domain_error 5.0.3

Domain errors with Result, DomainResult, and captureResult for expected errors in Dart applications.

domain_error

Domain errors and explicit operation results for Dart and Flutter. Model an expected failure with DomainError, capture a throwing operation at its boundary, and handle or compose its Result without nested try/catch blocks.

The package is pure Dart. It has no Flutter SDK dependency and does not choose state management, logging or application layers for you.

Contents

• When to use it
• Install
• Quick start
• Model domain errors
• Capture operations
  • Synchronous capture and observation
  • Asynchronous capture
• Handle results
• Compose operations
  • Map and chain successful values
  • Translate errors and recover
  • Compose asynchronous work
• Exceptions and asynchronous boundaries
  • Callback failures and failed futures
  • Late Future and Stream errors
• Equality and mutable payloads
• Diagnostics and reporting
• API map
• Compatibility
• Migrate to 5.x
• Package skill
• Development
• License

When to use it

Use a domain error when a caller needs to distinguish an expected application outcome, such as an empty order ID, an unavailable order or a rejected order. A feature-specific error hierarchy lets the caller make that decision without parsing exception messages.

Use Result<T, E> when an operation returns either a value of T or an error of E. The error can be any type. DomainResult<T> specializes it to DomainError; FutureDomainResult<T> is Future<DomainResult<T>>.

Use capture at a boundary where a throwing operation should become a domain result. Map other thrown objects into your vocabulary there. Keep that mapping selective so programming failures are not silently presented as expected outcomes. An ordinary return value or exception remains appropriate when the caller does not need a result value to inspect or compose.

Capture handles an operation and its returned future. Composition transforms existing result values. The package supplies no retries, cancellation, parallel execution, stream operator, logger or reporting integration.

Install

dart pub add domain_error
# In a Flutter project:
flutter pub add domain_error

Or add this dependency:

dependencies:
  domain_error: ^5.0.0

Import package:domain_error/domain_error.dart. Use the public import for all types and extensions; internal src paths are not separate supported entry points. Requires Dart >=3.0.0 <4.0.0. See compatibility for the tested Dart and Flutter endpoints.

Quick start

Every Dart block in this README is an independent program. Save one as bin/example.dart in a project with the dependency, then run dart --enable-asserts run bin/example.dart. This program prints Order 42.

import 'package:domain_error/domain_error.dart';

/// Converts an unavailable order into the application's error vocabulary.
final class OrderUnavailableError extends DomainError {
  const OrderUnavailableError({
    super.cause,
    super.stackTrace,
  });

  @override
  String get typeIdentifier {
    return 'OrderUnavailableError';
  }
}

Future<void> main() async {
  final result = await captureResult<int>(
    () async {
      return 42;
    },
    mapToDomainError: (error, trace) {
      return OrderUnavailableError(
        cause: error,
        stackTrace: trace,
      );
    },
  );

  final description = result.fold(
    (error) {
      return error.typeIdentifier;
    },
    (value) {
      return 'Order $value';
    },
  );

  assert(description == 'Order 42');
  print(description);
}

example/domain_error_example.dart contains a larger order-loading example with success, domain errors, timeouts and other thrown objects.

Model domain errors

Extend DomainError in your own library. A sealed feature root groups related outcomes while allowing exhaustive handling within your application's model. Set typeIdentifier explicitly. Preserve the string when renaming the Dart class; for example, EmptyOrderIdError can retain the identifier OrderIdEmptyError. Runtime type names and hash codes are not stable identifiers.

By default, equality includes the concrete error type, typeIdentifier and message. Diagnostic cause and stackTrace do not participate. Include business fields through props, preserving super.props:

import 'package:domain_error/domain_error.dart';

/// Groups order failures without prescribing an application architecture.
sealed class OrderError extends DomainError {
  const OrderError({
    super.message,
    super.cause,
    super.stackTrace,
  });
}

/// The rejection reason is business data and participates in equality.
final class OrderRejectedError extends OrderError {
  final String reason;

  const OrderRejectedError({
    required this.reason,
    super.message,
    super.cause,
    super.stackTrace,
  });

