{ pkgs, lib, ... }:

# Write an execline script, represented as nested nix lists.
# Everything is escaped correctly.
# https://skarnet.org/software/execline/

# TODO(Profpatsch) upstream into nixpkgs

let
  # replaces " and \ to \" and \\ respectively and quote with "
  # e.g.
  #   a"b\c -> "a\"b\\c"
  #   a\"bc -> "a\\\"bc"
  escapeExeclineArg = arg:
    ''"${builtins.replaceStrings [ ''"'' ''\'' ] [ ''\"'' ''\\'' ] (toString arg)}"'';

  # Escapes an execline (list of execline strings) to be passed to execlineb
  # Give it a nested list of strings. Nested lists are interpolated as execline
  # blocks ({}).
  # Everything is quoted correctly.
  #
  # Example:
  #   escapeExecline [ "if" [ "somecommand" ] "true" ]
  #   == ''"if" { "somecommand" } "true"''
  escapeExecline = execlineList: lib.concatStringsSep " "
    (let
      go = arg:
        if      builtins.isString arg then [(escapeExeclineArg arg)]
        else if builtins.isPath arg then [(escapeExeclineArg "${arg}")]
        else if lib.isDerivation arg then [(escapeExeclineArg arg)]
        else if builtins.isList arg then [ "{" ] ++ builtins.concatMap go arg ++ [ "}" ]
        else abort "escapeExecline can only hande nested lists of strings, was ${lib.generators.toPretty {} arg}";
     in builtins.concatMap go execlineList);

in

name:
{
  # "var": substitute readNArgs variables and start $@
  # from the (readNArgs+1)th argument
  # "var-full": substitute readNArgs variables and start $@ from $0
  # "env": don’t substitute, set # and 0…n environment vaariables, where n=$#
  # "none": don’t substitute or set any positional arguments
  # "env-no-push": like "env", but bypass the push-phase. Not recommended.
  argMode ? "var",
  # Number of arguments to be substituted as variables (passed to "var"/"-s" or "var-full"/"-S"
  readNArgs ? 0,
}:
# Nested list of lists of commands.
# Inner lists are translated to execline blocks.
argList:

let
  env =
    if      argMode == "var" then "s${toString readNArgs}"
    else if argMode == "var-full" then "S${toString readNArgs}"
    else if argMode == "env" then ""
    else if argMode == "none" then "P"
    else if argMode == "env-no-push" then "p"
    else abort ''"${toString argMode}" is not a valid argMode, use one of "var", "var-full", "env", "none", "env-no-push".'';

in
  # TODO(Profpatsch): rewrite `writeScript` with `runExecline`
  pkgs.writeScript name ''
    #!${pkgs.execline}/bin/execlineb -W${env}
    ${escapeExecline argList}
  ''