option_result

Description:

option result

option_result #
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()
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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');
}
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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')
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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');
}
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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...
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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;
}
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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 #

Option and Result types provided by this library are immutable. All composition
methods either return new instances or the same instance unmodified if applicable, and
methods for inserting/replacing values are not provided.

The benefits of immutability speak for themselves, but this also allows compile-time
const Option and Result values which can help improve application performance.

This library lacks all of the methods Rust's Option and Result types have
that are related to ref, deref, mut, pin, clone, and copy due to not
being applicable to Dart as a higher-level language.

The Option.filter()
method has been renamed where() to be more Dart-idiomatic.

The Option and Result methods mapOr, mapOrElse return Option<U> and Result<U, E>
respectively to aid composition of Option and Result values. The encapsulated
values of these types should never leave the context of Option or Result unless
explicitly unwrapped via the designated methods (unwrap(), expect(), etc.).

None()/Err() propagation is not supported at the language-level in Dart since
there's no concept of it so it's not quite as ergonomic as Rust, but is still quite
comfy and easily managed via the provided helpers.

Getting started #
Add the dependency to your pubspec.yaml file in your Dart/Flutter project:
dependencies:
option_result: ^3.2.0
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Or via git:
dependencies:
option_result:
git: https://github.com/zajrik/option_result.git
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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';
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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
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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)) {}
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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)) {}
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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)) {}
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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
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These can also be used for rebinding:
if (result case Err(e: var foo)) {}
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The exhaustive list of value field shorthand getters for Option and Result
is as follows:

Option

Some: v, val


Result

Ok: v, val
Err: v, val, e, error

Ideally err would be included but is not possible due to the err method
found on Result types





Potential extension conflicts #
This library provides 4 total class extensions on the following types:

Future<Option<T>>
FutureOr<Option<T>>
Future<Result<T, E>>
FutureOr<Result<T, E>>

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:

OptionFutureUnwrap
OptionFutureOrUnwrap
ResultFutureUnwrap
ResultFutureOrUnwrap

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;
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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.


oxidized - Provides Option and Result
types and is smiliarly close to a 1:1 representation of Rust's implementation as
this library but with a much cooler name.

Supports Dart 3's exhaustive pattern matching as of v6.0.0. This feature was
not available at the time of starting this project and probably would have prevented
me from wanting to start it at all had it been 🤣




ruqe - Provides Option and Result types,
as well as an Either type, which is like a Result type with extra steps.



either_option - Provides Option and
Either types.



crab - Provides Option and Result types.
Has a cool name.



fpdart - Functional programming in Dart. Very
thorougly documented. Provides Option and Either types and so much more. sealed
type support is in the works so expect proper pattern matching soon. 😎



dartz - Another functional programming library.
Predates fpdart but appears to no longer be receiving updates. Provides Option,
and Either types as well.

Also has a cool name.


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.