nixpkgs / lib / customisation.nix

{ lib }:

let
  inherit (builtins)
    intersectAttrs
    unsafeGetAttrPos
    ;
  inherit (lib)
    functionArgs
    isFunction
    mirrorFunctionArgs
    isAttrs
    setFunctionArgs
    optionalAttrs
    attrNames
    filter
    elemAt
    concatStringsSep
    sortOn
    take
    length
    filterAttrs
    optionalString
    flip
    head
    pipe
    isDerivation
    listToAttrs
    mapAttrs
    seq
    flatten
    deepSeq
    extends
    toFunction
    id
    ;
  inherit (lib.strings) levenshtein levenshteinAtMost;

in
rec {

  /**
    `overrideDerivation drv f` takes a derivation (i.e., the result
    of a call to the builtin function `derivation`) and returns a new
    derivation in which the attributes of the original are overridden
    according to the function `f`.  The function `f` is called with
    the original derivation attributes.

    `overrideDerivation` allows certain "ad-hoc" customisation
    scenarios (e.g. in ~/.config/nixpkgs/config.nix).  For instance,
    if you want to "patch" the derivation returned by a package
    function in Nixpkgs to build another version than what the
    function itself provides.

    For another application, see build-support/vm, where this
    function is used to build arbitrary derivations inside a QEMU
    virtual machine.

    Note that in order to preserve evaluation errors, the new derivation's
    outPath depends on the old one's, which means that this function cannot
    be used in circular situations when the old derivation also depends on the
    new one.

    You should in general prefer `drv.overrideAttrs` over this function;
    see the nixpkgs manual for more information on overriding.

    # Inputs

    `drv`

    : 1\. Function argument

    `f`

    : 2\. Function argument

    # Type

    ```
    overrideDerivation :: Derivation -> ( Derivation -> AttrSet ) -> Derivation
    ```

    # Examples
    :::{.example}
    ## `lib.customisation.overrideDerivation` usage example

    ```nix
    mySed = overrideDerivation pkgs.gnused (oldAttrs: {
      name = "sed-4.2.2-pre";
      src = fetchurl {
        url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2;
        hash = "sha256-MxBJRcM2rYzQYwJ5XKxhXTQByvSg5jZc5cSHEZoB2IY=";
      };
      patches = [];
    });
    ```

    :::
  */
  overrideDerivation =
    drv: f:
    let
      newDrv = derivation (drv.drvAttrs // (f drv));
    in
    flip (extendDerivation (seq drv.drvPath true)) newDrv (
      {
        meta = drv.meta or { };
        passthru = if drv ? passthru then drv.passthru else { };
      }
      // (drv.passthru or { })
      // optionalAttrs (drv ? __spliced) {
        __spliced = { } // (mapAttrs (_: sDrv: overrideDerivation sDrv f) drv.__spliced);
      }
    );

  /**
    `makeOverridable` takes a function from attribute set to attribute set and
    injects `override` attribute which can be used to override arguments of
    the function.

    Please refer to  documentation on [`<pkg>.overrideDerivation`](#sec-pkg-overrideDerivation) to learn about `overrideDerivation` and caveats
    related to its use.

    # Inputs

    `f`

    : 1\. Function argument

    # Type

    ```
    makeOverridable :: (AttrSet -> a) -> AttrSet -> a
    ```

    # Examples
    :::{.example}
    ## `lib.customisation.makeOverridable` usage example

    ```nix
    nix-repl> x = {a, b}: { result = a + b; }

    nix-repl> y = lib.makeOverridable x { a = 1; b = 2; }

    nix-repl> y
    { override = «lambda»; overrideDerivation = «lambda»; result = 3; }

    nix-repl> y.override { a = 10; }
    { override = «lambda»; overrideDerivation = «lambda»; result = 12; }
    ```

    :::
  */
  makeOverridable =
    f:
    let
      # Creates a functor with the same arguments as f
      mirrorArgs = mirrorFunctionArgs f;
    in
    mirrorArgs (
      origArgs:
      let
        result = f origArgs;

        # Changes the original arguments with (potentially a function that returns) a set of new attributes
        overrideWith = newArgs: origArgs // (if isFunction newArgs then newArgs origArgs else newArgs);

        # Re-call the function but with different arguments
        overrideArgs = mirrorArgs (
          /**
            Change the arguments with which a certain function is called.