  @override
  String get typeIdentifier {
    return 'OrderRejectedError';
  }

  @override
  List<Object?> get props {
    return [
      ...super.props,
      reason,
    ];
  }
}

void main() {
  const first = OrderRejectedError(reason: 'closed');
  final diagnosticCopy = OrderRejectedError(
    reason: 'closed',
    cause: StateError('warehouse unavailable'),
    stackTrace: StackTrace.current,
  );
  const different = OrderRejectedError(reason: 'invalid quantity');

  assert(first == diagnosticCopy);
  assert(first.hashCode == diagnosticCopy.hashCode);
  assert(first != different);
}

Keep business fields and their nested collections immutable. A changing field in props can change the error's equality and hash while it is stored in a set or used as a map key.

Capture operations

Both capture functions take the operation as the first positional argument and require the named mapToDomainError callback. Observers are optional.

Operation outcome	Order	Returned result
Value	No observer or mapper	Result.success(value)
Thrown DomainError	onDomainError	Result.error holding the same error instance
Other thrown object	onRawError, then mapToDomainError	Result.error holding the mapped error

The mapped error does not trigger onDomainError again. Observers receive the caught stack trace. Stored diagnostic fields remain the mapper's responsibility; capture preserves an existing domain error without filling or replacing its fields.

Synchronous capture and observation

captureResultSync<T> returns DomainResult<T> immediately. The operation and mapper are synchronous. All three observers return Null: an async function or a void tear-off does not match the required type. Wrap a synchronous void function in a block ending with return null, as below.

Observer failures reach optional onObserverError. Its absence or its own failure never replaces the operation result, and it is not called recursively. A mapper failure propagates; it is not an observer failure.

import 'package:domain_error/domain_error.dart';

/// Keeps a technical cause without making it part of error equality.
final class OrderUnavailableError extends DomainError {
  const OrderUnavailableError({
    super.cause,
    super.stackTrace,
  });

  @override
  String get typeIdentifier {
    return 'OrderUnavailableError';
  }
}

void main() {
  final events = <String>[];
  final trace = StackTrace.fromString('order.load');
  const original = OrderUnavailableError();
  final raw = StateError('warehouse unavailable');

  /// Uses the same boundary to demonstrate both error routes.
  DomainResult<int> capture(Object failure) {
    return captureResultSync<int>(
      () {
        Error.throwWithStackTrace(failure, trace);
      },
      mapToDomainError: (error, stackTrace) {
        events.add('map');
        return OrderUnavailableError(
          cause: error,
          stackTrace: stackTrace,
        );
      },
      onDomainError: (error, stackTrace) {
        events.add('domain');
        throw StateError('observer failed');
      },
      onRawError: (error, stackTrace) {
        events.add('raw');
        return null;
      },
      onObserverError: (error, stackTrace) {
        events.add('report');
        throw StateError('reporting failed');
      },
    );
  }

  final preserved = capture(original);
  final mapped = capture(raw);

  assert(identical(preserved.domainError, original));
  assert(identical(mapped.domainError.cause, raw));
  assert(identical(mapped.domainError.stackTrace, trace));
  assert(events.join(',') == 'domain,report,raw,map');
}

Return-type checks cannot prevent a synchronous observer from starting unawaited work internally. Use asynchronous capture when observation must await work.

Asynchronous capture

captureResult<T> returns Future<DomainResult<T>>. It accepts FutureOr callbacks and awaits the operation, selected observer, observer failure reporter and mapper in order. This example verifies that reporting completes before mapping.

import 'package:domain_error/domain_error.dart';

/// Stores the original failure and trace supplied to the mapper.
final class OrderUnavailableError extends DomainError {
  const OrderUnavailableError({
    super.cause,
    super.stackTrace,
  });

  @override
  String get typeIdentifier {
    return 'OrderUnavailableError';
  }
}

Future<void> main() async {
  final events = <String>[];
  final raw = StateError('warehouse unavailable');

  final result = await captureResult<int>(
    () async {
      await Future<void>.delayed(Duration.zero);
      throw raw;
    },
    onRawError: (error, trace) async {
      await Future<void>.delayed(Duration.zero);
      events.add('raw');
      throw StateError('observer failed');
    },
    onObserverError: (error, trace) async {
      await Future<void>.delayed(Duration.zero);
      events.add('report');
    },
    mapToDomainError: (error, trace) async {
      await Future<void>.delayed(Duration.zero);
      events.add('map');
      return OrderUnavailableError(
        cause: error,
        stackTrace: trace,
      );
    },
  );

  assert(result.isError);
  assert(identical(result.domainError.cause, raw));
  assert(events.join(',') == 'raw,report,map');
}

