Hey team, How can I flatten List<Either<Exc...

# arrow

[Sagar Suri]

Hey team,

How can I flatten List<Either<Exception, Value>> to List<Value>??

[Jannis]

The method you are looking for is sequence from the Traverse typeclass. We have quite good docs for this typeclass it is well worth a read: https://arrow-kt.io/docs/0.10/apidocs/arrow-core-data/arrow.typeclasses/-traverse/

[Jannis]

For you method in particular you can do sequence(Either.applicative()).fold({ emptyList() }, ::identity) (might need some type parameters though)

[Sagar Suri]

Awesome. Thanks for the quick help. I will check the document and implement it. 👏👏

[Sagar Suri]

One quick question, what if one of the Result in the List have an exception? Can I still return the List with valid data?

[Sagar Suri]

@Jannis

[Jannis]

ah, sequence will use ap to combine the Either 's in the list. This means it will short circuit on a Left . Note that it returns Either<E, List<A>>. If you want to concatenate all values ignoring failure you can use filterMap instead to do something like this: filterMap { it.toOption() }

[Sagar Suri]

Ohh! Got it! I forgot that I could have used filterMap..thanks a lot 👏

[Sagar Suri]

Just a long tiring day and brain has stopped working 😂

[Jakub Pi]

Just an observation, I've been lurking this group for over a month and I've seen this question asked 5 times (I've also asked it myself). Is there a FAQ?