            In some cases, you may find a list of possible attributes to pass in this function's `__functionArgs` attribute, but it will not be complete for an original function like `args@{foo, ...}: ...`, which accepts arbitrary attributes.

            This function was provided by `lib.makeOverridable`.
          */
          newArgs: makeOverridable f (overrideWith newArgs)
        );
        # Change the result of the function call by applying g to it
        overrideResult = g: makeOverridable (mirrorArgs (args: g (f args))) origArgs;
      in
      if isAttrs result then
        result
        // {
          override = overrideArgs;
          overrideDerivation = fdrv: overrideResult (x: overrideDerivation x fdrv);
          ${if result ? overrideAttrs then "overrideAttrs" else null} =
            /**
              Override the attributes that were passed to `mkDerivation` in order to generate this derivation.

              This function is provided by `lib.makeOverridable`, and indirectly by `callPackage` among others, in order to make the combination of `override` and `overrideAttrs` work.
              Specifically, it re-adds the `override` attribute to the result of `overrideAttrs`.

              The real implementation of `overrideAttrs` is provided by `stdenv.mkDerivation`.
            */
            # NOTE: part of the above documentation had to be duplicated in `mkDerivation`'s `overrideAttrs`.
            #       design/tech debt issue: https://github.com/NixOS/nixpkgs/issues/273815
            fdrv: overrideResult (x: x.overrideAttrs fdrv);
        }
      else if isFunction result then
        # Transform the result into a functor while propagating its arguments
        setFunctionArgs result (functionArgs result)
        // {
          override = overrideArgs;
        }
      else
        result
    );

  /**
    Call the package function in the file `fn` with the required
    arguments automatically.  The function is called with the
    arguments `args`, but any missing arguments are obtained from
    `autoArgs`.  This function is intended to be partially
    parameterised, e.g.,

      ```nix
      callPackage = callPackageWith pkgs;
      pkgs = {
        libfoo = callPackage ./foo.nix { };
        libbar = callPackage ./bar.nix { };
      };
      ```

    If the `libbar` function expects an argument named `libfoo`, it is
    automatically passed as an argument.  Overrides or missing
    arguments can be supplied in `args`, e.g.

      ```nix
      libbar = callPackage ./bar.nix {
        libfoo = null;
        enableX11 = true;
      };
      ```

    <!-- TODO: Apply "Example:" tag to the examples above -->

    # Inputs

    `autoArgs`

    : 1\. Function argument

    `fn`

    : 2\. Function argument

    `args`

    : 3\. Function argument

    # Type

    ```
    callPackageWith :: AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a
    ```
  */
  callPackageWith =
    autoArgs: fn: args:
    let
      f = if isFunction fn then fn else import fn;
      fargs = functionArgs f;

      # All arguments that will be passed to the function
      # This includes automatic ones and ones passed explicitly
      allArgs = intersectAttrs fargs autoArgs // args;

      # a list of argument names that the function requires, but
      # wouldn't be passed to it
      missingArgs =
        # Filter out arguments that have a default value
        (
          filterAttrs (name: value: !value)
            # Filter out arguments that would be passed
            (removeAttrs fargs (attrNames allArgs))
        );

      # Get a list of suggested argument names for a given missing one
      getSuggestions =
        arg:
        pipe (autoArgs // args) [
          attrNames
          # Only use ones that are at most 2 edits away. While mork would work,
          # levenshteinAtMost is only fast for 2 or less.
          (filter (levenshteinAtMost 2 arg))
          # Put strings with shorter distance first
          (sortOn (levenshtein arg))
          # Only take the first couple results
          (take 3)
          # Quote all entries
          (map (x: "\"" + x + "\""))
        ];

      prettySuggestions =
        suggestions:
        if suggestions == [ ] then
          ""
        else if length suggestions == 1 then
          ", did you mean ${elemAt suggestions 0}?"
        else
          ", did you mean ${concatStringsSep ", " (lib.init suggestions)} or ${lib.last suggestions}?";

      errorForArg =
        arg:
        let
          loc = unsafeGetAttrPos arg fargs;
          loc' = if loc != null then loc.file + ":" + toString loc.line else "<unknown location>";
        in
        "Function called without required argument \"${arg}\" at "
        + "${loc'}${prettySuggestions (getSuggestions arg)}";

