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zajrik / option_result

A lightweight Dart library for Rust-like Option/Result types and associated pattern matching
https://pub.dev/packages/option_result
MIT License
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dart library option pattern-matching result

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option_result is a lightweight library with the goal of bringing Rust's Option and Result types to Dart.

This library aims to provide as close to a 1:1 experience in Dart as possible to Rust's implementation of these types, carrying over all of the methods for composing Option and Result values (and_then(), or_else(), map(), etc.) and allowing the use of Dart 3's new exhaustive pattern matching to provide a familiar experience while working with Option and Result type values.

Overview

Option

Option types represent the presence (Some) or absence (None) of a value.

Dart handles this pretty well on its own via null and a focus on null-safety built in to the compiler and analyzer, but we can do better.

The advantage of Option types over nullable types lies in their composability. Option type values have many methods that allow composing many Option-returning operations together and helpers for propagating None values in larger operations without the need for repetitive null-checking.

This supports writing clean, concise, and most importantly, safe code.

Option<int> multiplyBy5(int i) => Some(i * 5);
Option<int> divideBy2(int i) => switch (i) {
  0 => None(),
  _ => Some(i ~/ 2)
};

Option<int> a = Some(10);
Option<int> b = Some(0);
Option<int> c = None();

Option<int> d = a.andThen(divideBy2).andThen(multiplyBy5); // Some(25)
Option<int> e = b.andThen(divideBy2).andThen(multiplyBy5); // None()
Option<int> f = c.andThen(divideBy2).andThen(multiplyBy5); // None()

For safety, operations culminating in an Option that make use of other Option values in their logic where the outcome is dependent on those Option values can benefit from None value propagation via catchOption():

// If user or email is None when unwrapped, catchOption will exit early, returning None
Option<String> getUserEmailLowerCase(int id) => catchOption(() {
  Option<User> user = getUser(id);
  // Unwrap user easily here by calling it like a function, as an alternative to:
  // Option<String> email = user.unwrap().email;
  Option<String> email = user().email;

  return Some(email().toLowerCase());
});

Option<String> email = getUserEmailLowerCase(12345);

switch (email) {
  case Some(value: String value): print('User email: $value');
  case None(): print('User does not have a valid email');
}

Result

Result types represent the result of some operation, either success (Ok), or failure (Err), and both variants can hold data.

This promotes safe handling of error values without the need for try/catch blocks while also providing composability like Option via methods for composing Result-returning operations together and helpers for propagating Err values within larger operations without the need for repetitive error catching, checking, and rethrowing.

Again, like Option, this helps promote clean, concise, and safe code.

Result<int, String> multiplyBy5(int i) => Ok(i * 5);
Result<int, String> divideBy2(int i) => switch (i) {
  0 => Err('divided by 0'),
  _ => Ok(i ~/ 2)
};

Result<int, String> a = Ok(10);
Result<int, String> b = Ok(0);
Result<int, String> c = Err('foo');

Result<int, String> d = a.andThen(divideBy2).andThen(multiplyBy5); // Ok(25)
Result<int, String> e = b.andThen(divideBy2).andThen(multiplyBy5); // Err('divided by 0')
Result<int, String> f = c.andThen(divideBy2).andThen(multiplyBy5); // Err('foo')

And, you guessed it, like Option, Result types can also benefit from safe propagation of their Err values using catchResult():

// If user or email is Err when unwrapped, catchResult will exit early, returning Err
Result<String, String> getUserEmailLowerCase(int id) => catchResult(() {
  Result<User, String> user = getUser(id);
  // Unwrap user easily here by calling it like a function, as an alternative to:
  // Result<String, String> email = user.unwrap().getEmail();
  Result<String, String> email = user().getEmail();

  return Ok(email().toLowerCase());
});

Result<String, String> email = getUserEmailLowerCase(12345);

switch (email) {
  case Ok(value: String value): print('User email: $value');
  case Err(value: String err): print('Error fetching email: $err');
}

But Result doesn't always have to concern data. A Result can be used strictly for error handling, where an Ok simply means there was no error and you can safely continue. In Rust this is typically done by returning the unit type () as Result<(), E> and the same can be done in Dart with an empty Record via ().

Result<(), String> failableOperation() {
  if (someReasonToFail) {
    return Err('Failure');
  }

  return Ok(());
}

Result<(), String> err = failableOperation();

if (err case Err(value: String error)) {
  print(error);
  return;
}

// No error, continue...

To further support this, just like how you can unwrap Option and Result values by calling them like a function, an extension for Future<Option<T>> and Future<Result<T, E>> is provided to allow calling them like a function as well which will transform the future into a future that unwraps the resulting Option or Result when completing.

