lib: Add splicing utilities

lib/customisation.nix

@@ -863,4 +863,139 @@ rec {
         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;
+    };
 }

lib/default.nix

@@ -398,6 +398,9 @@ let
         makeScopeWithSplicing
         makeScopeWithSplicing'
         extendMkDerivation
+        renameCrossIndexFrom
+        renameCrossIndexTo
+        mapCrossIndex
         ;
       inherit (self.derivations) lazyDerivation optionalDrvAttr warnOnInstantiate;
       inherit (self.generators) mkLuaInline;

lib/tests/misc.nix

@@ -4432,4 +4432,82 @@ runTests {
     expected = "/non-existent/this/does/not/exist/for/real/please-dont-mess-with-your-local-fs/default.nix";
   };
 
+  # Tests for cross index utilities
+
+  testRenameCrossIndexFrom = {
+    expr = lib.renameCrossIndexFrom "pkgs" {
+      pkgsBuildBuild = "dummy-build-build";
+      pkgsBuildHost = "dummy-build-host";
+      pkgsBuildTarget = "dummy-build-target";
+      pkgsHostHost = "dummy-host-host";
+      pkgsHostTarget = "dummy-host-target";
+      pkgsTargetTarget = "dummy-target-target";
+    };
+    expected = {
+      buildBuild = "dummy-build-build";
+      buildHost = "dummy-build-host";
+      buildTarget = "dummy-build-target";
+      hostHost = "dummy-host-host";
+      hostTarget = "dummy-host-target";
+      targetTarget = "dummy-target-target";
+    };
+  };
+
+  testRenameCrossIndexTo = {
+    expr = lib.renameCrossIndexTo "self" {
+      buildBuild = "dummy-build-build";
+      buildHost = "dummy-build-host";
+      buildTarget = "dummy-build-target";
+      hostHost = "dummy-host-host";
+      hostTarget = "dummy-host-target";
+      targetTarget = "dummy-target-target";
+    };
+    expected = {
+      selfBuildBuild = "dummy-build-build";
+      selfBuildHost = "dummy-build-host";
+      selfBuildTarget = "dummy-build-target";
+      selfHostHost = "dummy-host-host";
+      selfHostTarget = "dummy-host-target";
+      selfTargetTarget = "dummy-target-target";
+    };
+  };
+
+  testMapCrossIndex = {
+    expr = lib.mapCrossIndex (x: x * 10) {
+      buildBuild = 1;
+      buildHost = 2;
+      buildTarget = 3;
+      hostHost = 4;
+      hostTarget = 5;
+      targetTarget = 6;
+    };
+    expected = {
+      buildBuild = 10;
+      buildHost = 20;
+      buildTarget = 30;
+      hostHost = 40;
+      hostTarget = 50;
+      targetTarget = 60;
+    };
+  };
+
+  testMapCrossIndexString = {
+    expr = lib.mapCrossIndex (x: "prefix-${x}") {
+      buildBuild = "bb";
+      buildHost = "bh";
+      buildTarget = "bt";
+      hostHost = "hh";
+      hostTarget = "ht";
+      targetTarget = "tt";
+    };
+    expected = {
+      buildBuild = "prefix-bb";
+      buildHost = "prefix-bh";
+      buildTarget = "prefix-bt";
+      hostHost = "prefix-hh";
+      hostTarget = "prefix-ht";
+      targetTarget = "prefix-tt";
+    };
+  };
+
 }

pkgs/top-level/splice.nix

@@ -17,48 +17,30 @@
 lib: pkgs: actuallySplice:
 
 let
+  inherit (lib.customisation) mapCrossIndex renameCrossIndexFrom;
 
   spliceReal =
-    {
-      pkgsBuildBuild,
-      pkgsBuildHost,
-      pkgsBuildTarget,
-      pkgsHostHost,
-      pkgsHostTarget,
-      pkgsTargetTarget,
-    }:
+    inputs:
     let
       mash =
         # Other pkgs sets
-        pkgsBuildBuild
-        // pkgsBuildTarget
-        // pkgsHostHost
-        // pkgsTargetTarget
+        inputs.buildBuild
+        // inputs.buildTarget
+        // inputs.hostHost
+        // inputs.targetTarget
         # The same pkgs sets one probably intends
-        // pkgsBuildHost
-        // pkgsHostTarget;
+        // inputs.buildHost
+        // inputs.hostTarget;
       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 = pkgsHostHost.${name} or { };
-            valueHostTarget = pkgsHostTarget.${name} or { };
-            valueTargetTarget = pkgsTargetTarget.${name} or { };
+            value' = mapCrossIndex (x: x.${name} or { }) inputs;
+
             augmentedValue = defaultValue // {
-              __spliced =
-                (lib.optionalAttrs (pkgsBuildBuild ? ${name}) { buildBuild = valueBuildBuild; })
-                // (lib.optionalAttrs (pkgsBuildHost ? ${name}) { buildHost = valueBuildHost; })
-                // (lib.optionalAttrs (pkgsBuildTarget ? ${name}) { buildTarget = valueBuildTarget; })
-                // (lib.optionalAttrs (pkgsHostHost ? ${name}) { hostHost = valueHostHost; })
-                // (lib.optionalAttrs (pkgsHostTarget ? ${name}) { hostTarget = valueHostTarget; })
-                // (lib.optionalAttrs (pkgsTargetTarget ? ${name}) {
-                  targetTarget = valueTargetTarget;
-                });
+              __spliced = lib.filterAttrs (k: v: inputs.${k} ? ${name}) value';
             };
             # 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
@@ -76,27 +58,12 @@ let
           # 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
-            }
+            // spliceReal (mapCrossIndex tryGetOutputs value' // { hostTarget = getOutputs value'.hostTarget; })
           else if lib.isAttrs defaultValue then
-            spliceReal {
-              pkgsBuildBuild = valueBuildBuild;
-              pkgsBuildHost = valueBuildHost;
-              pkgsBuildTarget = valueBuildTarget;
-              pkgsHostHost = valueHostHost;
-              pkgsHostTarget = valueHostTarget;
-              pkgsTargetTarget = valueTargetTarget;
-              # Don't be fancy about non-derivations. But we could have used used
-              # `__functor__` for functions instead.
-            }
+            spliceReal value'
           else
+            # Don't be fancy about non-derivations. But we could have used used
+            # `__functor__` for functions instead.
             defaultValue;
       };
     in
@@ -111,7 +78,7 @@ let
       pkgsHostTarget,
       pkgsTargetTarget,
     }@args:
-    if actuallySplice then spliceReal args else pkgsHostTarget;
+    if actuallySplice then spliceReal (renameCrossIndexFrom "pkgs" args) else pkgsHostTarget;
 
   splicedPackages =
     splicePackages {