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A Dart implementation of the Result Monad which allows for more expressive result generation and processing without using exceptions.


Examples #

The project ships with examples which you can find here.

Simple Usage #

The simplest usage begins with defining the function return to be a Result Monad. The monad takes two types, the value type for when it succeeds and an error type for if it fails. The value type is always whatever the natural return of the function would be. The error type can be literally anything. For more complex cases you may have a reusable object type. For cases where you use the runCatching method to wrap exception-generating code in a way that returns a Result monad it'll be whatever the Exception type that was thrown. In the below case it is simply an error String. Since the monad can be queried for if it is encapsulating a success isSuccess or failed isFailure options the type chosen is more for how the developer decides to propagate their errors.

In the below case we aren't directly querying it but instead using the match method to have two different code paths depending on whether we are dealing with a succeeded or failed result. See the API documentation for more details.

import 'package:result_monad/result_monad.dart';

Result<double, String> invert(double value) {
  if (value == 0) {
    return Result.error('Cannot invert zero');

  return Result.ok(1.0/value);

void main() {
  // Prints 'Inverse is: 0.5'
      onSuccess: (value) => print("Inverse is: $value"),
      onError: (error) => print(error));

  // Prints 'Cannot invert zero'
      onSuccess: (value) => print("Inverse is: $value"),
      onError: (error) => print(error));

Intermediate Example: Temporary File Creation/Writing #

The example below is a larger example to show a more real world example. This program has methods for figuring out the temporary folder for the user and then creating a temporary file in that folder that the user can write to. It then attempts to write to that file. This shows how comprehensively one can use the various parts of the library, from the runCatching surrounding file I/O, to the mapError to create uniform return error types, and finally showing off andThen chaining and fold for final value extraction.

import 'dart:io';

import 'package:result_monad/result_monad.dart';

enum ErrorEnum { environment, fileAccess }

void main(List<String> arguments) {
  final stringToWrite = 'Data written to the temp file ${}';
  final tmpFileResult = getTempFile();

      onSuccess: (file) => print('Temp file: ${file.path}'),
      onError: (error) => print('Error getting temp file: $error'));

  // Probably would check if failure and stop here normally but want to show
  // that even starting with an error Result Monad flows correctly.
  final writtenSuccessfully = tmpFileResult
      .andThen((file) => runCatching(() {
    return Result.ok(file);
      .andThen((file) => runCatching(() => Result.ok(file.readAsStringSync())))
      onSuccess: (text) => text == stringToWrite,
      onError: (_) => false);

  print('Successfully wrote to temp file? $writtenSuccessfully');

Result<File, ErrorEnum> getTempFile(
    {String prefix = '', String suffix = '.tmp'}) {
  String tmpName = '$prefix${}$suffix';
  return getTempFolder()
      .andThen((tempFolder) =>
      .andThen((tmpPath) => Result.ok(File(tmpPath)))
      .mapError((error) => error is ErrorEnum ? error : ErrorEnum.fileAccess);

Result<String, ErrorEnum> getTempFolder() {
  String folderName = '';
  if (Platform.isMacOS || Platform.isWindows) {
    final varName = Platform.isMacOS ? 'TMPDIR' : 'TEMP';
    final tempDirPathFromEnv = Platform.environment[varName];
    if (tempDirPathFromEnv != null) {
      folderName = tempDirPathFromEnv;
    } else {
      return Result.error(ErrorEnum.environment);
  } else if (Platform.isLinux) {
    folderName = '/tmp';

  if (folderName.isEmpty) {
    return Result.error(ErrorEnum.environment);

  final Result<bool, dynamic> canWriteResult = runCatching(() {
    if (!Directory(folderName).existsSync()) {
      return Result.ok(false);

    final testFilePath =
    final tmpFile = File(testFilePath);

    return Result.ok(true);

  return canWriteResult
      .andThen<String, ErrorEnum>((canWrite) =>
  canWrite ? Result.ok(folderName) : Result.error(ErrorEnum.fileAccess))
      .mapError((_) => ErrorEnum.fileAccess);
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A Dart implementation of the Result Monad which allows for more expressive result generation and processing without using exceptions.

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