philiprehberger_result_type

Rust-style Result type for type-safe error handling with pattern matching

Requirements

• Dart >= 3.6

Installation

Add to your pubspec.yaml:

dependencies:
  philiprehberger_result_type: ^0.4.0

Then run:

dart pub get

Usage

import 'package:philiprehberger_result_type/philiprehberger_result_type.dart';

Result<int, String> divide(int a, int b) {
  if (b == 0) return Result.err('Division by zero');
  return Result.ok(a ~/ b);
}

Pattern Matching

final result = divide(10, 2);

final message = result.when(
  ok: (value) => 'Result: $value',
  err: (error) => 'Error: $error',
);
print(message); // Result: 5

Fold

A positional alternative to when() for terser destructuring.

final message = divide(10, 2).fold(
  (value) => 'Result: $value',
  (error) => 'Error: $error',
);

Swap

Swap the Ok and Err variants — useful for inverting validation results.

final swapped = Result<int, String>.ok(42).swap();
print(swapped); // Err(42)

final inverted = Result<int, String>.err('missing').swap();
print(inverted); // Ok(missing)

Transformations

// Map success values
final doubled = Result<int, String>.ok(5).map((v) => v * 2);
print(doubled); // Ok(10)

// Chain operations
final chained = Result<int, String>.ok(10)
    .flatMap((v) => v > 0 ? Result.ok(v) : Result.err('must be positive'));

// Transform errors
final mapped = Result<int, String>.err('not found')
    .mapErr((e) => 'Error: $e');

Async Error Handling

final result = await Result.tryAsync(
  () async => fetchDataFromApi(),
  (error, stack) => 'Request failed: $error',
);

Collecting Results

final results = [Result<int, String>.ok(1), Result.ok(2), Result.ok(3)];
final collected = Result.collect(results);
print(collected); // Ok([1, 2, 3])

final mixed = [Result<int, String>.ok(1), Result.err('bad')];
final failed = Result.collect(mixed);
print(failed); // Err(bad)

Unwrapping

final ok = Result<int, String>.ok(42);
print(ok.unwrap());      // 42
print(ok.unwrapOr(0));   // 42

final err = Result<int, String>.err('fail');
print(err.unwrapOr(0));  // 0
// err.unwrap();          // throws StateError

Unwrap with Lazy Default

final result = Err<int, String>('missing');
final value = result.unwrapOrElse((e) => e.length);
print(value); // 7

Expect

final result = Result<int, String>.ok(42);
print(result.expect('value must exist')); // 42

// On Err, throws StateError with custom message:
// Result<int, String>.err('fail').expect('value must exist');

Sync Error Handling

final result = Result.trySync(
  () => int.parse('42'),
  (error) => 'parse failed: $error',
);
print(result); // Ok(42)

Filtering

final result = Result<int, String>.ok(5)
    .filter((v) => v > 0, (v) => 'expected positive, got $v');
print(result); // Ok(5)

final filtered = Result<int, String>.ok(-1)
    .filter((v) => v > 0, (v) => 'expected positive, got $v');
print(filtered); // Err(expected positive, got -1)

Flattening Nested Results

final nested = Result<Result<int, String>, String>.ok(Result.ok(42));
final flat = Result.flatten(nested);
print(flat); // Ok(42)

final nestedErr = Result<Result<int, String>, String>.err('outer error');
final flatErr = Result.flatten(nestedErr);
print(flatErr); // Err(outer error)

Zipping Results

final a = Result<int, String>.ok(1);
final b = Result<int, String>.ok(2);
final zipped = Result.zip(a, b);
print(zipped); // Ok((1, 2))

API

Method	Description
Result.ok(T value)	Create a success result
Result.err(E error)	Create a failure result
isOk	Whether this is an Ok result
isErr	Whether this is an Err result
okOrNull	Success value or null
errOrNull	Error value or null
unwrap()	Get value or throw StateError
unwrapOr(T default)	Get value or return default
unwrapOrElse(T Function(E) fn)	Get value or compute default from error
expect(String message)	Get value or throw StateError with message
map(U Function(T) f)	Transform success value
mapErr(F Function(E) f)	Transform error value
flatMap(Result<U,E> Function(T) f)	Chain Result-producing operation
filter(predicate, orElse)	Keep Ok if predicate passes, else convert to Err
when({ok, err})	Exhaustive pattern match
fold(onOk, onErr)	Reduce to a single value using positional callbacks
swap()	Convert Ok↔Err
Result.trySync(fn, onError)	Wrap sync operation into Result
Result.tryAsync(fn, onError)	Wrap async operation into Result
Result.collect(results)	Combine list of Results
Result.flatten(nested)	Unwrap nested Result<Result<T,E>,E> into Result<T,E>
Result.zip(a, b)	Combine two Results into a record

Development

dart pub get
dart analyze --fatal-infos
dart test

Support

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License

MIT