devshell/lib/modules/main.nix

{
  pkgs,
  lib ? pkgs.lib,
  config,
  ...
}: let
  inherit (lib) mapAttrs mkOption types textClosureMap id attrNames;
  mkNakedShell = pkgs.callPackage ./../utils/mkNakedShell.nix {};

  addAttributeName = prefix:
    mapAttrs (
      k: v:
        v
        // {
          text = ''
            #### ${prefix}.${k}
            ${v.text}
          '';
        }
    );

  envBash =
    pkgs.writeText "devshell-env.bash"
    # sh
    ''
      export DEVSHELL_DIR=@DEVSHELL_DIR@
      # nicely handle it at the front or between/end
      PATH=''${PATH#/path-not-set:}
      PATH=''${PATH#:/path-not-set}
      export PATH=$DEVSHELL_DIR/bin:$PATH

      ${textClosureMap
        id
        (addAttributeName "startup" config.enterShellCommands)
        (attrNames config.enterShellCommands)}

      # interactive session
      if [[ $- == *i* ]]; then
        true
        ${textClosureMap
        id
        (addAttributeName "interactive" config.interactiveShellCommands)
        (attrNames config.interactiveShellCommands)}
      fi # interactive session

      unset DEVSHELL_DIR
    '';

  entryType = types.submodule {
    options = {
      text = mkOption {
        type = types.str;
        description = ''
          Script to run.
        '';
      };

      deps = mkOption {
        type = types.listOf types.str;
        default = [];
        description = ''
          A list of other steps that this one depends on.
        '';
      };
    };
  };
in {
  options = {
    shell = {
      finalPackage = mkOption {
        type = types.package;
        internal = true;
      };
      finalProfile = mkOption {
        type = types.package;
        internal = true;
      };
    };

    name = mkOption {
      type = types.str;
      default = "devshell";
      description = ''
        Name of the shell.
      '';
    };
    packages = mkOption {
      type = types.listOf types.package;
      default = [];
      description = ''
        Packages to install into the devshell.
      '';
    };
    enterShellCommands = mkOption {
      type = types.attrsOf entryType;
      default = {};
      description = ''
        Commands to run when entering the shell.
      '';
      example = {
        "hello".text = "echo Hello world";
      };
    };
    interactiveShellCommands = mkOption {
      type = types.attrsOf entryType;
      default = {};
      description = ''
        Commands to run when entering an interactive shell.
      '';
      example = {
        "hello".text = "echo Hello world";
      };
    };
  };

  config.shell = {
    finalProfile = pkgs.buildEnv {
      name = "${config.name}-profile";
      paths = config.packages;
      postBuild = ''
        substitute ${envBash} $out/env.bash --subst-var-by DEVSHELL_DIR $out
        # NOTE: maybe also add an entrypoint shell script at $out/bin/devshell-${config.name}?
      '';
      meta.mainProgram = "devshell-${config.name}";
    };
    finalPackage = mkNakedShell {
      inherit (config) name;
      profile = config.shell.finalProfile;
      passthru = {
        metadata = config.meta.finalMetadata;
        metadataJson = config.meta.finalJson;
      };
    };
  };
}