Even though, this compiles it’s clear that there a...

# arrow-contributors

[Imran/Malic]

Even though, this compiles it’s clear that there are cornerCases where this doesn’t work.

fun <B> mapFilter(f: (A) -> Option<B>): NonEmptyList<B> =
    map(f).foldLeft(NonEmptyList(listOf())) { acc, o -> o.fold({ acc }, { acc + it }) }

Well I suggest to change the returnType to something like Option<NonEmptyList<B>>, because if there doesn’t exist any value which fulfills the f function and returns an Some<B> the return List is empty thus it’s not an NonEmptyList<B>.

I also consider something like an Either<List<Nothing>, NonEmptyList<B>>, which is in my opinion more granular.
I am happy for Feedback 🙂

[raulraja]

Is this already on Arrow?

[Imran/Malic]

nop I am working on it here https://github.com/arrow-kt/arrow/pull/1513

[Imran/Malic]

I wrote tests and the expected behavior as ascribed above happend

[Imran/Malic]

In order to add the FunctorFilterLaws I thought that would make sense

[raulraja]

in order to filter you need an empty identity to bail when no predicate is match

[raulraja]

Nel does not have that as Paco said, there is no empty value for Nel

[raulraja]

not sure if there is a way around it

[Imran/Malic]

I am not sure how to read this.
Do anyone agree on what I propose or is there another way to do this.

[raulraja]

Why does Nel need mapFilter or what is the relationship between Nel and mapFilter in what you are doing?

[simon.vergauwen]

Nel doesn't have a FunctorFilter instance. A Nel with 1 element that is filtered makes it an empty list. Or when mapFilter { None}