Dartz - Functional programming in Dart

I want to use Either in this way:

Either<Failure, MyResponse> result = await restClient.request() // Future<MyResponse> request();
    .then((response) => Right(response))
    .catchError((failure) => Left(Failure()));

But it seems that I can't do this:

error: A value of type 'Right< dynamic, MyResponse>' can't be assigned to a variable of type 'Either< Failure, MyResponse>'.

I necessarily need to use explicit type casting, like this:

Either<Failure, MyResponse> result = await restClient.request() // Future<MyResponse> request();
    .then((response) => Right(response))
    .catchError((failure) => Left(Failure())) as Either<Failure, MyResponse>; 

even if I already return Left(Failure()) in the catchError(). So, is explicit casting the most concise and elegant solution?

Hi spebbe,

In the first place, I want to thank you to bring functional programing to Dart. However I have a question related to Task and Future. If you got time to take a look at my problem I will be thankfull.

I am running this code with no success and I don't have any clue left.

Task<Option<Product>> getProduct(int id) =>
      Task(() => this.httpClient.get(someUrl))
        .map((resp) => Some(ProductFromResp(resp)));

As I understand in this case Task takes a Function0 Future<Response> as a parameter and map the http response to produce an Option<Product>. However when I explicitly run getProduct(id).run() the task it doesn't seem to be fired.

Am I missing something ?

Regards

I would like to add default values to optional function parameters such as:

void process([Option<X> param = none()]) { ... }

This results in the error Default values of an optional parameter must be constant.

Enhancement request - add "const constructors" to dartz for simple "empty" values such as Option none and IList nil.

My current error prone workaround is to leave the default as null and use the "if null" operator ?? to replace the parameter param ?? none().

First of all, thanks for such extensive library. Usually it’s helpful to use a Reader<Future> monad stack to:

• inject dependencies
• perform asynchronous operations
• and return either a result or an error

Is it possible to achieve this in dartz?

Hi,

I wondered if anybody has considered having a Result - similar to Either but already has Ok, Err constructs.

More info here https://github.com/hqoss/monads/tree/master/lib/result. (typescript implementation)

and original rust implementation https://doc.rust-lang.org/std/result/enum.Result.html

Cheers

Hi, I'm currently working on a clean architecture project using Flutter. I find dartz package is useful to bring either success or failure values.

But, i have issues when it comes to testing.

This is is my test case which won't passed:

test('should load list of cast from repository', () async {
    // arrange
    final tId = 1;
    final tCast = Cast(...);
    final tCastList = <Cast>[tCast];

    when(mockCreditsRepository.loadCastByMovie(tId))
        .thenAnswer((_) async => Right(<Cast>[tCast]));
    // act
    final result = await usecase.execute(tId);
    // assert
    expect(result, Right(<Cast>[tCast]));
});

Those test will return as follows:

Expected: Right<dynamic, List<Cast>>:<Right([Cast(1, department, name, originalName, 0.0, profilePath, character)])>
  Actual: Right<Failure, List<Cast>>:<Right([Cast(1, department, name, originalName, 0.0, profilePath, character)])>

But, somehow it passed when I put both value in the same variable.

final tCastList = <Cast>[tCast];

when(mockCreditsRepository.loadCastByMovie(tId))
        .thenAnswer((_) async => Right(tCastList));
    // act
    final result = await usecase.execute(tId);
    // assert
    expect(result, Right(tCastList));

I think it is because i create 2 difference instance for the test. Is there a way to evaluate only the value of either instead of comparing the instance?

The Option method optionOf needs to accept a nullable value since its function is to remove said null...

Option<A> optionOf<A>(A? value) => value != null ? some(value) : none();

Using dartz: ^0.10.0-nullsafety.0

I'm a Scala developer who's starting to use Dart and am excited to see your dartz library. I'm living on the bleeding edge and using Dart 2.0.0-dev.39.0. Option and Either work but when I try to use IList I get a runtime CastError. For example, this code from you test suite final l1 = ilist([1,2,3,4]); compiles okay but throws the following error when compiled for Dart Angular with DDC and running in Chrome:

_js_helper.CastErrorImplementation.new {message: "CastError: Casting value of type '_Nil' to type 'IList<int>' which is incompatible in strong mode"

I've tried other variations of using IList but all result in the above error.

Edit: The runtime CastError also happens for Option None, however I can explicitly cast the object creation like Option<String> s4 = new None<String>();. This cast isn't possible for Nil since the _Nil class constructor is private.

Maybe this issue is a more general question of when do you expect to be Dart 2.x compatible?

Can you please merge this PR and publish the release version of dartz to pub? the test plugin version has been changed to the released version and flutter test runs correctly:

flutter test
Changing current working directory to: /home/idelub/Repository/Flutter/forks/dartz
Running "flutter pub get" in dartz...                              43.3s
00:02 +258: All tests passed!

Thanks

I need some help here. I learned this library from this tutorial https://resocoder.com/2019/12/14/functional-error-handling-in-flutter-dart-2-either-task-fp/ and single API call seemed to work.

However I have this scenario

1st API - https://ip/v1/programs - this API will return all programs

Then, in order to get the programs' photo, I need to call another API and do a for loop.

