Adding toErrorIf method (#11)

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 - Breaking change: Raised minimum Dart to >=3.0.0
 - Added `partition`, `getAll` and `getAllErrors` for interacting with a list of results
+- Added `toErrorIf` method
 
 ## 1.0.2
 

lib/src/functions/map.dart

@@ -34,4 +34,22 @@ extension MapResult<T, E> on Result<T, E> {
       return Err<F>(failure((result as Err<E>).error));
     }
   }
+
+  /// Transforms a success into an error if it satisfies the given [predicate],
+  /// otherwise return this Result.
+  Result<T, E> toErrorIf(
+    bool Function(T value) predicate,
+    E Function(T value) transform,
+  ) {
+    final result = this;
+    if (result is Ok<T>) {
+      if (predicate(result.value)) {
+        return Err(transform(result.value));
+      } else {
+        return result;
+      }
+    } else {
+      return result;
+    }
+  }
 }

test/functions/map_test.dart

@@ -50,6 +50,25 @@ void main() {
     });
   });
 
+  group('toErrorIf', () {
+    test('should not transform success if does not satisfies the condition',
+        () {
+      final Result<int, String> result = const Ok(1);
+      expect(result.toErrorIf((_) => false, (_) => "error"), const Ok(1));
+    });
+
+    test('should transform success if satisfies the condition', () {
+      final Result<int, String> result = const Ok(1);
+      expect(result.toErrorIf((_) => true, (_) => "error"), const Err("error"));
+    });
+
+    test('should not transform failures', () {
+      final Result<int, String> result = const Err("original");
+      expect(result.toErrorIf((_) => true, (_) => "modified"),
+          const Err("original"));
+    });
+  });
+
   test('Map extension name should not conflict with Dart Map', () {
     Result<Map<String, String>, Never> result = const Ok(0).map((value) => {});
     expect(result, isA<Ok<Map<String, String>>>());