Handle results

Result<T, E> is sealed, with ResultSuccess<T, E> and ResultError<T, E> branches. Construct them with Result.success / Result.error or the concrete branch constructors. These constructors are const when their payload is const. The success type comes first.

Use fold(onError, onSuccess) or a switch to handle both branches. Only the selected callback runs. isSuccess and isError test the branch. Reading value from an error, or error from a success, throws StateError. domainError is the error getter on DomainResult<T> and also throws on success.

A null payload is a valid success or error; branch identity does not depend on whether the payload is null. A Result<void, E> can represent successful completion without a useful value.

import 'package:domain_error/domain_error.dart';

/// Matches the public concrete branches with Dart 3 object patterns.
String describe(Result<int, String> result) {
  switch (result) {
    case ResultError(:final error):
      return 'Error: $error';
    case ResultSuccess(:final value):
      return 'Order $value';
  }
}

void main() {
  const success = Result<int, String>.success(42);
  const error = Result<int, String>.error('missing');
  const nullableSuccess = ResultSuccess<int?, String?>(null);
  const nullableError = ResultError<int?, String?>(null);
  const completed = Result<void, String>.success(null);

  final folded = success.fold(
    (error) {
      return 'Error: $error';
    },
    (value) {
      return 'Order $value';
    },
  );

  assert(folded == describe(success));
  assert(describe(error) == 'Error: missing');
  assert(success.isSuccess && success.value == 42);
  assert(error.isError && error.error == 'missing');
  assert(nullableSuccess.isSuccess && nullableSuccess.value == null);
  assert(nullableError.isError && nullableError.error == null);
  assert(nullableSuccess != nullableError);
  assert(completed.isSuccess);
}

Compose operations

Composition selects one branch at each step. Skipped callbacks do not run; selected steps run once, in order. Unchanged payloads retain their references; wrapper identity is not a contract. Composition does not capture exceptions.

Map and chain successful values

map<U> turns a successful value into a value of U. andThen<U> runs an operation returning Result<U, E>, so it can choose success or error. Both skip an existing error and preserve its payload.

import 'package:domain_error/domain_error.dart';

void main() {
  final events = <String>[];

  final total = const Result<int, String>.success(2).map(
    (quantity) {
      events.add('price');
      return quantity * 20;
    },
  ).andThen(
    (subtotal) {
      events.add('limit');
      if (subtotal > 100) {
        return const Result<int, String>.error('limit exceeded');
      }
      return Result<int, String>.success(subtotal);
    },
  );

  final skipped = const Result<int, String>.error('sold out').andThen(
    (quantity) {
      events.add('unexpected');
      return Result<int, String>.success(quantity);
    },
  );

  assert(total.value == 40);
  assert(skipped.error == 'sold out');
  assert(events.join(',') == 'price,limit');
}

Translate errors and recover

mapError<F> changes the error type from E to F, preserving a success. recover runs only for an error and returns another Result<T, E>. Return a success for a selected fallback or an error to preserve or replace the failure. Recovery preserves both type arguments; use mapError to change error vocabulary.

import 'package:domain_error/domain_error.dart';

/// A small application vocabulary for a boundary's numeric error codes.
enum OrderFailure {
  missing,
  unavailable,
}

void main() {
  final translated = const Result<int, int>.error(404).mapError<OrderFailure>(
    (code) {
      if (code == 404) {
        return OrderFailure.missing;
      }
      return OrderFailure.unavailable;
    },
  );

  final recovered = translated.recover(
    (error) {
      if (error == OrderFailure.missing) {
        return const Result<int, OrderFailure>.success(0);
      }
      return Result<int, OrderFailure>.error(error);
    },
  );

  var called = false;
  final unchanged = const Result<int, OrderFailure>.success(42).recover(
    (error) {
      called = true;
      return Result<int, OrderFailure>.error(error);
    },
  );

  assert(translated.error == OrderFailure.missing);
  assert(recovered.value == 0);
  assert(unchanged.value == 42 && !called);
}