      # Only show the error for the first missing argument
      error = errorForArg (head (attrNames missingArgs));

    in
    if missingArgs == { } then
      makeOverridable f allArgs
    # This needs to be an abort so it can't be caught with `builtins.tryEval`,
    # which is used by nix-env and ofborg to filter out packages that don't evaluate.
    # This way we're forced to fix such errors in Nixpkgs,
    # which is especially relevant with allowAliases = false
    else
      abort "lib.customisation.callPackageWith: ${error}";

  /**
    Like callPackage, but for a function that returns an attribute
    set of derivations. The override function is added to the
    individual attributes.

    # Inputs

    `autoArgs`

    : 1\. Function argument

    `fn`

    : 2\. Function argument

    `args`

    : 3\. Function argument

    # Type

    ```
    callPackagesWith :: AttrSet -> ((AttrSet -> AttrSet) | Path) -> AttrSet -> AttrSet
    ```
  */
  callPackagesWith =
    autoArgs: fn: args:
    let
      f = if isFunction fn then fn else import fn;
      auto = intersectAttrs (functionArgs f) autoArgs;
      mirrorArgs = mirrorFunctionArgs f;
      origArgs = auto // args;
      pkgs = f origArgs;
      mkAttrOverridable = name: _: makeOverridable (mirrorArgs (newArgs: (f newArgs).${name})) origArgs;
    in
    if isDerivation pkgs then
      throw (
        "function `callPackages` was called on a *single* derivation "
        + ''"${pkgs.name or "<unknown-name>"}";''
        + " did you mean to use `callPackage` instead?"
      )
    else
      mapAttrs mkAttrOverridable pkgs;

  /**
    Add attributes to each output of a derivation without changing
    the derivation itself and check a given condition when evaluating.

    # Inputs

    `condition`

    : 1\. Function argument

    `passthru`

    : 2\. Function argument

    `drv`

    : 3\. Function argument

    # Type

    ```
    extendDerivation :: Bool -> Any -> Derivation -> Derivation
    ```
  */
  extendDerivation =
    condition: passthru: drv:
    let
      outputs = drv.outputs or [ "out" ];

      commonAttrs =
        drv // (listToAttrs outputsList) // { all = map (x: x.value) outputsList; } // passthru;

      outputToAttrListElement = outputName: {
        name = outputName;
        value =
          commonAttrs
          // {
            inherit (drv.${outputName}) type outputName;
            outputSpecified = true;
            drvPath =
              assert condition;
              drv.${outputName}.drvPath;
            outPath =
              assert condition;
              drv.${outputName}.outPath;
          }
          //
            # TODO: give the derivation control over the outputs.
            #       `overrideAttrs` may not be the only attribute that needs
            #       updating when switching outputs.
            optionalAttrs (passthru ? overrideAttrs) {
              # TODO: also add overrideAttrs when overrideAttrs is not custom, e.g. when not splicing.
              overrideAttrs = f: (passthru.overrideAttrs f).${outputName};
            };
      };

      outputsList = map outputToAttrListElement outputs;
    in
    commonAttrs
    // {
      drvPath =
        assert condition;
        drv.drvPath;
      outPath =
        assert condition;
        drv.outPath;
    };

  /**
    Strip a derivation of all non-essential attributes, returning
    only those needed by hydra-eval-jobs. Also strictly evaluate the
    result to ensure that there are no thunks kept alive to prevent
    garbage collection.

    # Inputs

    `drv`

    : 1\. Function argument

    # Type

    ```
    hydraJob :: (Derivation | Null) -> (Derivation | Null)
    ```
  */
  hydraJob =
    drv:
    let
      outputs = drv.outputs or [ "out" ];

      commonAttrs = {
        inherit (drv) name system meta;
        inherit outputs;
      }
      // optionalAttrs (drv._hydraAggregate or false) {
        _hydraAggregate = true;
        constituents = map hydraJob (flatten drv.constituents);
      }
      // (listToAttrs outputsList);

      makeOutput =
        outputName:
        let
          output = drv.${outputName};
        in
        {
          name = outputName;
          value = commonAttrs // {
            outPath = output.outPath;
            drvPath = output.drvPath;
            type = "derivation";
            inherit outputName;
          };
        };

      outputsList = map makeOutput outputs;

      drv' = (head outputsList).value;
    in
    if drv == null then null else deepSeq drv' drv';

  /**
    Make an attribute set (a "scope") from functions that take arguments from that same attribute set.
    See [](#ex-makeScope) for how to use it.