(This also applies to FutureOr values.)

// Here we have two functions that return Result<(), String>, one of which is a Future.
// We can wrap them in a catchResult block (async in this case) and call them like a function
// to unwrap them, discarding the unit value if Ok, or propagating the Err value otherwise.
Result<(), String> err = await catchResultAsync(() async {
  await failableOperation1()();
  failableOperation2()();

  return Ok(());
});

if (err case Err(value: String error)) {
  print(error);
  return;
}

Note that just like how unit has one value in Rust, empty Record values in Dart are optimized to the same runtime constant reference so there is no performance or memory overhead when using () as a unit type.

Key differences from Rust

Getting started

Add the dependency to your pubspec.yaml file in your Dart/Flutter project:

dependencies:
  option_result: ^3.2.0

Or via git:

dependencies:
  option_result:
    git: https://github.com/zajrik/option_result.git

Then run dart pub get or flutter pub get and import the library:

import 'package:option_result/option_result.dart';
// or import the separate types individually:
import 'package:option_result/option.dart';
import 'package:option_result/result.dart';

Basic Usage

// Assume getUser() returns some sort of User object
Result<User, String> user = await getUser(id: 12345);

if (user case Err(value: String error)) {
  print('Error retrieving user: $error');
  return;
}

// Assume the User object has an email field of type Option<String>
Option<String> email = user.unwrap().email;

if (email case Some(value: String address)) {
  print('User email: $address');
} else {
  print('User has no email set.');
}

// Alternative to the above using a switch expression for pattern matching
print(switch (email) {
  Some(value: String address) => 'User email: $address',
  None() => 'User has no email set.'
});

// Pattern matching with switch is exhaustive for Option and Result, so the compiler
// will give you warnings/errors to make sure you're providing cases for all potential
// values for Some()/Ok(), either directly or via a default case, and for None()/Err(),
// again either directly or via a default case

Pattern-matching verbosity

All of the prior examples in this document are using the most verbose syntax for pattern matching possible, but Dart does provide some sugar to make our lives as developers a little easier and this library provides a bit of its own sugar too.

Consider the following if-case:

if (result case Err(value: String value)) {}

This example checks if result is Err and that its value field contains a String type value, which it binds to the scoped variable of the same name, value.

This level of verbosity is necessary if you want to rebind the value field to a scoped variable of a different name like so:

if (result case Err(value: String foo)) {}

But if you're comfortable with your scoped variable being named value then you can make use of the field-access shorthand that Dart provides:

if (result case Err(:String value)) {}
if (result case Err(:final value)) {}
if (result case Err(:var value)) {}

These are all functionally identical but the lack of repetition really cleans things up.

To clean things up even further, the Option and Result types have a few shorthand getters you can take advantage of:

if (result case Ok(:var v)) {}
if (result case Ok(:var val)) {}
if (result case Err(:var e)) {}
if (result case Err(:var error)) {}
// Err types also have v, val

These can also be used for rebinding:

if (result case Err(e: var foo)) {}

The exhaustive list of value field shorthand getters for Option and Result is as follows:

Potential extension conflicts

This library provides 4 total class extensions on the following types:

These extensions provide a call() method to allow calling these types like functions to unwrap the underlying Option and Result types. In the event that any other library provides a call() method on Future/FutureOr types that you need to make use of instead, you can hide the following extensions from this library:

You can accomplish this like so:

// Whitespace doesn't matter here, just keeping it clean
import 'package:option_result/option_result.dart'
  hide
    OptionFutureUnwrap,
    OptionFutureOrUnwrap,
    ResultFutureUnwrap,
    ResultFutureOrUnwrap;

// Or if you're only importing one of the types from the package:
import 'package:option_result/option.dart'
  hide
    OptionFutureUnwrap,
    OptionFutureOrUnwrap;

Similar packages

I started writing this library because there are many options (pun-intended) out there that accomplished similar goals but none of them stuck out to me at a glance as something that fit my needs. Pretty much all of them provided faux-pattern-matching via higher-order functions which I didn't care for, and I wanted to be able to make use of Dart 3's new exhaustive pattern matching which none of the libraries that I could find provided at the time of starting this project.

Final thoughts

I've had a lot of fun writing this library. I haven't had a good project to work on in quite some time so even if I'm the only person to ever end up using this, I'm still content that I took the time to write it and put it out there. It was a nice exercise.

Functional programming in Dart is not my goal and never really was. I just like Option and Result types for null/error handling. I always find myself thinking about them whenever I try new languages without a similar concept.

With all of that said, if you're reading this: Thank you for taking the time to explore this library, even if it's not what you need for your projects.