Compose asynchronous work

Receiver	Methods	Callback completion	Return
Result<T, E>	map, andThen, mapError, recover	Synchronous	Result
Result<T, E>	mapAsync, andThenAsync, mapErrorAsync, recoverAsync	Awaited FutureOr	Future<Result>
Future<Result<T, E>>	map, andThen, mapError, recover	Source and selected FutureOr callback awaited	Future<Result>

On a future, the unsuffixed methods come from the public FutureResult<T, E> extension. A chain needs one final await; call fold on the completed result. The generic future has no FutureResult typedef. Use FutureDomainResult<T> for the domain specialization or Future<Result<T, E>> for other error types.

import 'package:domain_error/domain_error.dart';

Future<void> main() async {
  final mapped = await const Result<int, String>.success(20).mapAsync(
    (value) async {
      return value + 1;
    },
  );
  final chained = await mapped.andThenAsync(
    (value) async {
      return Result<int, String>.success(value * 2);
    },
  );
  final translated = await const Result<int, int>.error(404).mapErrorAsync(
    (code) async {
      return 'HTTP $code';
    },
  );
  final recovered = await translated.recoverAsync(
    (error) async {
      return const Result<int, String>.success(0);
    },
  );

  final events = <String>[];
  final pipeline = await Future<Result<int, String>>.value(
    const Result.success(20),
  ).andThen(
    (value) async {
      events.add('chain');
      return Result<int, String>.success(value + 1);
    },
  ).map(
    (value) {
      events.add('map');
      return value * 2;
    },
  ).mapError(
    (error) {
      events.add('unexpected translation');
      return error;
    },
  ).recover(
    (error) async {
      events.add('unexpected recovery');
      return const Result<int, String>.success(-1);
    },
  );

  assert(chained.value == 42);
  assert(translated.error == 'HTTP 404');
  assert(recovered.value == 0);
  assert(pipeline.value == 42);
  assert(events.join(',') == 'chain,map');
}

The maintained composition example also combines capture, observation, nullable recovery and chaining.

Exceptions and asynchronous boundaries

Callback failures and failed futures

Observer failure isolation belongs to capture. Mapper failures propagate with their original objects and stack traces. A thrown transform or recovery failure also propagates. recover handles an error payload inside a result; it does not handle a failed source future. Capture a throwing operation explicitly when its failure should become a domain error.

import 'package:domain_error/domain_error.dart';

Future<void> main() async {
  final failure = StateError('callback failed');
  final trace = StackTrace.fromString('original callback trace');
  var recovered = false;
  var propagated = 0;

  try {
    captureResultSync<int>(
      () {
        throw StateError('operation failed');
      },
      mapToDomainError: (error, stackTrace) {
        Error.throwWithStackTrace(failure, trace);
      },
    );
  } on Object catch (error, stackTrace) {
    assert(identical(error, failure));
    assert(identical(stackTrace, trace));
    propagated++;
  }

  try {
    const Result<int, String>.success(42).map<int>(
      (value) {
        Error.throwWithStackTrace(failure, trace);
      },
    );
  } on Object catch (error, stackTrace) {
    assert(identical(error, failure));
    assert(identical(stackTrace, trace));
    propagated++;
  }

  try {
    await Future<Result<int, String>>.error(failure, trace).recover(
      (error) {
        recovered = true;
        return const Result<int, String>.success(0);
      },
    );
  } on Object catch (error, stackTrace) {
    assert(identical(error, failure));
    assert(identical(stackTrace, trace));
    propagated++;
  }

  assert(propagated == 3);
  assert(!recovered);
}

Late Future and Stream errors

captureResultSync<T> stores its returned T unchanged. If T is a Future or Stream, later errors occur outside that synchronous call. Likewise, capturing the creation of a stream does not observe subscription errors. Await the future or consume the stream inside an asynchronous capture boundary:

import 'package:domain_error/domain_error.dart';

