Hey guys. I want a combination of find and map in a single function. Find a first element that satis...

# announcements

[Бежан Александр]

Hey guys. I want a combination of find and map in a single function. Find a first element that satisfies a predicate and map it over function at the same time. Scala has collectFirst for that. How to do it in Kotlin ?

[murphy]

You could use mapNotNull with your partial transformation function and then firstOrNull on the result.

[murphy]

However, that computes the full intermediate result.

[murphy]

You could also use firstOrNull with a predicate and then transform the result, if any.

[U75957]

You can write your own extension function collectFirst

[murphy]

If you only have a partial function, no associated predicate and you want to evaluate this lazily, then I think you'll have to write your own method.

[bezrukov]

You can try to play with a sequence to avoid intermediate result, something like that:
list.asSequence().filter { }.map { }.first()

[murphy]

The generic implementation of collectFirst is also not hard to write, for example like this:

fun <T, R> Iterable<T>.collectFirst(transform: (T) -> R?) : R? {
    for (i in this) {
        val o = transform(i)
        if (o != null) return o
    }
    return null
}

[Pavlo Liapota]

Or for example:

list
    .find { it % 2 == 0 }
    ?.let { abs(it) }

If you need separate lambdas for predicate and transformation, then one function call will not look nice:

list.collectFirst({ it % 2 == 0 }, { abs(it) })

[murphy]

@Pavlo Liapota I would also recommend to split the function into predicate and transformation, because that will be more clear and will allow the use of existing library functions. But the OP specifically asked for an analogon of a Scala library method and that method only takes one partial function as its argument, so I replicated that behaviour.

[Бежан Александр]

Thank you guys. I think @murphy version is exactly what I’m looking for. The thing is that I don’t wanna recompute value, since filtering function already does the mapping, I just want this result.

[Pavlo Liapota]

@murphy Ah, I see, sorry, I am not familiar with Scala 🙂
@Бежан Александр You can also make that extension function inline.

[murphy]

The solution using asSequence before mapNotNull and firstOrNull instead of a dedicated extension method would also avoid computing too many intermediate results.

[Бежан Александр]

Yeah, this is also good

[Бежан Александр]

I would create collectFirst extension function in anyway. Since I want this level of abstraction and don’t wanna know how it’s implemented.

[murphy]

Sure, if this functionality is used frequently an extension function definitely makes sense.

[Бежан Александр]

@Pavlo Liapota thanks for inline suggestion

[Бежан Александр]

@murphy Yeah, exactly