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Either, tipos de fallo y gestión funcional de errores

Modele los errores recuperables como valores mediante Either de dartz y tipos de fallo sellados.

Either, tipos de fallo y gestión funcional de errores es una lección gratuita de Flutter Mobile Development en CoddyKit. Esta es la lección 4 de 4. Puedes leer la lección completa abajo gratuitamente — luego la practicas en el navegador con un editor de código integrado y un tutor de IA 24/7. Forma parte de la ruta de aprendizaje de Flutter Mobile Development, y tu progreso se sincroniza en la web y la app de CoddyKit. El curso de Flutter Mobile Development incluye 4 lecciones en total.

Partes de esta lección aún no han sido traducidas y se muestran en inglés.

Errors as Values

In Clean Architecture, your domain and data layers should not crash the UI with raw exceptions. Instead of throwing across layer boundaries, you model recoverable errors as return values.

The dartz package gives you Either<L, R>: a value that is either a Left (by convention, the failure) or a Right (the success). The caller is forced to handle both.

  • Left = the error path (a Failure)
  • Right = the happy path (your data)

This turns invisible throw/catch control flow into an explicit, type-checked contract.

// A repository method signature in the domain layer
// Future<Either<Failure, User>> getUser(String id);
//
// Left  -> something went wrong (a Failure)
// Right -> the success value (a User)

Modeling Either Ourselves

Before reaching for dartz, it helps to see what Either actually is. It is just a sealed type with two cases. Dart 3 sealed classes make the idea concrete and exhaustive.

The key property: a function returning Either always returns one of two shapes, and the compiler can verify you handled both in a switch.

sealed class Either<L, R> {}

class Left<L, R> extends Either<L, R> {
  final L value;
  Left(this.value);
}

class Right<L, R> extends Either<L, R> {
  final R value;
  Right(this.value);
}

Either<String, int> parsePositive(String s) {
  final n = int.tryParse(s);
  if (n == null) return Left('not a number');
  if (n <= 0) return Left('must be positive');
  return Right(n);
}

void main() {
  final result = parsePositive('42');
  final msg = switch (result) {
    Left(value: final e) => 'Error: $e',
    Right(value: final v) => 'Got $v',
  };
  print(msg);
}

Sealed Failure Types

Rather than putting a raw String on the Left, Clean Architecture uses a sealed hierarchy of Failure types. Each subtype names a distinct, recoverable error your domain cares about.

  • ServerFailure — backend returned an error
  • CacheFailure — local storage failed
  • NetworkFailure — device is offline
  • ValidationFailure — input was invalid

Because the base is sealed, a switch over a Failure is checked for exhaustiveness — add a new failure and the compiler points you at every place that must handle it.

sealed class Failure {
  final String message;
  const Failure(this.message);
}

class ServerFailure extends Failure {
  final int? statusCode;
  const ServerFailure(super.message, {this.statusCode});
}

class CacheFailure extends Failure {
  const CacheFailure(super.message);
}

class NetworkFailure extends Failure {
  const NetworkFailure(super.message);
}

class ValidationFailure extends Failure {
  const ValidationFailure(super.message);
}

Returning Either from a Repository

Here is the classic Clean Architecture repository pattern. The data source throws exceptions; the repository catches them at the boundary and converts each into a typed Failure on the Left.

This is the only place try/catch lives. Above this line, the rest of your app deals exclusively in Either<Failure, T>.

import 'package:dartz/dartz.dart';

class UserRepository {
  final RemoteDataSource remote;
  UserRepository(this.remote);

  Future<Either<Failure, User>> getUser(String id) async {
    try {
      final user = await remote.fetchUser(id);
      return Right(user);
    } on ServerException catch (e) {
      return Left(ServerFailure(e.message, statusCode: e.code));
    } on SocketException {
      return const Left(NetworkFailure('No internet connection'));
    }
  }
}

fold: Collapsing Either into One Value

The primary way to consume an Either is fold. It takes two functions — one for Left, one for Right — and runs whichever case applies, returning a single value of the same type.

