Add handleErr method

src/main/scala/dotty/result/package.scala

@@ -361,6 +361,16 @@ object Result:
     def flatMap[U](f: T => Result[U, E]) = r match
       case Ok(value)    => f(value)
       case err @ Err(_) => err
+
+    /** Returns the output of `f` from applying it to the [[Err]] case error,
+      * otherwise keeping the [[Ok]] case. Similar to [[flatMap]], but on the
+      * [[Err]] case.
+      * @group transform
+      */
+    def handleErr[E1](f: E => Result[T, E1]) = r match
+      case ok: Ok[T]  => ok
+      case Err(error) => f(error)
+
   end extension
 
   extension [T, E](r: Result[Result[T, E], E])

src/test/scala/result/Result.scala

@@ -49,6 +49,9 @@ class ResultTest extends munit.FunSuite {
     assertEquals(ok.flatMap(v => Err("pew")), Err("pew"))
     assertEquals(err.flatMap(v => Ok(v + 1)), err)
 
+    assertEquals(ok.handleErr(_ => err), ok)
+    assertEquals(err.handleErr(_ => ok), ok)
+
     val sum = for
       a <- ok
       b <- ok