Add tap, tapLeft and tapBoth to Either

lib/src/either.dart

@@ -19,6 +19,28 @@ abstract class Either<L, R> implements TraversableMonadOps<Either<L, dynamic>, R
   Either<LL, RR> bimap<LL, RR>(LL ifLeft(L l), RR ifRight(R r)) =>
     fold((l) => left(ifLeft(l)), (r) => right(ifRight(r)));
 
+  Either<L, R> tap(void Function(R) f) =>
+    flatMap((r) {
+      f(r);
+      return right(r);
+    });
+
+  Either<L, R> tapLeft(void Function(L) f) =>
+    fold((l) {
+      f(l);
+      return left(l);
+    }, (r) => right(r));
+
+  Either<L, R> tapBoth(void Function(L) f1, void Function(R) f2) =>
+    fold((l) {
+      f1(l);
+      return left(l);
+    }, (r) {
+      f2(r);
+      return right(r);
+    }
+  );
+
   @override Either<L, R2> map<R2>(R2 f(R r)) => fold(left, (R r) => right(f(r)));
   @override Either<L, R2> bind<R2>(Function1<R, Either<L, R2>> f) => fold(left, f);
   @override Either<L, R2> flatMap<R2>(Function1<R, Either<L, R2>> f) => fold(left, f);

test/either_test.dart

@@ -103,4 +103,80 @@ void main() {
       expect(right.value, 2);
     });
   });
+
+  group('tap', () {
+    test('executes function with value if right', () {
+      final either = right(42);
+      final results = [];
+
+      final result = either.tap((value) => results.add(value));
+
+      expect(results, [42]);
+      expect(result, equals(either));
+    });
+
+    test('does not execute function if left', () {
+      final either = left('error');
+      final results = [];
+
+      final result = either.tap((value) => results.add(value));
+
+      expect(results, isEmpty);
+      expect(result, equals(either));
+    });
+  });
+
+  group('tapLeft', () {
+    test('executes function with error if left', () {
+      final either = left('error');
+      final results = [];
+
+      final result = either.tapLeft((error) => results.add(error));
+
+      expect(results, ['error']);
+      expect(result, equals(either));
+    });
+
+    test('does not execute function if right', () {
+      final either = right(42);
+      final results = [];
+
+      final result = either.tapLeft((error) => results.add(error));
+
+      expect(results, isEmpty);
+      expect(result, equals(either));
+    });
+  });
+
+  group('tapBoth', () {
+    test('executes left function if left', () {
+      final either = left('error');
+      final resultsLeft = [];
+      final resultsRight = [];
+
+      final result = either.tapBoth(
+        (error) => resultsLeft.add(error),
+        (value) => resultsRight.add(value),
+      );
+
+      expect(resultsLeft, ['error']);
+      expect(resultsRight, isEmpty);
+      expect(result, equals(either));
+    });
+
+    test('executes right function if right', () {
+      final either = right(42);
+      final resultsLeft = [];
+      final resultsRight = [];
+
+      final result = either.tapBoth(
+        (error) => resultsLeft.add(error),
+        (value) => resultsRight.add(value),
+      );
+
+      expect(resultsLeft, isEmpty);
+      expect(resultsRight, [42]);
+      expect(result, equals(either));
+    });
+  });
 }