nixpkgs/lib/options.nix

# Nixpkgs/NixOS option handling.
{ lib }:

with lib.trivial;
with lib.lists;
with lib.attrsets;
with lib.strings;

rec {

  /* Returns true when the given argument is an option

     Type: isOption :: a -> bool

     Example:
       isOption 1             // => false
       isOption (mkOption {}) // => true
  */
  isOption = lib.isType "option";

  /* Creates an Option attribute set. mkOption accepts an attribute set with the following keys:

     All keys default to `null` when not given.

     Example:
       mkOption { }  // => { _type = "option"; }
       mkOption { defaultText = "foo"; } // => { _type = "option"; defaultText = "foo"; }
  */
  mkOption =
    {
    # Default value used when no definition is given in the configuration.
    default ? null,
    # Textual representation of the default, for the manual.
    defaultText ? null,
    # Example value used in the manual.
    example ? null,
    # String describing the option.
    description ? null,
    # Related packages used in the manual (see `genRelatedPackages` in ../nixos/doc/manual/default.nix).
    relatedPackages ? null,
    # Option type, providing type-checking and value merging.
    type ? null,
    # Function that converts the option value to something else.
    apply ? null,
    # Whether the option is for NixOS developers only.
    internal ? null,
    # Whether the option shows up in the manual.
    visible ? null,
    # Whether the option can be set only once
    readOnly ? null,
    } @ attrs:
    attrs // { _type = "option"; };

  /* Creates an Option attribute set for a boolean value option i.e an
     option to be toggled on or off:

     Example:
       mkEnableOption "foo"
       => { _type = "option"; default = false; description = "Whether to enable foo."; example = true; type = { ... }; }
  */
  mkEnableOption =
    # Name for the created option
    name: mkOption {
    default = false;
    example = true;
    description = "Whether to enable ${name}.";
    type = lib.types.bool;
  };

  /* This option accepts anything, but it does not produce any result.

     This is useful for sharing a module across different module sets
     without having to implement similar features as long as the
     values of the options are not accessed. */
  mkSinkUndeclaredOptions = attrs: mkOption ({
    internal = true;
    visible = false;
    default = false;
    description = "Sink for option definitions.";
    type = mkOptionType {
      name = "sink";
      check = x: true;
      merge = loc: defs: false;
    };
    apply = x: throw "Option value is not readable because the option is not declared.";
  } // attrs);

  mergeDefaultOption = loc: defs:
    let list = getValues defs; in
    if length list == 1 then head list
    else if all isFunction list then x: mergeDefaultOption loc (map (f: f x) list)
    else if all isList list then concatLists list
    else if all isAttrs list then foldl' lib.mergeAttrs {} list
    else if all isBool list then foldl' lib.or false list
    else if all isString list then lib.concatStrings list
    else if all isInt list && all (x: x == head list) list then head list
    else throw "Cannot merge definitions of `${showOption loc}' given in ${showFiles (getFiles defs)}.";

  mergeOneOption = loc: defs:
    if defs == [] then abort "This case should never happen."
    else if length defs != 1 then
      throw "The unique option `${showOption loc}' is defined multiple times, in ${showFiles (getFiles defs)}."
    else (head defs).value;

  /* "Merge" option definitions by checking that they all have the same value. */
  mergeEqualOption = loc: defs:
    if defs == [] then abort "This case should never happen."
    else foldl' (val: def:
      if def.value != val then
        throw "The option `${showOption loc}' has conflicting definitions, in ${showFiles (getFiles defs)}."
      else
        val) (head defs).value defs;

  /* Extracts values of all "value" keys of the given list.

     Type: getValues :: [ { value :: a } ] -> [a]

     Example:
       getValues [ { value = 1; } { value = 2; } ] // => [ 1 2 ]
       getValues [ ]                               // => [ ]
  */
  getValues = map (x: x.value);

  /* Extracts values of all "file" keys of the given list

     Type: getFiles :: [ { file :: a } ] -> [a]

     Example:
       getFiles [ { file = "file1"; } { file = "file2"; } ] // => [ "file1" "file2" ]
       getFiles [ ]                                         // => [ ]
  */
  getFiles = map (x: x.file);

  # Generate documentation template from the list of option declaration like
  # the set generated with filterOptionSets.
  optionAttrSetToDocList = optionAttrSetToDocList' [];

  optionAttrSetToDocList' = prefix: options:
    concatMap (opt:
      let
        docOption = rec {
          loc = opt.loc;
          name = showOption opt.loc;
          description = opt.description or (throw "Option `${name}' has no description.");
          declarations = filter (x: x != unknownModule) opt.declarations;
          internal = opt.internal or false;
          visible = opt.visible or true;
          readOnly = opt.readOnly or false;
          type = opt.type.description or null;
        }
        // optionalAttrs (opt ? example) { example = scrubOptionValue opt.example; }
        // optionalAttrs (opt ? default) { default = scrubOptionValue opt.default; }
        // optionalAttrs (opt ? defaultText) { default = opt.defaultText; }
        // optionalAttrs (opt ? relatedPackages && opt.relatedPackages != null) { inherit (opt) relatedPackages; };

        subOptions =
          let ss = opt.type.getSubOptions opt.loc;
          in if ss != {} then optionAttrSetToDocList' opt.loc ss else [];
      in
        [ docOption ] ++ subOptions) (collect isOption options);


  /* This function recursively removes all derivation attributes from
     `x` except for the `name` attribute.

     This is to make the generation of `options.xml` much more
     efficient: the XML representation of derivations is very large
     (on the order of megabytes) and is not actually used by the
     manual generator.
  */
  scrubOptionValue = x:
    if isDerivation x then
      { type = "derivation"; drvPath = x.name; outPath = x.name; name = x.name; }
    else if isList x then map scrubOptionValue x
    else if isAttrs x then mapAttrs (n: v: scrubOptionValue v) (removeAttrs x ["_args"])
    else x;


  /* For use in the `example` option attribute. It causes the given
     text to be included verbatim in documentation. This is necessary
     for example values that are not simple values, e.g., functions.
  */
  literalExample = text: { _type = "literalExample"; inherit text; };

  # Helper functions.

  /* Convert an option, described as a list of the option parts in to a
     safe, human readable version.

     Example:
       (showOption ["foo" "bar" "baz"]) == "foo.bar.baz"
       (showOption ["foo" "bar.baz" "tux"]) == "foo.\"bar.baz\".tux"
  */
  showOption = parts: let
    escapeOptionPart = part:
      let
        escaped = lib.strings.escapeNixString part;
      in if escaped == "\"${part}\""
         then part
         else escaped;
    in (concatStringsSep ".") (map escapeOptionPart parts);
  showFiles = files: concatStringsSep " and " (map (f: "`${f}'") files);
  unknownModule = "<unknown-file>";

}