splice.nix

# The `splicedPackages' package set, and its use by `callPackage`
#
# The `buildPackages` pkg set is a new concept, and the vast majority package
# expression (the other *.nix files) are not designed with it in mind. This
# presents us with a problem with how to get the right version (build-time vs
# run-time) of a package to a consumer that isn't used to thinking so cleverly.
#
# The solution is to splice the package sets together as we do below, so every
# `callPackage`d expression in fact gets both versions. Each# derivation (and
# each derivation's outputs) consists of the run-time version, augmented with a
# `nativeDrv` field for the build-time version, and `crossDrv` field for the
# run-time version.
#
# We could have used any names we want for the disambiguated versions, but
# `crossDrv` and `nativeDrv` were somewhat similarly used for the old
# cross-compiling infrastructure. The names are mostly invisible as
# `mkDerivation` knows how to pull out the right ones for `buildDepends` and
# friends, but a few packages use them directly, so it seemed efficient (to
# @Ericson2314) to reuse those names, at least initially, to minimize breakage.
#
# For performance reasons, rather than uniformally splice in all cases, we only
# do so when `pkgs` and `buildPackages` are distinct. The `actuallySplice`
# parameter there the boolean value of that equality check.
lib: pkgs: actuallySplice:

let

  spliceReal = { pkgsBuildBuild, pkgsBuildHost, pkgsBuildTarget
               , pkgsHostHost, pkgsHostTarget
               , pkgsTargetTarget
               }: let
    mash =
      # Other pkgs sets
      pkgsBuildBuild // pkgsBuildTarget // pkgsHostHost // pkgsTargetTarget
      # The same pkgs sets one probably intends
      // pkgsBuildHost // pkgsHostTarget;
    merge = name: {
      inherit name;
      value = let
        defaultValue = mash.${name};
        # `or {}` is for the non-derivation attsert splicing case, where `{}` is the identity.
        valueBuildBuild = pkgsBuildBuild.${name} or {};
        valueBuildHost = pkgsBuildHost.${name} or {};
        valueBuildTarget = pkgsBuildTarget.${name} or {};
        valueHostHost = throw "`valueHostHost` unimplemented: pass manually rather than relying on splice.";
        valueHostTarget = pkgsHostTarget.${name} or {};
        valueTargetTarget = pkgsTargetTarget.${name} or {};
        augmentedValue = defaultValue
          # TODO(@Ericson2314): Stop using old names after transition period
          // (lib.optionalAttrs (pkgsBuildHost ? ${name}) { nativeDrv = valueBuildHost; })
          // (lib.optionalAttrs (pkgsHostTarget ? ${name}) { crossDrv = valueHostTarget; })
          // {
            __spliced =
                 (lib.optionalAttrs (pkgsBuildBuild ? ${name}) { buildBuild = valueBuildBuild; })
              // (lib.optionalAttrs (pkgsBuildTarget ? ${name}) { buildTarget = valueBuildTarget; })
              // { hostHost = valueHostHost; }
              // (lib.optionalAttrs (pkgsTargetTarget ? ${name}) { targetTarget = valueTargetTarget;
          });
        };
        # Get the set of outputs of a derivation. If one derivation fails to
        # evaluate we don't want to diverge the entire splice, so we fall back
        # on {}
        tryGetOutputs = value0: let
          inherit (builtins.tryEval value0) success value;
        in getOutputs (lib.optionalAttrs success value);
        getOutputs = value: lib.genAttrs
          (value.outputs or (lib.optional (value ? out) "out"))
          (output: value.${output});
      in
        # The derivation along with its outputs, which we recur
        # on to splice them together.
        if lib.isDerivation defaultValue then augmentedValue // spliceReal {
          pkgsBuildBuild = tryGetOutputs valueBuildBuild;
          pkgsBuildHost = tryGetOutputs valueBuildHost;
          pkgsBuildTarget = tryGetOutputs valueBuildTarget;
          pkgsHostHost = tryGetOutputs valueHostHost;
          pkgsHostTarget = getOutputs valueHostTarget;
          pkgsTargetTarget = tryGetOutputs valueTargetTarget;
        # Just recur on plain attrsets
        } else if lib.isAttrs defaultValue then spliceReal {
          pkgsBuildBuild = valueBuildBuild;
          pkgsBuildHost = valueBuildHost;
          pkgsBuildTarget = valueBuildTarget;
          pkgsHostHost = {};
          pkgsHostTarget = valueHostTarget;
          pkgsTargetTarget = valueTargetTarget;
        # Don't be fancy about non-derivations. But we could have used used
        # `__functor__` for functions instead.
        } else defaultValue;
    };
  in lib.listToAttrs (map merge (lib.attrNames mash));

  splicePackages = { pkgsBuildBuild, pkgsBuildHost, pkgsBuildTarget
                   , pkgsHostHost, pkgsHostTarget
                   , pkgsTargetTarget
                   } @ args:
    if actuallySplice then spliceReal args else pkgsHostTarget;

  splicedPackages = splicePackages {
    inherit (pkgs)
      pkgsBuildBuild pkgsBuildHost pkgsBuildTarget
      pkgsHostHost pkgsHostTarget
      pkgsTargetTarget
      ;
  } // {
    # These should never be spliced under any circumstances
    inherit (pkgs)
      pkgsBuildBuild pkgsBuildHost pkgsBuildTarget
      pkgsHostHost pkgsHostTarget
      pkgsTargetTarget
      buildPackages pkgs targetPackages
      ;
    inherit (pkgs.stdenv) buildPlatform targetPlatform hostPlatform;
  };

  splicedPackagesWithXorg = splicedPackages // builtins.removeAttrs splicedPackages.xorg [
    "callPackage" "newScope" "overrideScope" "packages"
  ];

in

{
  inherit splicePackages;

  # We use `callPackage' to be able to omit function arguments that can be
  # obtained `pkgs` or `buildPackages` and their `xorg` package sets. Use
  # `newScope' for sets of packages in `pkgs' (see e.g. `gnome' below).
  callPackage = pkgs.newScope {};

  callPackages = lib.callPackagesWith splicedPackagesWithXorg;

  newScope = extra: lib.callPackageWith (splicedPackagesWithXorg // extra);

  # Haskell package sets need this because they reimplement their own
  # `newScope`.
  __splicedPackages = splicedPackages // { recurseForDerivations = false; };
}