i'm trying to make a flake that tangles most of its own source from a literate org-mode file at evaluation time, but to do that it needs emacs available so it needs to know what system it's being evaluated on, and builtins.currentSystem doesn't seem to be available at the point where outputs is being evaluated. hardcoding it as nixpkgs.legacyPackages.x86_64-linux.emacs works for now, but hard-coding the system there seems fragile.

is there a better way of doing this?

this is what i have working:

    outputs = inputs:
      let
        pkgs = inputs.nixpkgs.legacyPackages.x86_64-linux;
        emacs = pkgs.emacsWithPackages (epkgs: [ epkgs.s epkgs.f epkgs.dash ]);
        mkSource = src: pkgs.stdenvNoCC.mkDerivation {
          name = "boxen-source";
          inherit src;

          buildPhase = ''
            ${emacs}/bin/emacs --batch --quick \
                --eval "(require 'org)" \
                --eval "(let ((org-confirm-babel-evaluate nil)) (org-babel-tangle-file \"boxen.org\"))"
          '';
          installPhase = ''
            mkdir $out
            cp -r lib packages homes modules systems files $out/
            cp boxen.org $out/
          '';
        };
        src = mkSource ./.;
      in …

In reply to @petrichor:envs.net

this is what i have working:

    outputs = inputs:
      let
        pkgs = inputs.nixpkgs.legacyPackages.x86_64-linux;
        emacs = pkgs.emacsWithPackages (epkgs: [ epkgs.s epkgs.f epkgs.dash ]);
        mkSource = src: pkgs.stdenvNoCC.mkDerivation {
          name = "boxen-source";
          inherit src;

          buildPhase = ''
            ${emacs}/bin/emacs --batch --quick \
                --eval "(require 'org)" \
                --eval "(let ((org-confirm-babel-evaluate nil)) (org-babel-tangle-file \"boxen.org\"))"
          '';
          installPhase = ''
            mkdir $out
            cp -r lib packages homes modules systems files $out/
            cp boxen.org $out/
          '';
        };
        src = mkSource ./.;
      in …

In reply to @petrichor:envs.net
Artturin: thanks, i'm aware of this, but i still need to then instantiate this derivation while defining the flake's outputs, which requires a specific system rather than mapping over a range of possible systems