Add flatMapErr method

lib/src/result.dart

@@ -38,6 +38,8 @@ abstract class Result<T, E> {
   /// Calls `f` if the result is `Ok`, otherwise returns the `Err` value of self.
   ///
   /// This method can be used for control flow based on Result values.
+  ///
+  /// Example:
   /// ```
   /// Result<double, String> getReciprocal(double n) => tryCatch(() => 1 / n);
   ///
@@ -51,6 +53,14 @@ abstract class Result<T, E> {
         (error) => Err(error),
       );
 
+  /// Calls `f` if the result is `Err`, otherwise returns the `Ok` value of self.
+  ///
+  /// This method can be used for control flow based on Result values.
+  Result<T, F> flatMapErr<F>(Result<T, F> Function(E error) f) => fold(
+        (value) => Ok(value),
+        (error) => f(error),
+      );
+
   /// Converts from `Result<T, E>` to `B` by applying a function `ifOk` to a contained `Ok` value and a function `ifErr` to a contained `Err` value.
   B fold<B>(B Function(T value) ifOk, B Function(E error) ifErr);
 

test/result_test.dart

@@ -53,6 +53,17 @@ void main() {
       expect(err.flatMap((value) => Ok(value)), err);
     });
 
+    test('flatMapErr works correctly', () {
+      final ok = Ok<int, String>(42);
+      final err = Err<int, String>('error');
+
+      expect(ok.flatMapErr((value) => Err(value)), Ok(42));
+      expect(
+        err.flatMapErr((value) => Err('Error: $value')),
+        Err('Error: error'),
+      );
+    });
+
     test('mapErr works correctly', () {
       final ok = Ok(42);
       final err = Err('error');