layered.nix

{ pkgs ? import <nixpkgs> { }, ... }: rec {
  appendDerivation = f: f // {
    __functor = self: arg: pkgs.runCommand "${f.pname}/S" self arg;
  };

  runList = prev: cmd: pkgs.runCommand "command-list"
    {
      # __functor = self: nextCommand: (if nextCommand == null then self else (runList self nextCommand));
    } ''
    ${restoreEnv prev}
    ${startEnv}
    ${cmd}
    ${saveEnv}
  '';

  dockerlike = { first ? startList, list }:
    if list == [ ]
    then first
    else dockerlike { first = runList first (builtins.head list); list = builtins.tail list; }
  ;

  startList = pkgs.runCommand "command-name"
    { }
    ''
      ${startEnv}
      ${saveEnv}
    '';

  testList = dockerlike
    {
      list = [
        "ls -hal > $out/test"
        "export a=12"
        "export b=42"
        "echo $((a + b)) > $out/test2"
      ];
    };

  buildSomething = [
    "touch test"
    "export A=12"
    "echo $A > $out"
  ];
  system = "x86_64-linux";

  startEnv = ''
    env | sort > $TMP/.startenv
  '';

  saveEnv = ''
    mkdir -p $out
    env | sort | comm - $TMP/.startenv -23 >> $out/.envdiff
  '';

  restoreEnv = from: ''
    # having a fuse-overlayfs would be way better.
    cp -r ${from} $out
    chmod -R u+w $out
    source ${from}/.envdiff
  '';

  testSaving = with pkgs; runCommand "agdaa"
    { } ''
    ${startEnv}
    export ALEPH=mowmow
    ${saveEnv}
  '';
  runScript = { shell ? "/bin/sh", script, name ? "script", extra ? { } }: derivation
    {
      inherit name system;
      builder = shell;
      args = [
        (builtins.toFile "builder-${name}.sh" script)
      ];
    } // extra;

  runShScript = runScript {
    name = "asdf";
    script = ''
      export PATH=$PATH:${pkgs.coreutils}
      touch test
    '';
    extra = {
      buildInputs = [ pkgs.coreutils ];
    };
  };
}
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