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.
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
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(:String v): print('User email: $v');
case Err(:String e): print('Error fetching email: $e');
}
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(e: 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(e: 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.
Option
andResult
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-timeconst
Option
andResult
values which can help improve application performance.- This library lacks all of the methods Rust's
Option
andResult
types have that are related toref
,deref
,mut
,pin
,clone
, andcopy
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
andResult
methodsmapOr
,mapOrElse
returnOption<U>
andResult<U, E>
respectively to aid composition ofOption
andResult
values. The encapsulated values of these types should never leave the context ofOption
orResult
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.
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';
// Assume getUser() returns some sort of User object
Result<User, String> user = await getUser(id: 12345);
if (user case Err(e: 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(v: 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(v: 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
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(e: String value)) {}
This example checks if result
is Err
and that its e
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 e
field to
a scoped variable of a different name like so:
if (result case Err(e: String foo)) {}
But if you're comfortable with your scoped variable being named e
then you can
make use of the field-access shorthand that Dart provides:
if (result case Err(:String e)) {}
if (result case Err(:final e)) {}
if (result case Err(:var e)) {}
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;
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
andResult
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 ๐คฃ
- 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 ๐คฃ
-
ruqe - Provides
Option
andResult
types, as well as anEither
type, which is like aResult
type with extra steps. -
either_option - Provides
Option
andEither
types. -
crab - Provides
Option
andResult
types. Has a cool name. -
fpdart - Functional programming in Dart. Very thorougly documented. Provides
Option
andEither
types and so much more.sealed
type support is in the works so expect proper pattern matching soon. ๐ -
dartz - Another functional programming library. Predates
fpdart
but appears to no longer be receiving updates. ProvidesOption
, andEither
types as well.
Also has a cool name.
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.