{ lib }:

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, you can do something like this:

       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;
           sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k";
         };
         patches = [];
       });

     For another application, see build-support/vm, where this
     function is used to build arbitrary derivations inside a QEMU
     virtual machine.
  */
  overrideDerivation = drv: f:
    let
      newDrv = derivation (drv.drvAttrs // (f drv));
    in lib.flip (extendDerivation true) newDrv (
      { meta = drv.meta or {};
        passthru = if drv ? passthru then drv.passthru else {};
      }
      //
      (drv.passthru or {})
      //
      (if (drv ? crossDrv && drv ? nativeDrv)
       then {
         crossDrv = overrideDerivation drv.crossDrv f;
         nativeDrv = overrideDerivation drv.nativeDrv f;
       }
       else { }));


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

       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; }

     Please refer to "Nixpkgs Contributors Guide" section
     "<pkg>.overrideDerivation" to learn about `overrideDerivation` and caveats
     related to its use.
  */
  makeOverridable = f: origArgs:
    let
      result = f origArgs;

      # Creates a functor with the same arguments as f
      copyArgs = g: lib.setFunctionArgs g (lib.functionArgs f);
      # Changes the original arguments with (potentially a function that returns) a set of new attributes
      overrideWith = newArgs: origArgs // (if lib.isFunction newArgs then newArgs origArgs else newArgs);

      # Re-call the function but with different arguments
      overrideArgs = copyArgs (newArgs: makeOverridable f (overrideWith newArgs));
      # Change the result of the function call by applying g to it
      overrideResult = g: makeOverridable (copyArgs (args: g (f args))) origArgs;
    in
      if builtins.isAttrs result then
        result // {
          override = overrideArgs;
          overrideDerivation = fdrv: overrideResult (x: overrideDerivation x fdrv);
          ${if result ? overrideAttrs then "overrideAttrs" else null} = fdrv:
            overrideResult (x: x.overrideAttrs fdrv);
        }
      else if lib.isFunction result then
        # Transform the result into a functor while propagating its arguments
        lib.setFunctionArgs result (lib.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.,

      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.

      libbar = callPackage ./bar.nix {
        libfoo = null;
        enableX11 = true;
      };
  */
  callPackageWith = autoArgs: fn: args:
    let
      f = if lib.isFunction fn then fn else import fn;
      auto = builtins.intersectAttrs (lib.functionArgs f) autoArgs;
    in makeOverridable f (auto // args);


  /* Like callPackage, but for a function that returns an attribute
     set of derivations. The override function is added to the
     individual attributes. */
  callPackagesWith = autoArgs: fn: args:
    let
      f = if lib.isFunction fn then fn else import fn;
      auto = builtins.intersectAttrs (lib.functionArgs f) autoArgs;
      origArgs = auto // args;
      pkgs = f origArgs;
      mkAttrOverridable = name: _: makeOverridable (newArgs: (f newArgs).${name}) origArgs;
    in
      if lib.isDerivation pkgs then throw
        ("function `callPackages` was called on a *single* derivation "
          + ''"${pkgs.name or "<unknown-name>"}";''
          + " did you mean to use `callPackage` instead?")
      else lib.mapAttrs mkAttrOverridable pkgs;


  /* Add attributes to each output of a derivation without changing
     the derivation itself and check a given condition when evaluating. */
  extendDerivation = condition: passthru: drv:
    let
      outputs = drv.outputs or [ "out" ];

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

      outputToAttrListElement = outputName:
        { name = outputName;
          value = commonAttrs // {
            inherit (drv.${outputName}) type outputName;
            drvPath = assert condition; drv.${outputName}.drvPath;
            outPath = assert condition; drv.${outputName}.outPath;
          };
        };

      outputsList = map outputToAttrListElement outputs;
    in commonAttrs // {
      outputUnspecified = true;
      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. */
  hydraJob = drv:
    let
      outputs = drv.outputs or ["out"];

      commonAttrs =
        { inherit (drv) name system meta; inherit outputs; }
        // lib.optionalAttrs (drv._hydraAggregate or false) {
          _hydraAggregate = true;
          constituents = map hydraJob (lib.flatten drv.constituents);
        }
        // (lib.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' = (lib.head outputsList).value;
    in lib.deepSeq drv' drv';

  /* Make a set of packages with a common scope. All packages called
     with the provided `callPackage' will be evaluated with the same
     arguments. Any package in the set may depend on any other. The
     `overrideScope'` function allows subsequent modification of the package
     set in a consistent way, i.e. all packages in the set will be
     called with the overridden packages. The package sets may be
     hierarchical: the packages in the set are called with the scope
     provided by `newScope' and the set provides a `newScope' attribute
     which can form the parent scope for later package sets. */
  makeScope = newScope: f:
    let self = f self // {
          newScope = scope: newScope (self // scope);
          callPackage = self.newScope {};
          overrideScope = g: lib.warn
            "`overrideScope` (from `lib.makeScope`) is deprecated. Do `overrideScope' (self: super: { … })` instead of `overrideScope (super: self: { … })`. All other overrides have the parameters in that order, including other definitions of `overrideScope`. This was the only definition violating the pattern."
            (makeScope newScope (lib.fixedPoints.extends (lib.flip g) f));
          overrideScope' = g: makeScope newScope (lib.fixedPoints.extends g f);
          packages = f;
        };
    in self;

  /* Like the above, but aims to support cross compilation. It's still ugly, but
     hopefully it helps a little bit. */
  makeScopeWithSplicing = splicePackages: newScope: otherSplices: keep: extra: 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
          splicePackages
          newScope
          otherSplices
          keep
          extra
          (lib.fixedPoints.extends g f);
        packages = f;
      };
    in self;

}