Result to handle errors in Dart.

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[Changelog] | [License]

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Introduction

It's while exploring the new features of Dart 3 and its sealed classes that I felt like packaging this little piece of code to exploit the power of pattern matching.

Because no matter what people say, functional programming can be really cool and powerful 😏

This package is a simple implementation of the Result Monad, which is a way to handle errors in a functional way. It's a simple wrapper around a value that can be either a success or a failure. It's a way to avoid throwing exceptions and to handle errors in a more explicit way.

The used naming convention is inspired by the Rust Result enum. (So you will find Ok and Err ).

Features

Note those code are pseudo-code, check tests and examples for more details and real code.

Different ways to create a Result:

• Result.ok(value) to create a successful result
• Ok(value) same as Result.ok(value)
• Result.success(value) same as Result.ok(value)
• Result.err(error, [stackTrace]) to create a failed result
• Err(error, [stackTrace]) same as Result.err(error, [stackTrace])
• Result.error(error, [stackTrace]) same as Result.err(error, [stackTrace])
• Result.failure(error, [stackTrace]) same as Result.err(error, [stackTrace])
• Result.from(() => value) to create a result from a function that can throw
• Result.fromAsync(() => future) to create a result from a future that can fail
• Result.fromCondition({condition, value, error}) to create a result from a condition
• Result.fromConditionLazy({condition: () => condition, value: () => value, error: () => error}) to create a result from a condition with lazy evaluation

Different ways to extract the value from a Result:

• result.ok to get the value if the result is successful and discard the error if it's a failure
• result.err to get the error if the result is a failure and discard the value if it's a success
• result.stackTrace to get the stack trace if the result is a failure
• result.expect() to get the value if the result is successful or throw an exception if it's a failure
• result.expectErr() to get the error if the result is a failure or throw an exception if it's a success
• result.unwrap() same as result.expect()
• result.unwrapErr() same as result.expectErr()
• result.unwrapOr(defaultValue) to get the value contained in the result or a default value if it's a failure
• result.unwrapOrElse((error) => defaultValue) to get the value contained in the result or a default value if it's a failure with lazy evaluation

Different ways to inspect the value of a Result:

• result.isOk to check if the result is successful
• result.isErr to check if the result is a failure
• result.contains(value) to check if the result contains a specific value
• result.containsErr(error) to check if the result contains a specific error
• result.containsLazy(() => value) to check if the result contains a specific value with lazy evaluation
• result.containsErrLazy(() => error) to check if the result contains a specific error with lazy evaluation
• result.inspect((value) => void) to inspect the value of the result
• result.inspectErr((error) => void) to inspect the error of the result
• result1 == result2 to check if two results are equal
• result1 != result2 to check if two results are not equal

Different ways to transform a Result:

• result.map<U>(U Function(value) transform) to transform the value of the result
• result.mapAsync<U>(FutureOr<U> Function(value) transform) to transform the value of the result asynchronously
• result.mapErr<U>(U Function(error) transform) to transform the error of the result
• result.mapErrAsync<U>(FutureOr<U> Function(error) transform) to transform the error of the result asynchronously
• result.mapOr<U>(U defaultValue, U Function(value) transform) to transform the value of the result or return a default value if it's a failure
• result.mapOrAsync<U>(FutureOr<U> defaultValue, FutureOr<U> Function(value) transform) to transform the value of the result or return a default value if it's a failure asynchronously
• result.mapOrElse<U>(U Function(value) defaultFn, U Function(error) transform) to transform the value and the error of the result
• result.mapOrElseAsync<U>(FutureOr<U> Function(value) defaultFn, FutureOr<U> Function(error) transform) to transform the value and the error of the result asynchronously
• result.fold<U>(U Function(value) okFn, U Function(error) errFn) same as mapOrElse with a different syntax
• result.foldAsync<U>(FutureOr<U> Function(value) okFn, FutureOr<U> Function(error) errFn) same as mapOrElseAsync with a different syntax
• result.flatten() to flatten a result of type Result<Result<T,E>,E> into a result of type Result<T,E>
• result.and<U>(Result<U,E> other) to combine two results
• result.andThen<U>(Result<U,E> Function(value) transform) to combine two results with a function
• result.or<F>(Result<T,F> other) to combine two results
• result.orElse<F>(Result<T,F> Function(error) transform) to combine two results with a function
• result1 & result2 to combine two results with the and operator
• result1 | result2 to combine two results with the or operator

Usage

Import the package:

import 'package:sealed_result/sealed_result.dart';

Create a function that can fail:

enum Version { version1, version2 }

Result<Version, ResultException> parseVersion(List<int> header) =>
  switch (header) {
    final header when header.isEmpty =>
      const Result.err(ResultException('invalid header length')),
    final header when header[0] == 1 => const Result.ok(Version.version1),
    final header when header[0] == 2 => const Result.ok(Version.version2),
    _ => const Result.err(ResultException('invalid version')),
  };

Use the function:

final version = parseVersion([1, 2, 3, 4]);
print(
  switch (version) {
    Ok(ok: final value) => 'working with version: $value',
    Err(err: final error) => 'error parsing header: $error',
  },
);

If, version = parseVersion([1, 2, 3, 4]) , then the output will be:

working with version: Version.version1

if, version = parseVersion([3, 2, 3, 4]) , then the output will be:

error parsing header: ResultException: invalid version

and, if version = parseVersion([]) , then the output will be:

error parsing header: ResultException: invalid header length

See more examples in example and test folders.

Notes

Result is a sealed class, so you can exhaustively match it with switch . So if you just want to get the value of the result, prefer using Dart 3 pattern matching instead of the fold method:

final result = Result.ok(42);
final value = switch (result) {
  Ok(ok: final value) => value,
  Err(err: final error) => 0,
};

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Multiple factory constructors are available to create a Result :

Result.ok(42);
Ok(42);
Result.success(42);

are equivalent, and

Result.err('error');
Err('error');
Result.error('error');
Result.failure('error');

are equivalent too.

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All Result methods can be used with Future thanks to the extension methods:

final Future<Result<int, Exception>> result = Result.fromAsync(() => Future.value(42));
final Future<Result<int, Exception>> mappedResult = result.map((value) => value * 2);

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This project uses Just for managing tasks, so you can run tests with:

just test

and generate coverage with:

just coverage

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