fold forces you to handle the failure. You cannot accidentally read the success while ignoring the error, the way a forgotten try/catch lets you.

import 'package:dartz/dartz.dart';

String describe(Either<Failure, int> result) {
  return result.fold(
    (failure) => 'Failed: ${failure.message}',
    (value) => 'Success: $value',
  );
}

void main() {
  final ok = const Right<Failure, int>(7);
  final err = const Left<Failure, int>(CacheFailure('disk full'));
  print(describe(ok));   // Success: 7
  print(describe(err));  // Failed: disk full
}

fold Returns a Value, Not Side Effects

A common confusion: fold is for producing a value, not for branching side effects. In Flutter you typically fold an Either into a UI state object.

Notice both branches return the same type (here, a ViewState). If the branches returned different types, the result would be Object and you would lose type safety — a frequent beginner mistake.

sealed class ViewState {}
class ErrorState extends ViewState {
  final String message;
  ErrorState(this.message);
}
class LoadedState extends ViewState {
  final int data;
  LoadedState(this.data);
}

// In a Bloc/Cubit/Notifier:
// final ViewState next = result.fold(
//   (f) => ErrorState(f.message),
//   (v) => LoadedState(v),
// );

Transforming the Right with map

Either is right-biased: map transforms the success value and leaves a Left untouched. This lets a use case reshape data without unwrapping and rewrapping.

If the value is a Left failure, map is a no-op — the failure flows straight through. This is how you build pipelines that short-circuit on the first error.

import 'package:dartz/dartz.dart';

void main() {
  Either<String, int> success = const Right(10);
  Either<String, int> failure = const Left('boom');

  // map only touches the Right side
  print(success.map((v) => v * 2)); // Right(20)
  print(failure.map((v) => v * 2)); // Left(boom) — unchanged
}

Chaining with flatMap (bind)

When each step itself returns an Either, plain map would nest you into Either<F, Either<F, T>>. Use flatMap (also spelled bind in dartz) to chain failure-producing steps so the first Left short-circuits the rest.

This is the functional replacement for a chain of try/catch blocks: validation, then parsing, then a lookup — any failure stops the pipeline and surfaces as the final Left.

sealed class Either<L, R> {
  Either<L, T> flatMap<T>(Either<L, T> Function(R) f) =>
      switch (this) {
        Left(value: final l) => Left(l),
        Right(value: final r) => f(r),
      };
}

class Left<L, R> extends Either<L, R> {
  final L value;
  Left(this.value);
}
class Right<L, R> extends Either<L, R> {
  final R value;
  Right(this.value);
}

Either<String, int> parse(String s) =>
    int.tryParse(s) is int n ? Right(n) : Left('bad int');
Either<String, int> mustBePositive(int n) =>
    n > 0 ? Right(n) : Left('not positive');

void main() {
  final r = parse('5').flatMap(mustBePositive);
  print(switch (r) {
    Left(value: final e) => 'fail: $e',
    Right(value: final v) => 'ok: $v',
  });
}

Use Cases Return Either Too

In Clean Architecture, a use case sits between the presentation layer and the repository. It also returns Either<Failure, T>, often adding domain-level validation that produces its own ValidationFailure.

The use case never knows or cares whether the failure came from the network or from its own checks — both are just Failure values flowing on the Left.

import 'package:dartz/dartz.dart';

class GetUser {
  final UserRepository repo;
  GetUser(this.repo);

  Future<Either<Failure, User>> call(String id) async {
    if (id.trim().isEmpty) {
      return const Left(ValidationFailure('id must not be empty'));
    }
    return repo.getUser(id);
  }
}

Consuming Either in the UI Layer

At the edge — inside a Bloc, Cubit, or Riverpod notifier — you finally fold the Either into emitted UI states. The widget tree then reacts to those states.

