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 🙂

Is this already on Arrow?

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

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

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

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

Nel doesn't have a FunctorFilter instance. A Nel with 1 element that is filtered makes it an empty list. Or when

mapFilter { None}