Introduction

In Flutter mobile development, uncaught exceptions and loosely-handled futures create brittle apps and poor user experiences. A Result type—an explicit wrapper representing success or failure—makes error flow visible, composable, and testable. This tutorial shows how to design a lightweight Result in Dart, use it with async APIs, and integrate it into Flutter UI patterns for robust error handling.

Why Use Result Types

Exceptions in Dart are easy to throw but easy to forget to catch. Result types force callers to handle outcomes, reducing silent failures. Benefits for mobile development include:

• Predictable error paths for networking, storage, and platform channels.

• Simpler unit tests: functions return deterministic values rather than throwing.

• Easier mapping of domain errors to user-facing messages.

Result types also enable functional composition (map/flatMap) so you can chain operations without try/catch spaghetti. They are particularly useful with Future> which models asynchronous operations that may fail.

Designing a Result Type

Keep the Result type small and pragmatic. A generic Result with Success and Failure variants plus convenient helpers is enough for most apps.

abstract class Result<T, E> {
  const Result();
  R when<R>({required R Function(T) ok, required R Function(E) err});
}

class Ok<T, E> extends Result<T, E> {
  final T value; const Ok(this.value);
  R when<R>({required R Function(T) ok, required R Function(E) err}) => ok(value);
}

class Err<T, E> extends Result<T, E> {
  final E error; const Err(this.error);
  R when<R>({required R Function(T) ok, required R Function(E) err}) => err(error);
}

This compact API provides a single entry point (when) for consumers to handle both branches. You can add map, flatMap, or fold helpers as needed.

Using Result With Async APIs

Wrap network, database, or platform calls so they return Future>. Convert exceptions into domain errors at the boundary, so upstream code deals with a single error type.

Future<Result<User, AppError>> fetchUser(String id) async {
  try {
    final resp = await httpClient.get('/users/$id');
    if (resp.statusCode == 200) return Ok(User.fromJson(resp.body));
    return Err(AppError.server('Status ${resp.statusCode}'));
  } catch (e) {
    return Err(AppError.network(e.toString()));
  }
}

Key considerations:

• Use a domain AppError enum/class that captures recoverable and unrecoverable cases.

• Convert low-level exceptions (socket, timeout, parsing) into AppError at the boundary.

• Keep the async flow non-throwing so callers can chain operations with flatMap/map.

Integrating With Flutter Widgets

In UI code, Result helps separate presentation from error mapping. Avoid try/catch in widgets; instead handle Result variants and show appropriate visuals.

Pattern: call service -> get Future -> update state with Ok/Err -> render via when/fold.

Example usage in a stateful widget or a provider: store Result in state and build UI from it. Use when to map the result to widgets: show data, a friendly error message, or a loading indicator while awaiting.

Best practices:

• Map AppError to localized messages in a single place (error translator) rather than sprinkling strings across widgets.

• Use small helper widgets for error states (RetryButton that calls the same action).

• In lists or pagination, keep Result around per item or page to preserve partial success state.

Result types also play well with state management libraries (Provider, Riverpod, Bloc). They make state transitions explicit and reduce the need for custom loading/error flags.

Common Patterns And Extensions

• map/flatMap: Add these methods to Result to transform values without unwrapping.

• Combine results: When multiple independent operations must succeed, short-circuit on the first Err and aggregate errors when needed.

• Testing: Unit tests become straightforward; assert Ok or Err variants and inspect payloads.

Example: combining two results

• Use flatMap to chain dependent calls.

• Use a combine helper to run independent calls and collect successes or the first error.

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Conclusion

Implementing a Result type in Dart makes error handling explicit and composable, which is crucial for robust Flutter mobile development. Wrap boundaries where exceptions occur, convert low-level errors into domain types, and expose Future> from services. In the UI, render results deterministically and map domain errors centrally to user-friendly messages. This approach reduces runtime surprises, simplifies testing, and improves maintainability of mobile apps.