    # Inputs

    1. `newScope` (`AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a`)

       A function that takes an attribute set `attrs` and returns what ends up as `callPackage` in the output.

       Typical values are `callPackageWith` or the output attribute `newScope`.

    2. `f` (`AttrSet -> AttrSet`)

       A function that takes an attribute set as returned by `makeScope newScope f` (a "scope") and returns any attribute set.

       This function is used to compute the fixpoint of the resulting scope using `callPackage`.
       Its argument is the lazily evaluated reference to the value of that fixpoint, and is typically called `self` or `final`.

       See [](#ex-makeScope) for how to use it.
       See [](#sec-functions-library-fixedPoints) for details on fixpoint computation.

    # Output

    `makeScope` returns an attribute set of a form called `scope`, which also contains the final attributes produced by `f`:

    ```
    scope :: {
      callPackage :: ((AttrSet -> a) | Path) -> AttrSet -> a
      newScope = AttrSet -> scope
      overrideScope = (scope -> scope -> AttrSet) -> scope
      packages :: AttrSet -> AttrSet
    }
    ```

    - `callPackage` (`((AttrSet -> a) | Path) -> AttrSet -> a`)

      A function that

      1. Takes a function `p`, or a path to a Nix file that contains a function `p`, which takes an attribute set and returns value of arbitrary type `a`,
      2. Takes an attribute set `args` with explicit attributes to pass to `p`,
      3. Calls `f` with attributes from the original attribute set `attrs` passed to `newScope` updated with `args`, i.e. `attrs // args`, if they match the attributes in the argument of `p`.

      All such functions `p` will be called with the same value for `attrs`.

      See [](#ex-makeScope-callPackage) for how to use it.

    - `newScope` (`AttrSet -> scope`)

      Takes an attribute set `attrs` and returns a scope that extends the original scope.

    - `overrideScope` (`(scope -> scope -> AttrSet) -> scope`)

      Takes a function `g` of the form `final: prev: { # attributes }` to act as an overlay on `f`, and returns a new scope with values determined by `extends g f`.
      See [](https://nixos.org/manual/nixpkgs/unstable/#function-library-lib.fixedPoints.extends) for details.

      This allows subsequent modification of the final attribute set in a consistent way, i.e. all functions `p` invoked with `callPackage` will be called with the modified values.

    - `packages` (`AttrSet -> AttrSet`)

      The value of the argument `f` to `makeScope`.

    - final attributes

      The final values returned by `f`.

    # Examples

    :::{#ex-makeScope .example}
    # Create an interdependent package set on top of `pkgs`

    The functions in `foo.nix` and `bar.nix` can depend on each other, in the sense that `foo.nix` can contain a function that expects `bar` as an attribute in its argument.

    ```nix
    let
      pkgs = import <nixpkgs> { };
    in
    pkgs.lib.makeScope pkgs.newScope (self: {
      foo = self.callPackage ./foo.nix { };
      bar = self.callPackage ./bar.nix { };
    })
    ```

    evaluates to

    ```nix
    {
      callPackage = «lambda»;
      newScope = «lambda»;
      overrideScope = «lambda»;
      packages = «lambda»;
      foo = «derivation»;
      bar = «derivation»;
    }
    ```
    :::

    :::{#ex-makeScope-callPackage .example}
    # Using `callPackage` from a scope

    ```nix
    let
      pkgs = import <nixpkgs> { };
      inherit (pkgs) lib;
      scope = lib.makeScope lib.callPackageWith (self: { a = 1; b = 2; });
      three = scope.callPackage ({ a, b }: a + b) { };
      four = scope.callPackage ({ a, b }: a + b) { a = 2; };
    in
    [ three four ]
    ```

    evaluates to

    ```nix
    [ 3 4 ]
    ```
    :::

