test: 🧪 add tests for either class

Author: Nidal-Bakir

test/either_test.dart

@@ -0,0 +1,70 @@
+import 'package:test/test.dart';
+import 'package:exponential_back_off/src/either.dart';
+
+void main() {
+  test('test Left Either', () {
+    // arrange
+    final error = Exception('error');
+    Either<Exception, int> leftEither = Left(error);
+
+    // act
+    final isLeft = leftEither.isLeft();
+    final isRight = leftEither.isRight();
+
+    final getLeftLeftEither = leftEither.getLeft;
+    final getLeftRightEither = leftEither.getRight;
+
+    final getLeftValue = leftEither.getLeftValue;
+    final getRightValue = leftEither.getRightValue;
+
+    final foldResult = leftEither.fold(
+      (l) => l,
+      (r) => 'NO OP',
+    );
+
+    // assert
+    expect(isLeft, isTrue);
+    expect(isRight, isFalse);
+
+    expect(getLeftLeftEither(), equals(leftEither));
+    expect(() => getLeftRightEither(), throwsA(isA<NotRightException>()));
+
+    expect(getLeftValue(), equals(error));
+    expect(() => getRightValue(), throwsA(isA<NotRightException>()));
+
+    expect(foldResult, equals(error));
+  });
+
+  test('test Right Either', () {
+    // arrange
+    final data = 1;
+    Either<Exception, int> rightEither = Right(data);
+
+    // act
+    final isLeft = rightEither.isLeft();
+    final isRight = rightEither.isRight();
+
+    final getLeftLeftEither = rightEither.getLeft;
+    final getLeftRightEither = rightEither.getRight;
+
+    final getLeftValue = rightEither.getLeftValue;
+    final getRightValue = rightEither.getRightValue;
+
+    final foldResult = rightEither.fold(
+      (l) => 'NO OP',
+      (r) => r,
+    );
+
+    // assert
+    expect(isLeft, isFalse);
+    expect(isRight, isTrue);
+
+    expect(() => getLeftLeftEither(), throwsA(isA<NotLeftException>()));
+    expect(getLeftRightEither(), equals(rightEither));
+
+    expect(getRightValue(), equals(data));
+    expect(() => getLeftValue(), throwsA(isA<NotLeftException>()));
+
+    expect(foldResult, equals(data));
+  });
+}