/// Represents a failure observed while awaiting or consuming work.
final class OrderUnavailableError extends DomainError {
  const OrderUnavailableError({
    super.cause,
    super.stackTrace,
  });

  @override
  String get typeIdentifier {
    return 'OrderUnavailableError';
  }
}

Future<void> main() async {
  final failure = StateError('late failure');
  final trace = StackTrace.fromString('asynchronous operation');

  final pending = captureResultSync<Future<int>>(
    () {
      return Future<int>.error(failure, trace);
    },
    mapToDomainError: (error, stackTrace) {
      return OrderUnavailableError(
        cause: error,
        stackTrace: stackTrace,
      );
    },
  );

  final awaited = await captureResult<int>(
    () {
      return pending.value;
    },
    mapToDomainError: (error, stackTrace) {
      return OrderUnavailableError(
        cause: error,
        stackTrace: stackTrace,
      );
    },
  );
  final consumed = await captureResult<int>(
    () async {
      return await Stream<int>.error(failure, trace).first;
    },
    mapToDomainError: (error, stackTrace) {
      return OrderUnavailableError(
        cause: error,
        stackTrace: stackTrace,
      );
    },
  );

  assert(pending.isSuccess);
  assert(identical(awaited.domainError.cause, failure));
  assert(identical(consumed.domainError.cause, failure));
}

For a long-lived stream subscription, own error handling and cancellation at the subscription boundary. Capture is not a stream transformer or a zone-wide handler.

Equality and mutable payloads

Result equality requires the same concrete branch and both type arguments, plus deeply equal payloads. The payload comparison checks both directions. For example, Result<int, String>.success(7) differs from Result<num, String>.success(7), even though the numeric values compare equal. Hashing includes the concrete result type and deep payload hash.

Result stores payloads by reference without copying them. Mutating a payload can change equality and hashing and make an existing map key or set element unfindable. An unmodifiable outer collection does not protect nested mutable collections. Snapshot every mutable layer you own:

import 'package:domain_error/domain_error.dart';

void main() {
  final source = <String, List<int>>{
    'items': [1, 2],
  };
  final snapshot = Map<String, List<int>>.unmodifiable({
    for (final entry in source.entries) entry.key: List<int>.unmodifiable(entry.value),
  });
  final result = Result<Map<String, List<int>>, String>.success(snapshot);
  final results = {result};

  source['items']!.add(3);

  assert(identical(result.value, snapshot));
  assert(result.value['items']!.length == 2);
  assert(results.contains(result));
  assert(
    result ==
        Result<Map<String, List<int>>, String>.success({
          'items': [1, 2],
        }),
  );
  assert(
    const Result<int, String>.success(7) != const Result<num, String>.success(7),
  );
}

The same immutability discipline applies to domain error business fields in props. Diagnostic fields excluded from equality do not make mutable business payloads safe as keys.

Diagnostics and reporting

DomainError.toString() uses typeIdentifier and the first non-empty description: message, then cause. It trims whitespace and keeps the first line, recognizing LF, CRLF and CR. A nested domain error contributes its identifier. An empty, opaque or throwing cause description becomes unknown; without a description, only the identifier remains. The original fields remain available.

This is display formatting, not redaction. Messages, causes, stack traces and toString() output can contain private data. Define an allowlist for remote reporting, and prepare user-facing messages separately from technical causes. The package does not send reports or decide which fields are safe.

import 'package:domain_error/domain_error.dart';

/// Keeps the display message separate from technical diagnostic context.
final class OrderUnavailableError extends DomainError {
  const OrderUnavailableError({
    super.message,
    super.cause,
    super.stackTrace,
  });

  @override
  String get typeIdentifier {
    return 'OrderUnavailableError';
  }
}

void main() {
  final error = OrderUnavailableError(
    message: '  Try again later\nAdditional internal detail',
    cause: StateError('private diagnostic context'),
    stackTrace: StackTrace.current,
  );

  /// Only explicitly selected fields cross the reporting boundary.
  final report = <String, String>{
    'type': error.typeIdentifier,
    'operation': 'order.load',
  };

  assert(error.toString() == 'OrderUnavailableError(Try again later)');
  assert(error.message!.contains('Additional internal detail'));
  assert(report.length == 2);
  assert(report['type'] == 'OrderUnavailableError');
}

API map

All entries are exported from package:domain_error/domain_error.dart.

