bahaghari/lib: add trivial.optionalNull

subprojects/bahaghari/lib/trivial.nix

@@ -195,4 +195,22 @@ rec {
   */
   scale = { inMin, inMax, outMin, outMax }: v:
     ((v - inMin) * (outMax - outMin)) / ((inMax - inMin) + outMin);
+
+  /* Returns a null value if the condition fails. Otherwise, returns the given
+     value `as`.
+
+     Type: optionalNull :: Bool -> Any -> Any
+
+     Example:
+      optionalNull true "HELLO"
+      => "HELLO"
+
+      optionalNull (null != null) "HELLO"
+      => null
+  */
+  optionalNull = cond: as:
+    if cond then
+      as
+    else
+      null;
 }

subprojects/bahaghari/tests/lib/trivial/default.nix

@@ -260,4 +260,14 @@ lib.runTests {
     expr = self.trivial.isNumber true;
     expected = false;
   };
+
+  testOptionalNull = {
+    expr = self.trivial.optionalNull true "HELLO";
+    expected = "HELLO";
+  };
+
+  testOptionalNull2 = {
+    expr = self.trivial.optionalNull false "HELLO";
+    expected = null;
+  };
 }