Because every failure is a typed subtype of the sealed Failure, you can switch on it to show tailored messages: a retry button for NetworkFailure, a form error for ValidationFailure, and so on.

Future<void> loadUser(String id) async {
  emit(UserLoading());
  final result = await getUser(id);
  result.fold(
    (failure) => emit(UserError(_mapFailure(failure))),
    (user) => emit(UserLoaded(user)),
  );
}

String _mapFailure(Failure f) => switch (f) {
  NetworkFailure() => 'You are offline. Tap to retry.',
  ServerFailure(:final statusCode) => 'Server error ($statusCode).',
  CacheFailure() => 'Could not read local data.',
  ValidationFailure(:final message) => message,
};

Either vs Throwing Exceptions

Why prefer Either over exceptions across layers?

  • Explicit contracts — the return type tells callers a failure is possible; exceptions are invisible in the signature.
  • Exhaustiveness — sealed Failure + switch means a new error variant is a compile-time prompt, not a runtime surprise.
  • Composability — map/flatMap chain steps and short-circuit cleanly.

Reserve real throw for unrecoverable, programmer-error situations (e.g. assert, invariant violations). Use Either<Failure, T> for expected, recoverable outcomes like network or validation errors.

Quick Check

Test your understanding of functional error handling in Clean Architecture.

Recap

You learned to model recoverable errors as values:

  • Either<Failure, T> makes failure an explicit part of every repository and use case signature — Left for errors, Right for success.
  • A sealed Failure hierarchy (ServerFailure, CacheFailure, NetworkFailure, ValidationFailure) gives exhaustive, type-checked error handling.
  • The repository is the one place that catches raw exceptions and converts them to typed failures.
  • fold collapses an Either into a single value (often a UI state); map and flatMap/bind build short-circuiting pipelines that stop on the first Left.
  • Reserve throw for unrecoverable programmer errors; use Either for expected, recoverable ones.

Preguntas frecuentes

¿La lección «Either, tipos de fallo y gestión funcional de errores» es gratis?

Sí — el texto completo de «Either, tipos de fallo y gestión funcional de errores» es gratis para leer aquí en la web. Para practicarla de forma interactiva (editor de código integrado y tutor de IA 24/7) y desbloquear el resto del curso de Flutter Mobile Development, actualiza a CoddyKit PRO. El curso de Flutter Mobile Development incluye 4 lecciones en total.

¿Qué aprenderé en «Either, tipos de fallo y gestión funcional de errores»?

Modele los errores recuperables como valores mediante Either de dartz y tipos de fallo sellados. Practicas Flutter Mobile Development con código real que ejecutas directamente en el navegador, y un tutor de IA 24/7 responde tus preguntas mientras trabajas en la lección.

¿Necesito experiencia previa para empezar Flutter Mobile Development?

No se requiere experiencia previa. Flutter Mobile Development en CoddyKit está estructurado para principiantes hasta estudiantes avanzados, así que puedes empezar aquí o desde el inicio y avanzar a tu ritmo. Esta es la lección 4 de 4.

¿Cuánto tiempo toma la lección «Either, tipos de fallo y gestión funcional de errores»?

La mayoría de las lecciones de CoddyKit toman alrededor de 5–10 minutos. Cada una es compacta e interactiva, así que avanzas constantemente y retomas exactamente por donde dejaste en la web y la app.

¿Puedo escribir y ejecutar código en esta lección de Flutter Mobile Development?

Sí. Cada lección de Flutter Mobile Development incluye un editor de código integrado, así que escribes y ejecutas código real directamente en tu navegador y obtienes retroalimentación instantánea de IA — sin configuración local necesaria.

Todas las lecciones de este curso

  1. Límites entre las capas de dominio, datos y presentación
  2. Inyección de dependencias con get_it e injectable
  3. Estructura de carpetas por funcionalidad y monorepositorios Melos
  4. Either, tipos de fallo y gestión funcional de errores
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