Public interface	Purpose and constraints
DomainError	Abstract Exception model; const constructor with optional message, cause, stackTrace
typeIdentifier, message, cause, stackTrace	Stable error vocabulary, description and diagnostic context
props, equality, hashCode, toString()	Business identity and diagnostic display; see the sections above
Result<T, E>	Sealed generic outcome, success type first; its unnamed base constructor is not directly instantiable
Result.success(value), Result.error(error)	Const factories for the two branches
ResultSuccess<T, E>, ResultError<T, E>	Const concrete constructors and object-pattern types; public value / error fields
isSuccess, isError, value, error	Branch predicates and checked branch access
fold<U>(onError, onSuccess)	Error callback first; returns the selected callback's U
map<U>, andThen<U>	Change the success type; preserve E
mapError<F>	Change the error type; preserve T
recover	Return another result on error; preserve T and E
mapAsync, andThenAsync, mapErrorAsync, recoverAsync	Await a selected FutureOr callback on an existing result
FutureResult<T, E>	Extension with unsuffixed composition methods on Future<Result<T, E>>
DomainResult<T>	Type alias for Result<T, DomainError>
FutureDomainResult<T>	Type alias for Future<DomainResult<T>>
DomainResultError<T>.domainError	Extension getter for the error of a domain result
captureResult<T>	Await an operation and callbacks; return Future<DomainResult<T>>
captureResultSync<T>	Run synchronous work; return DomainResult<T> without awaiting a future payload
operation, mapToDomainError	Required positional operation and required named mapper
onDomainError, onRawError, onObserverError	Optional domain/raw observers and isolated observer failure reporter

Extension names matter for explicit extension overrides and show/hide imports; ordinary instance syntax is usually sufficient. Read the published API reference for full signatures.

Compatibility

The package declares Dart >=3.0.0 <4.0.0 and no Flutter SDK constraint. Dart 3 syntax is required by its class modifiers and patterns. A separate Flutter consumer resolves the library without dependency overrides.

Environment	Verification
Dart 3.0.0	Minimum SDK: library checks, examples and README programs
Current stable Dart	Library checks, examples and README programs
Flutter 3.10.0 / Dart 3.0.0	Consumer analysis/tests with normal and lowest runtime dependency resolution
Current stable Flutter	Consumer analysis/tests with its independent resolution

The CI workflow checks these endpoints. Flutter 3.10.0 resolves collection 1.17.1 and meta 1.9.1; its downgrade check also selects equatable 2.0.7. Intermediate SDK versions are not each tested by this matrix. Platform declarations describe a pure Dart library and are not physical-device test results.

The current API guide is for 5.x. The portable skill also maintains separate 3.x and 4.x contracts. Historical migrations below do not imply that every old package version was tested with every current SDK.

Migrate to 5.x

Keep the package name and public import. The table maps legacy spelling to the current interface; choose the rows for your starting version. There are no compatibility aliases.

Starting API	Current 5.x API
2.x Either<L, R>, Either.err / Either.ok	Result<R, L>, Result.error / Result.success
2.x EitherErr<L, R> / EitherOk<L, R>	ResultError<R, L> / ResultSuccess<R, L>
2.x isErr / isOk, errValue	isError / isSuccess, error
2.x DomainError.rawError, including constructor forwarding	cause, including super.cause
2.x executeSafely / executeSafelySync	captureResult / captureResultSync
2.x ExecuteSafelyOptions / ExecuteSafelySyncOptions	Move fields directly into named capture arguments; Options classes are removed
2.x mapRawErrorToDomain	mapToDomainError
2.x positional method, fold parameters ifErr / ifOk	operation, onError / onSuccess; positional call syntax stays the same
3.x CaptureResultOptions / CaptureResultSyncOptions	Move fields directly into named capture arguments
3.x / 4.x Either<L, R>, Either.error / Either.success	Result<R, L>, Result.error / Result.success
3.x / 4.x EitherError<L, R> / EitherSuccess<L, R>	ResultError<R, L> / ResultSuccess<R, L>
2.x / 3.x / 4.x domain Result<T> / FutureResult<T> aliases	DomainResult<T> / FutureDomainResult<T>
2.x / 3.x / 4.x successValue; 3.x / 4.x errorValue	value; error, including named pattern fields
2.x / 3.x / 4.x ResultDomainError<T>	DomainResultError<T>; domainError remains
4.x FutureEitherOperations<L, R>	FutureResult<R, L> extension; it is not a typedef

Reorder both generic arguments, including inside collections, futures, bounds, casts, type tests, branch constructors and explicit extension overrides.