    # Type

    ```
    makeScope :: (AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a) -> (AttrSet -> AttrSet) -> scope
    ```
  */
  makeScope =
    newScope: f:
    let
      self = f self // {
        newScope = scope: newScope (self // scope);
        callPackage = self.newScope { };
        overrideScope = g: makeScope newScope (extends g f);
        packages = f;
      };
    in
    self;

  /**
    backward compatibility with old uncurried form; deprecated

    # Inputs

    `splicePackages`

    : 1\. Function argument

    `newScope`

    : 2\. Function argument

    `otherSplices`

    : 3\. Function argument

    `keep`

    : 4\. Function argument

    `extra`

    : 5\. Function argument

    `f`

    : 6\. Function argument
  */
  makeScopeWithSplicing =
    splicePackages: newScope: otherSplices: keep: extra: f:
    makeScopeWithSplicing' { inherit splicePackages newScope; } {
      inherit
        otherSplices
        keep
        extra
        f
        ;
    };

  /**
    Like makeScope, but aims to support cross compilation. It's still ugly, but
    hopefully it helps a little bit.

    # Type

    ```
    makeScopeWithSplicing' ::
      { splicePackages :: Splice -> AttrSet
      , newScope :: AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a
      }
      -> { otherSplices :: Splice, keep :: AttrSet -> AttrSet, extra :: AttrSet -> AttrSet }
      -> AttrSet

    Splice ::
      { pkgsBuildBuild :: AttrSet
      , pkgsBuildHost :: AttrSet
      , pkgsBuildTarget :: AttrSet
      , pkgsHostHost :: AttrSet
      , pkgsHostTarget :: AttrSet
      , pkgsTargetTarget :: AttrSet
      }
    ```
  */
  makeScopeWithSplicing' =
    {
      splicePackages,
      newScope,
    }:
    {
      otherSplices,
      # Attrs from `self` which won't be spliced.
      # Avoid using keep, it's only used for a python hook workaround, added in PR #104201.
      # ex: `keep = (self: { inherit (self) aAttr; })`
      keep ? (_self: { }),
      # Additional attrs to add to the sets `callPackage`.
      # When the package is from a subset (but not a subset within a package IS #211340)
      # within `spliced0` it will be spliced.
      # When using an package outside the set but it's available from `pkgs`, use the package from `pkgs.__splicedPackages`.
      # If the package is not available within the set or in `pkgs`, such as a package in a let binding, it will not be spliced
      # ex:
      # ```
      # nix-repl> darwin.apple_sdk.frameworks.CoreFoundation
      #   «derivation ...CoreFoundation-11.0.0.drv»
      # nix-repl> darwin.CoreFoundation
      #   error: attribute 'CoreFoundation' missing
      # nix-repl> darwin.callPackage ({ CoreFoundation }: CoreFoundation) { }
      #   «derivation ...CoreFoundation-11.0.0.drv»
      # ```
      extra ? (_spliced0: { }),
      f,
    }:
    let
      spliced0 = splicePackages {
        pkgsBuildBuild = otherSplices.selfBuildBuild;
        pkgsBuildHost = otherSplices.selfBuildHost;
        pkgsBuildTarget = otherSplices.selfBuildTarget;
        pkgsHostHost = otherSplices.selfHostHost;
        pkgsHostTarget = self; # Not `otherSplices.selfHostTarget`;
        pkgsTargetTarget = otherSplices.selfTargetTarget;
      };
      spliced = extra spliced0 // spliced0 // keep self;
      self = f self // {
        newScope = scope: newScope (spliced // scope);
        callPackage = newScope spliced; # == self.newScope {};
        # N.B. the other stages of the package set spliced in are *not*
        # overridden.
        overrideScope =
          g:
          (makeScopeWithSplicing' { inherit splicePackages newScope; } {
            inherit otherSplices keep extra;
            f = extends g f;
          });
        packages = f;
      };
    in
    self;

  /**
    Define a `mkDerivation`-like function based on another `mkDerivation`-like function.

    [`stdenv.mkDerivation`](#part-stdenv) gives access to
    its final set of derivation attributes when it is passed a function,
    or when it is passed an overlay-style function in `overrideAttrs`.

    Instead of composing new `stdenv.mkDerivation`-like build helpers
    using normal function composition,
    `extendMkDerivation` makes sure that the returned build helper
    supports such first class recursion like `mkDerivation` does.