2nd API - https://ip/v1/programs/id/photo - the id is passed from 1st API and photo is in binary bytes

Basically I need to call 2nd API after I call 1st API. Then assign the photo into each program from the List return from 1st API.

Note: program.dart (used freezed package) and it's immutable. Has id, photo, etc fields.

My codes

Either<NetworkExceptions, List<Program>> _result;
  Either<NetworkExceptions, List<Program>> get result => _result;
  void _setResult(Either<NetworkExceptions, List<Program>> result) {
    _result = result;
  }

Future<void> getPrograms() async {
await Task(() => _service.getPrograms(token))
.attempt()
.mapLeftToFailure()
.run()
.then((value) => _setResult(value));

result.fold(
(failure) => {}
(programs) async {
// call 2nd API here ???
}
);

Future<List<Program>> getProgramPhoto(Token token, List<Program> old) async {
List<Program> newProgram;

old.forEach(
(program) async {
 await Task(() => _service.getProgramPhoto(token, program.id))
.attempt()
.mapLeftToFailure()
.run()
.then((value) => ??);
}
);
}

Questions

1. Not sure the way I write my codes is correct. Any idea?
2. Any idea how do I assign photo (Uint8List) to an existing List as Program is immutable?

Since today in my Android Studio the dart compiler values the dartz import more than the material.dart import so I have to use

import 'package:dartz/dartz.dart' as dz;

when I want to use a StatefulWidget and some dartz objects or functions. Otherwise the app doesn't compile and throws this error message:

Compiler message:
lib/main.dart:31:32: Error: Expected 0 type arguments.
class _HomeWidgetState extends State<HomeWidget> {
                               ^
lib/main.dart:28:20: Error: The return type of the method 'HomeWidget.createState' is '_HomeWidgetState', which does not match the return type, 'State<StatefulWidget>', of the overridden method, 'StatefulWidget.createState'.
 - '_HomeWidgetState' is from 'package:stock_check/main.dart' ('lib/main.dart').
 - 'State' is from 'package:flutter/src/widgets/framework.dart' ('/C:/src/flutter/packages/flutter/lib/src/widgets/framework.dart').
 - 'StatefulWidget' is from 'package:flutter/src/widgets/framework.dart' ('/C:/src/flutter/packages/flutter/lib/src/widgets/framework.dart').
Change to a subtype of 'State<StatefulWidget>'.
  _HomeWidgetState createState() => _HomeWidgetState();

In the following scenario

https://stackoverflow.com/questions/75007868/with-dartz-is-it-possible-to-pass-a-failure-on-to-the-function-that-is-folding-m/75008089#75008089

it would be great to have an asLeft and asRight as was proposed in the answer.

Future<Either<Failure, AuthUser>> call() async {
    final userResponse = await _authRepository.getUser();

    if(userResponse.isLeft()){
       // do your stuffs       
     }

Inside this, you may like to access data directly, this extension will help

extension EitherX<L, R> on Either<L, R> {
  R asRight() => (this as Right).value; //
  L asLeft() => (this as Left).value;
}

I am trying to have Either<Failure, Concrete> where Concrete extends Abstract class, but I cannot get it to work. If I try to cast the left side I get the error:

type 'Right<Failure, Concrete>' is not a subtype of type 'Either<Failure, Abstract>' in type cast

if I try for the right side I get:

type 'Left<Object, Concrete>' is not a subtype of type 'Either<Failure, Concrete>' in type cast

And bimap does not seems to solve it. Is there an easy way to do it? tks

In the option.dart file a comment suggests that the code below the comment has side effects.

// PURISTS BEWARE: side effecty stuff below -- proceed with caution!

One Method there is the toNullable() method.

It looks like that:

A? toNullable() => fold(() => null, id);

This method does not seem to create a side effect and therefore it should be above the stated comment to avoid confusion.

I would create a PR that moves the method upwords, so that it is above the comment. Would this PR be merged?

I'm not sure if enough people would participate to make it work out, but it might be nice to enable Discussions so there is a place to ask for advice without creating an issue.

Unhandled Exception: 'package:bloc/src/emitter.dart': Failed assertion: '!_isCompleted'

emit was called after an event handler completed normally. This is usually due to an unawaited future in an event handler. Please make sure to await all asynchronous operations with event handlers and use emit.isDone after asynchronous operations before calling emit() to ensure the event handler has not completed.

BAD on((event, emit) { future.whenComplete(() => emit(...)); });

GOOD on((event, emit) async { await future.whenComplete(() => emit(...)); });

failureOrToken.map((token) async {
        final failureOrProfile =
            await profileRepository.getCurrentUserProfile(token);
        failureOrProfile.map(
          (profile) async {
            final failureOrTeams = await teamsRepository.getTeams(token);
            failureOrTeams.map(
              (teams) async {
                final failureOrLocations =
                    await locationsRepository.getLocations(token);
                failureOrLocations.map(
                  (locations) => emit(
                    SuccessState(
                      profile: profile,
                      teams: teams,
                      locations: locations,
                    ),
                  ),
                );
              },
            );
          },
        );
      },
    ).leftMap(
      (failure) => emit(
        ErrorState(
          message: mapFailureToMessage(failure),
        ),
      ),
    );