Review behavior as well as names:

• Upgrading from 2.x: observer failures are isolated, optional onObserverError receives them, and a failure in that reporter does not recurse. Diagnostics no longer affect default domain error equality. Different concrete generic result types compare unequal; payload comparison is symmetric. Description formatting consistently uses one line and tolerates failing causes.
• Upgrading from 3.x or earlier: synchronous observers return Null. Wrap synchronous void functions; choose async capture for asynchronous observers. Composition provides explicit Async methods on a result and unsuffixed methods on a result future. Recovery returns another result with the same types.
• Upgrading from 4.x: 5.0 changes names and generic ordering while retaining capture and composition behavior, nullable/void payloads, stable identifiers and the SDK minimums.

Keep typeIdentifier strings, messages and diagnostic context when renaming classes. Do not use runtime type names or hash codes as persisted identity. The two historical wrong-branch StateError messages remain Either.success has no errorValue and Either.error has no successValue; these strings are diagnostic text, not compatibility APIs.

For historical detail, see the 3.0 changelog, 3.x to 4.0 guide and 4.x to 5.0 guide. The 4.0 migration program uses its historical API; run current examples against 5.x.

Package skill

Install the portable flutter-domain-errors guidance:

npx skills add andrey-krasheninnikov/domain_error --skill flutter-domain-errors

Installation is project-local by default. Add --agent codex claude-code to select those clients or --global for user-wide installation. The existing skills CLI needs a compatible Node.js installation; there is no separate npm package to install.

The skill covers pure Dart and Flutter with separate 3.x, 4.x and 5.x references. It checks the applicable package's direct dependency and resolved API, including path/Git sources, before choosing rules. Older, newer or unresolved APIs require source inspection. It does not silently upgrade dependencies, migrate untouched code or impose application architecture.

Automatic selection depends on client support and task relevance. Installation alone does not establish selection. The validation notes name the exercised client versions, scenarios and limits.

Development

From a checkout with the selected Dart SDK on PATH:

make deps
make check
dart run example/domain_error_example.dart
dart --enable-asserts run example/composition_example.dart
python3 -B -m unittest discover -s tool/readme_checks
python3 -B tool/readme_checks/check.py
make publish-dry-run

make check runs formatting checks, analysis and tests without rewriting source. Use make format to apply the Dart 3.0 language style at a line length of 120. The README checker requires Python 3.9 or later and uses its standard library. It extracts every Dart block into a temporary package, analyzes and formats-checks the programs, then runs them with assertions against this checkout. It also checks local files, image descriptions and heading anchors. The Dart CI matrix runs these checks on minimum and stable SDKs.

With Flutter 3.10.0 or current stable on PATH, run make flutter-deps, then make flutter-check. To test the lowest runtime dependencies, run flutter pub downgrade inside tool/flutter_consumer and repeat make flutter-check from the root. This consumer has its own dependency resolution and does not add Flutter to the published library.

Check portable skill examples with:

python3 -B -m unittest discover -s tool/skill_checks
python3 -B tool/skill_checks/check.py

Editable artwork lives in assets/cover.html, assets/overview.html, assets/flow.html and assets/fold.html, sharing assets/readme-art.css. Run make diagrams to export a 1600×640 cover and three 1600×900 diagrams. The renderer uses isolated temporary Chrome profiles and local resources; set CHROME_BIN to select Chrome or Chromium. The want dark artwork uses bundled IBM Plex and Pixelify Sans fonts under their SIL Open Font Licenses.

Use GitHub Issues for defects or proposals. Public contract changes need tests and migration notes. Project vocabulary is in the glossary; architecture decisions record the reasoning behind identity, capture, composition and compatibility.

License

MIT.