    `extendMkDerivation` takes an extra attribute set to configure its behaviour.
    One can optionally specify
    `transformDrv` to specify a function to apply to the result derivation,
    or `inheritFunctionArgs` to decide whether to inherit the `__functionArgs`
    from the base build helper.

    # Inputs

    `extendMkDerivation`-specific configurations
    : `constructDrv` (required)
      : Base build helper, the `mkDerivation`-like build helper to extend.

      `excludeDrvArgNames` (default to `[ ]`)
      : Argument names not to pass from the input fixed-point arguments to `constructDrv`.
        It doesn't apply to the updating arguments returned by `extendDrvArgs`.

      `excludeFunctionArgNames` (default to `[ ]`)
      : `__functionArgs` attribute names to remove from the result build helper.
        `excludeFunctionArgNames` is useful for argument deprecation while avoiding ellipses.

      `extendDrvArgs` (required)
      : An extension (overlay) of the argument set, like the one taken by [overrideAttrs](#sec-pkg-overrideAttrs) but applied before passing to `constructDrv`.

      `inheritFunctionArgs` (default to `true`)
      : Whether to inherit `__functionArgs` from the base build helper.
        Set `inheritFunctionArgs` to `false` when `extendDrvArgs`'s `args` set pattern does not contain an ellipsis.

      `transformDrv` (default to `lib.id`)
      : Function to apply to the result derivation.

    # Type

    ```
    extendMkDerivation ::
      {
        constructDrv :: ((FixedPointArgs | AttrSet) -> a)
        excludeDrvArgNames :: [ String ],
        excludeFunctionArgNames :: [ String ]
        extendDrvArgs :: (AttrSet -> AttrSet -> AttrSet)
        inheritFunctionArgs :: Bool,
        transformDrv :: a -> a,
      }
      -> (FixedPointArgs | AttrSet) -> a

    FixedPointArgs = AttrSet -> AttrSet
    a = Derivation when defining a build helper
    ```

    # Examples

    :::{.example}
    ## `lib.customisation.extendMkDerivation` usage example
    ```nix-repl
    mkLocalDerivation = lib.extendMkDerivation {
      constructDrv = pkgs.stdenv.mkDerivation;
      excludeDrvArgNames = [ "specialArg" ];
      extendDrvArgs =
        finalAttrs: args@{ preferLocalBuild ? true, allowSubstitute ? false, specialArg ? (_: false), ... }:
        { inherit preferLocalBuild allowSubstitute; passthru = { inherit specialArg; } // args.passthru or { }; };
    }

    mkLocalDerivation.__functionArgs
    => { allowSubstitute = true; preferLocalBuild = true; specialArg = true; }

    mkLocalDerivation { inherit (pkgs.hello) pname version src; specialArg = _: false; }
    => «derivation /nix/store/xirl67m60ahg6jmzicx43a81g635g8z8-hello-2.12.1.drv»

    mkLocalDerivation (finalAttrs: { inherit (pkgs.hello) pname version src; specialArg = _: false; })
    => «derivation /nix/store/xirl67m60ahg6jmzicx43a81g635g8z8-hello-2.12.1.drv»

    (mkLocalDerivation (finalAttrs: { inherit (pkgs.hello) pname version src; passthru = { foo = "a"; bar = "${finalAttrs.passthru.foo}b"; }; })).bar
    => "ab"
    ```
    :::

    :::{.note}
    If `transformDrv` is specified,
    it should take care of existing attributes that perform overriding
    (e.g., [`overrideAttrs`](#sec-pkg-overrideAttrs))
    to ensure that the overriding functionality of the result derivation
    work as expected.
    Modifications that breaks the overriding include
    direct [attribute set update](https://nixos.org/manual/nix/stable/language/operators#update)
    and [`lib.extendDerivation`](#function-library-lib.customisation.extendDerivation).
    :::
  */
  extendMkDerivation =
    let
      extendsWithExclusion =
        excludedNames: g: f: final:
        let
          previous = f final;
        in
        removeAttrs previous excludedNames // g final previous;
    in
    {
      constructDrv,
      excludeDrvArgNames ? [ ],
      excludeFunctionArgNames ? [ ],
      extendDrvArgs,
      inheritFunctionArgs ? true,
      transformDrv ? id,
    }:
    setFunctionArgs
      # Adds the fixed-point style support
      (
        fpargs:
        transformDrv (
          constructDrv (extendsWithExclusion excludeDrvArgNames extendDrvArgs (toFunction fpargs))
        )
      )
      # Add __functionArgs
      (
        removeAttrs (
          # Inherit the __functionArgs from the base build helper
          optionalAttrs inheritFunctionArgs (removeAttrs (functionArgs constructDrv) excludeDrvArgNames)
          # Recover the __functionArgs from the derived build helper
          // functionArgs (extendDrvArgs { })
        ) excludeFunctionArgNames
      )
    // {
      inherit
        # Expose to the result build helper.
        constructDrv
        excludeDrvArgNames
        extendDrvArgs
        transformDrv
        ;
    };

