lib/attrsets: add cartesianProductOfSets function

doc/functions/library/attrsets.xml

@@ -1711,4 +1711,43 @@ recursiveUpdate
   </example>
  </section>
 
+ <section xml:id="function-library-lib.attrsets.cartesianProductOfSets">
+  <title><function>lib.attrsets.cartesianProductOfSets</function></title>
+
+  <subtitle><literal>cartesianProductOfSets :: AttrSet -> [ AttrSet ]</literal>
+  </subtitle>
+
+  <xi:include href="./locations.xml" xpointer="lib.attrsets.cartesianProductOfSets" />
+
+  <para>
+    Return the cartesian product of attribute set value combinations.
+  </para>
+
+  <variablelist>
+   <varlistentry>
+    <term>
+     <varname>set</varname>
+    </term>
+    <listitem>
+     <para>
+      An attribute set with attributes that carry lists of values.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+
+  <example xml:id="function-library-lib.attrsets.cartesianProductOfSets-example">
+   <title>Creating the cartesian product of a list of attribute values</title>
+<programlisting><![CDATA[
+cartesianProductOfSets { a = [ 1 2 ]; b = [ 10 20 ]; }
+=> [
+     { a = 1; b = 10; }
+     { a = 1; b = 20; }
+     { a = 2; b = 10; }
+     { a = 2; b = 20; }
+   ]
+]]></programlisting>
+  </example>
+ </section>
+
 </section>

lib/attrsets.nix

@@ -183,6 +183,24 @@ rec {
     else
       [];
 
+  /* Return the cartesian product of attribute set value combinations.
+
+    Example:
+      cartesianProductOfSets { a = [ 1 2 ]; b = [ 10 20 ]; }
+      => [
+           { a = 1; b = 10; }
+           { a = 1; b = 20; }
+           { a = 2; b = 10; }
+           { a = 2; b = 20; }
+         ]
+  */
+  cartesianProductOfSets = attrsOfLists:
+    lib.foldl' (listOfAttrs: attrName:
+      concatMap (attrs:
+        map (listValue: attrs // { ${attrName} = listValue; }) attrsOfLists.${attrName}
+      ) listOfAttrs
+    ) [{}] (attrNames attrsOfLists);
+
 
   /* Utility function that creates a {name, value} pair as expected by
      builtins.listToAttrs.
@@ -493,5 +511,4 @@ rec {
   zipWithNames = zipAttrsWithNames;
   zip = builtins.trace
     "lib.zip is deprecated, use lib.zipAttrsWith instead" zipAttrsWith;
-
 }

lib/default.nix

@@ -78,7 +78,7 @@ let
       zipAttrsWithNames zipAttrsWith zipAttrs recursiveUpdateUntil
       recursiveUpdate matchAttrs overrideExisting getOutput getBin
       getLib getDev getMan chooseDevOutputs zipWithNames zip
-      recurseIntoAttrs dontRecurseIntoAttrs;
+      recurseIntoAttrs dontRecurseIntoAttrs cartesianProductOfSets;
     inherit (self.lists) singleton forEach foldr fold foldl foldl' imap0 imap1
       concatMap flatten remove findSingle findFirst any all count
       optional optionals toList range partition zipListsWith zipLists

lib/tests/misc.nix

@@ -660,4 +660,71 @@ runTests {
     expected = [ [ "foo" ] [ "foo" "<name>" "bar" ] [ "foo" "bar" ] ];
   };
 
+  testCartesianProductOfEmptySet = {
+    expr = cartesianProductOfSets {};
+    expected = [ {} ];
+  };
+
+  testCartesianProductOfOneSet = {
+    expr = cartesianProductOfSets { a = [ 1 2 3 ]; };
+    expected = [ { a = 1; } { a = 2; } { a = 3; } ];
+  };
+
+  testCartesianProductOfTwoSets = {
+    expr = cartesianProductOfSets { a = [ 1 ]; b = [ 10 20 ]; };
+    expected = [
+      { a = 1; b = 10; }
+      { a = 1; b = 20; }
+    ];
+  };
+
+  testCartesianProductOfTwoSetsWithOneEmpty = {
+    expr = cartesianProductOfSets { a = [ ]; b = [ 10 20 ]; };
+    expected = [ ];
+  };
+
+  testCartesianProductOfThreeSets = {
+    expr = cartesianProductOfSets {
+      a = [   1   2   3 ];
+      b = [  10  20  30 ];
+      c = [ 100 200 300 ];
+    };
+    expected = [
+      { a = 1; b = 10; c = 100; }
+      { a = 1; b = 10; c = 200; }
+      { a = 1; b = 10; c = 300; }
+
+      { a = 1; b = 20; c = 100; }
+      { a = 1; b = 20; c = 200; }
+      { a = 1; b = 20; c = 300; }
+
+      { a = 1; b = 30; c = 100; }
+      { a = 1; b = 30; c = 200; }
+      { a = 1; b = 30; c = 300; }
+
+      { a = 2; b = 10; c = 100; }
+      { a = 2; b = 10; c = 200; }
+      { a = 2; b = 10; c = 300; }
+
+      { a = 2; b = 20; c = 100; }
+      { a = 2; b = 20; c = 200; }
+      { a = 2; b = 20; c = 300; }
+
+      { a = 2; b = 30; c = 100; }
+      { a = 2; b = 30; c = 200; }
+      { a = 2; b = 30; c = 300; }
+
+      { a = 3; b = 10; c = 100; }
+      { a = 3; b = 10; c = 200; }
+      { a = 3; b = 10; c = 300; }
+
+      { a = 3; b = 20; c = 100; }
+      { a = 3; b = 20; c = 200; }
+      { a = 3; b = 20; c = 300; }
+
+      { a = 3; b = 30; c = 100; }
+      { a = 3; b = 30; c = 200; }
+      { a = 3; b = 30; c = 300; }
+    ];
+  };
 }