lib: implement filterAttrs'

Uhh... I forgot that this is a thing.

lib/trivial.nix

@@ -14,11 +14,20 @@
     lib.count (attr: pred attr.name attr.value)
       (lib.mapAttrsToList lib.nameValuePair attrs);
 
-  /* Filters and groups the attribute set into two separate attribute.
+  /* Filters and groups the attribute set into two separate attribute where it
+     either accepted or denied from a given predicate function.
 
      Example:
        filterAttrs' (n: v: v == 4) { a = 4; b = 2; c = 6; }
-       => { ok = { a = 4; }; reject = { b = 2; c = 6; }; }
+       => { ok = { a = 4; }; notOk = { b = 2; c = 6; }; }
   */
-  filterAttrs' = f: attrs: { };
+  filterAttrs' = f: attrs:
+    lib.foldlAttrs (acc: name: value: let
+      isOk = f name value;
+    in {
+      ok = acc.ok // lib.optionalAttrs isOk { ${name} = value; };
+      notOk = acc.notOk // lib.optionalAttrs (!isOk) { ${name} = value; };
+    })
+    { ok = { }; notOk = { }; }
+    attrs;
 }

tests/lib/trivial.nix

@@ -12,4 +12,16 @@ lib.runTests {
     };
     expected = 2;
   };
+
+  testFilterAttrs' = {
+    expr = self.trivial.filterAttrs' (n: v: v == 4) {
+      e = 5;
+      f = 7;
+      a = 4;
+    };
+    expected = {
+      ok = { a = 4; };
+      notOk = { e = 5; f = 7; };
+    };
+  };
 }