How to extract Left or Right easily from Either type in Dart (Dartz)

Question

I am looking to extract a value easily from a method that return a type Either<Exception, Object>.

I am doing some tests but unable to test easily the return of my methods.

For example:

final Either<ServerException, TokenModel> result = await repository.getToken(...); 

To test I am able to do that

expect(result, equals(Right(tokenModelExpected))); // => OK 

Now how can I retrieve the result directly?

final TokenModel modelRetrieved = Left(result); ==> Not working.. 

I found that I have to cast like that:

final TokenModel modelRetrieved = (result as Left).value; ==> But I have some linter complain, that telling me that I shouldn't do as to cast on object... 

Also I would like to test the exception but it's not working, for example:

expect(result, equals(Left(ServerException()))); // => KO 

So I tried this

expect(Left(ServerException()), equals(Left(ServerException()))); // => KO as well, because it says that the instances are different. 

Answer 1

Ok here the solutions of my problems:

To extract/retrieve the data

final Either<ServerException, TokenModel> result = await repository.getToken(...); result.fold(  (exception) => DoWhatYouWantWithException,   (tokenModel) => DoWhatYouWantWithModel );  //Other way to 'extract' the data if (result.isRight()) {   final TokenModel tokenModel = result.getOrElse(null); } 

To test the exception

//You can extract it from below, or test it directly with the type expect(() => result, throwsA(isInstanceOf<ServerException>())); 

Answer 2

I can't post a comment... But maybe you could look at this post. It's not the same language, but looks like it's the same behaviour.

Good luck.