdenv.mkDerivation {
        name = "flake02";
        src = ./d;
        buildInputs = [ hello ];
        buildPhase = ''
          hello >> ./hello.txt
        '';
        installPhase = ''
          mkdir -p $out
          install -m 644 -t $out/ hello.txt
        '';
      };
  };
}

need feedback:
flox@c03252c

Or change the order here in stdenv? https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/generic/setup.sh#L72-L80

[tomjnixon]

I ran into this too, and it was a bit confusing. This also applies to using nix-shell. Three possible resolutions for this:

1. Add a function in the default builder that you could run with runPhase buildPhase (or runPhase build?), which would have the same behavior as this line, and update all the documentation to say to use this rather than buildPhase.

2. Change the default phase definitions to evaluate the contents of the corresponding variable, and have default definitions of those variables with the current behavior.

3. Just update the documentation to say that you might need to run eval "$buildPhase" rather than buildPhase if it's overridden, and add a change in nix to make --build etc. work properly.

IMO 1 is the cleanest, but is annoying to type and there will be lots of tutorials etc. that will never be fixed. 2 is pragmatic, but still results in some confusion with user-defined (or non-default) phases, or where buildPhase is re-defined as a function. 3 is lowest risk but is a bad user experience.

[Ninlives]

Any progress? I ran into this problem too, which is very confusing to me.

[thufschmitt]

Not really. I think this could be somewhat easily be worked around in Nix by replacing the runHook call here by something like what stdenv does. Do you want to open a PR with that?

[Ninlives]

But which implementation is preferred? The commit mentioned has already done this but it's not accepted.

[thufschmitt]

The commit mentioned has already done this but it's not accepted.

Oh, I'd forgotten about that 🤦‍♂️
It never made it to a PR actually. But it looks good to me. @tomberek was there any specific reason why you didn't submit it?

[Ninlives]

@thufschmitt I've created a PR for nixpkgs in preparation of fixing this issue, please take a look at it.