Arrow: What should List<A>.firstOption { A -> Boolean } return?

[aparmet-toast]

I have a list and I called firstOption on it, and was getting None when I thought I shouldn't be. I'd expect a function called firstOption that takes a predicate and is called on a Foldable to iterate through the foldable and return the first item matching the predicate or None if no items match, but instead I see here that it only checks the first element of the foldable.

Is this intended? If so, what's the usual use case?

Thanks!

[ps-feng]

I'd expect a function called firstOption that takes a predicate and is called on a Foldable to iterate through the foldable and return the first item matching the predicate or None if no items match

I wouldn't expect that because I think it matches the standard library's Iterable<T>.firstOrNull(). See here.

[aparmet-toast]

I think that's exactly what the standard library does:

Returns the first element matching the given predicate, or null if element was not found.

This is how I have it implemented right now, with a toOption() call tacked on the end.

[pakoito]

Just in case, what you want is called find. I agree that sharing the same signature, having different behavior is weird, although it's documented what it does.

This is a specialized get I believe we inherited from funktionale.

[aparmet-toast]

Is it documented? I'm using 0.0.10 and IDE hover showed no documentation and click-through didn't work at all, as I ended up in a recursive cycle in a file called ListKFoldable.kt (package arrow.core.extensions.list.foldable, which does not seem to appear on GitHub anymore), here:

@JvmName("firstOption")
@Suppress(
        "UNCHECKED_CAST",
        "USELESS_CAST",
        "EXTENSION_SHADOWED_BY_MEMBER",
        "UNUSED_PARAMETER"
)
fun <A> List<A>.firstOption(): Option<A> = arrow.core.extensions.list.foldable.List.foldable().run {
  arrow.core.ListK(this@firstOption).firstOption<A>() as arrow.core.Option<A>
}

@JvmName("firstOption")
@Suppress(
        "UNCHECKED_CAST",
        "USELESS_CAST",
        "EXTENSION_SHADOWED_BY_MEMBER",
        "UNUSED_PARAMETER"
)
fun <A> List<A>.firstOption(arg1: Function1<A, Boolean>): Option<A> =
        arrow.core.extensions.list.foldable.List.foldable().run {
  arrow.core.ListK(this@firstOption).firstOption<A>(arg1) as arrow.core.Option<A>
}

So this behavior as encountered was completely unexpected. I had to debug by printing a stack trace from the invoked closure.

[pakoito]

You're right. The docs aren't copied into code generated by kapt, so that's lost. Here's the original function: https://github.com/arrow-kt/arrow/blob/master/modules/core/arrow-core-data/src/main/kotlin/arrow/typeclasses/Foldable.kt#L210-L214

[lenguyenthanh]

To be clear ListKFoldable.kt is generated, so it is never on github.

[1Jajen1]

I opened https://github.com/arrow-kt/arrow-core/issues/22 a few days ago (forgot to check existing issues), imo changing to the behaviour of the stdlib is the best move.

[1Jajen1]

https://github.com/arrow-kt/arrow-core/pull/32 clarified this. firstOption behaves now like the standard lib firstOrNull. It is also deprecated and renamed to firstOrNone and it's an alias for find.