  /**
    Removes a prefix from the attribute names of a cross index.

    A cross index (short for "Cross Platform Pair Index") is a 6-field structure
    organizing values by cross-compilation platform relationships.

    # Inputs

    `prefix`
    : The prefix to remove from cross index attribute names

    `crossIndex`
    : A cross index with prefixed names

    # Type

    ```
    renameCrossIndexFrom :: String -> AttrSet -> AttrSet
    ```

    # Examples

    :::{.example}
    ## `lib.customisation.renameCrossIndexFrom` usage example

    ```nix
    renameCrossIndexFrom "pkgs" { pkgsBuildBuild = ...; pkgsBuildHost = ...; ... }
    => { buildBuild = ...; buildHost = ...; ... }
    ```
    :::
  */
  renameCrossIndexFrom = prefix: x: {
    buildBuild = x."${prefix}BuildBuild";
    buildHost = x."${prefix}BuildHost";
    buildTarget = x."${prefix}BuildTarget";
    hostHost = x."${prefix}HostHost";
    hostTarget = x."${prefix}HostTarget";
    targetTarget = x."${prefix}TargetTarget";
  };

  /**
    Adds a prefix to the attribute names of a cross index.

    A cross index (short for "Cross Platform Pair Index") is a 6-field structure
    organizing values by cross-compilation platform relationships.

    # Inputs

    `prefix`
    : The prefix to add to cross index attribute names

    `crossIndex`
    : A cross index to be prefixed

    # Type

    ```
    renameCrossIndexTo :: String -> AttrSet -> AttrSet
    ```

    # Examples

    :::{.example}
    ## `lib.customisation.renameCrossIndexTo` usage example

    ```nix
    renameCrossIndexTo "self" { buildBuild = ...; buildHost = ...; ... }
    => { selfBuildBuild = ...; selfBuildHost = ...; ... }
    ```
    :::
  */
  renameCrossIndexTo = prefix: x: {
    "${prefix}BuildBuild" = x.buildBuild;
    "${prefix}BuildHost" = x.buildHost;
    "${prefix}BuildTarget" = x.buildTarget;
    "${prefix}HostHost" = x.hostHost;
    "${prefix}HostTarget" = x.hostTarget;
    "${prefix}TargetTarget" = x.targetTarget;
  };

  /**
    Takes a function and applies it pointwise to each field of a cross index.

    A cross index (short for "Cross Platform Pair Index") is a 6-field structure
    organizing values by cross-compilation platform relationships.

    # Inputs

    `f`
    : Function to apply to each cross index value

    `crossIndex`
    : A cross index to transform

    # Type

    ```
    mapCrossIndex :: (a -> b) -> AttrSet -> AttrSet
    ```

    # Examples

    :::{.example}
    ## `lib.customisation.mapCrossIndex` usage example

    ```nix
    mapCrossIndex (x: x * 10) { buildBuild = 1; buildHost = 2; ... }
    => { buildBuild = 10; buildHost = 20; ... }
    ```

    ```nix
    # Extract a package from package sets
    mapCrossIndex (pkgs: pkgs.hello) crossIndexedPackageSets
    ```
    :::
  */
  mapCrossIndex =
    f:
    {
      buildBuild,
      buildHost,
      buildTarget,
      hostHost,
      hostTarget,
      targetTarget,
    }:
    {
      buildBuild = f buildBuild;
      buildHost = f buildHost;
      buildTarget = f buildTarget;
      hostHost = f hostHost;
      hostTarget = f hostTarget;
      targetTarget = f targetTarget;
    };
}