Explicit Outcome

Typed outcomes for explicit, predictable Dart development.

explicit_outcome provides Result and Option types — sealed, pattern-matchable containers that make success, failure, presence, and absence first-class citizens in your type signatures.

Installation

dart pub add explicit_outcome

Most users should install explicit instead, which re-exports these types and adds ergonomic utilities. Use explicit_outcome directly only when you need the low-level outcome types without additional utilities.

Result: Res<T, E>

Model operations that can succeed with a value or fail with an error — without throwing exceptions.

import 'package:explicit_outcome/explicit_outcome.dart';

Res<int, String> parsePort(String raw) {
  final value = int.tryParse(raw);
  if (value == null || value < 1 || value > 65535) {
    return Err('invalid port: $raw');
  }
  return Ok(value);
}

Creating results

Constructor	Purpose
Ok<T, E>(value)	Wrap a success value.
Err<T, E>(error)	Wrap an error value.
Res<T, E>	Compact typedef for Result<T, E>.
Success<T, E>	Compatibility alias for Ok.
Failure<T, E>	Compatibility alias for Err.

Consuming results

final result = parsePort('8080');

// Branch and return a value:
final message = result.fold(
  onSuccess: (port) => 'port=$port',
  onError: (error) => 'error=$error',
);

// Branch for side effects:
result.when(
  onSuccess: (port) => print('port=$port'),
  onError: (error) => print('error=$error'),
);

// Provide a default on error:
final port = result.getOrElse((error) => 8080);

// Pattern match:
switch (result) {
  case Ok(:final value):
    print('success: $value');
  case Err(:final error):
    print('error: $error');
}

Composing results

final pipeline = parsePort('8080')
    .map((port) => 'http://localhost:$port')       // transform success
    .mapError((e) => 'ConfigError: $e')            // transform error
    .next((url) => checkUrl(url))                  // chain another Result operation
    .or((error) => Ok('http://localhost:8080'));   // recover from error

Method	Signature	Behavior
map	Res<R, E> map<R>(R Function(T))	Transform the success value. Short-circuits on Err.
mapError	Res<T, R> mapError<R>(R Function(E))	Transform the error value. Short-circuits on Ok.
next	Res<R, E> next<R>(Res<R, E> Function(T))	Chain another result-returning operation. Short-circuits on Err.
or	Res<T, E> or(Res<T, E> Function(E))	Recover from error with an alternative result.
fold	R fold<R>({R Function(T), R Function(E)})	Branch on variant and return a value.
when	void when({void Function(T), void Function(E)})	Branch for side effects.
getOrElse	T getOrElse(T Function(E))	Return value or compute a default from the error.
isSuccess	bool	true if Ok.
isFailure	bool	true if Err.

Option: Opt<T>

Model presence and absence without null.

Opt<int> findIndex(List<String> items, String target) {
  final index = items.indexOf(target);
  if (index == -1) return Nil();
  return Val(index);
}

Creating options

Constructor	Purpose
Val<T>(value)	Wrap a present value.
Nil<T>()	Represent absence.
Opt<T>	Compact typedef for Option<T>.

Consuming options

final option = findIndex(names, 'Alice');

final label = option.fold(
  onVal: (i) => 'Found at $i',
  onNil: () => 'Not found',
);

option.when(
  onVal: (i) => print('index=$i'),
  onNil: () => print('absent'),
);

final index = option.getOrElse(() => -1);

Composing options

final result = findIndex(names, 'Alice')
    .map((i) => names[i].toUpperCase())
    .next((name) => validateName(name))
    .or(() => Val('DEFAULT'));

Method	Signature	Behavior
map	Opt<R> map<R>(R Function(T))	Transform the present value. Propagates Nil.
next	Opt<R> next<R>(Opt<R> Function(T))	Chain another option-returning operation. Propagates Nil.
or	Opt<T> or(Opt<T> Function())	Provide an alternative when Nil.
fold	R fold<R>({R Function(T), R Function()})	Branch on variant and return a value.
when	void when({void Function(T), void Function()})	Branch for side effects.
getOrElse	T getOrElse(T Function())	Return value or compute a default.
hasValue	bool	true if Val.
isNil	bool	true if Nil.

Async: AsyncRes<T, E> and AsyncOpt<T>

Lazy asynchronous wrappers. Work does not start until run() is called.

final pipeline = AsyncRes<int, String>(() async => Ok(2))
    .map((v) => v + 3)
    .next((v) => AsyncRes(() async => Ok(v * 10)));

final result = await pipeline.run(); // Res<int, String>

Semantics

Behavior	Detail
Lazy	No work happens until run() is called.
Short-circuit	After Err/Nil, downstream steps are not invoked.
No caching	Each run() re-executes the pipeline.
No retry	Failures are not retried.
No catching	Thrown exceptions propagate; use fold for expected failures.

AsyncRes methods

Method	Description
run()	Execute and return Future<Res<T, E>>.
map	Transform the success value (lazy).
next	Chain another AsyncRes operation (lazy).
mapError	Transform the error value (lazy).
or	Recover from error (lazy).

AsyncOpt methods

Method	Description
run()	Execute and return Future<Opt<T>>.
map	Transform the present value (lazy).
next	Chain another AsyncOpt operation (lazy).
or	Provide an alternative when Nil (lazy).
fold	Await and branch (convenience extension).
when	Await and side-effect (convenience extension).
getOrElse	Await and default (convenience extension).
hasValue	Await and check for Val (convenience extension).
isNil	Await and check for Nil (convenience extension).

ResultAsync<T, E> and OptionAsync<T> are convenience typedefs for Future<Res<T, E>> and Future<Opt<T>> respectively.

API Summary

Symbol	Kind	Description
Res<T, E>	typedef	Compact alias for Result<T, E>.
Result<T, E>	sealed class	Base type for success/error.
Ok<T, E>	class	Success variant.
Err<T, E>	class	Error variant.
Opt<T>	typedef	Compact alias for Option<T>.
Option<T>	sealed class	Base type for present/absent.
Val<T>	class	Present option variant.
Nil<T>	class	Absent option variant.
ResultAsync<T, E>	typedef	Future<Res<T, E>>.
AsyncRes<T, E>	class	Lazy async result.
OptionAsync<T>	typedef	Future<Opt<T>>.
AsyncOpt<T>	class	Lazy async option.
Success<T, E>	typedef	Compatibility alias for Ok.
Failure<T, E>	typedef	Compatibility alias for Err.

Compatibility

• Dart SDK: >=3.6.0 <4.0.0 (sealed classes, pattern matching, records).
• License: Mozilla Public License 2.0 (MPL-2.0).