lib: add defaultTo

(cherry picked from commit 8ba40fd)

lib/default.nix

@@ -71,7 +71,7 @@ let
     # these are the only ones that are currently not
     inherit (builtins) addErrorContext isPath trace typeOf unsafeGetAttrPos;
     inherit (self.trivial) id const pipe concat or and xor bitAnd bitOr bitXor
-      bitNot boolToString mergeAttrs flip mapNullable inNixShell isFloat min max
+      bitNot boolToString mergeAttrs flip defaultTo mapNullable inNixShell isFloat min max
       importJSON importTOML warn warnIf warnIfNot throwIf throwIfNot checkListOfEnum
       info showWarnings nixpkgsVersion version isInOldestRelease oldestSupportedReleaseIsAtLeast
       mod compare splitByAndCompare seq deepSeq lessThan add sub

lib/trivial.nix

@@ -316,6 +316,40 @@ in {
   */
   flip = f: a: b: f b a;
 
+  /**
+    Return `maybeValue` if not null, otherwise return `default`.
+
+
+    # Inputs
+
+    `default`
+
+    : 1\. Function argument
+
+    `maybeValue`
+
+    : 2\. Function argument
+
+
+    # Examples
+    :::{.example}
+    ## `lib.trivial.defaultTo` usage example
+
+    ```nix
+    defaultTo "default" null
+    => "default"
+    defaultTo "default" "foo"
+    => "foo"
+    defaultTo "default" false
+    => false
+    ```
+
+    :::
+  */
+  defaultTo = default: maybeValue:
+    if maybeValue != null then maybeValue
+    else default;
+
   /**
     Apply function if the supplied argument is non-null.