How to best implement a pattern where I want to compute a value for each element in a collection and...

# getting-started

[David Kubecka]

How to best implement a pattern where I want to compute a value for each element in a collection and return those element+value pairs where the value is not null)? Currently, I have simply this

someCollection.map { it to someExpensiveComputation(it) }.filter { it.second != null }

which works but the return type is List<T, R?> and I of course want List<T, R> .

[ephemient]

someCollection.mapNotNull {
    val result = someExpensiveComputation(it) ?: return@mapNotNull null
    it to result
}

[ephemient]

you could golf that down to

someCollection.mapNotNull { someExpensiveComputation(it)?.let(it::to) }

but I don't think it's clearer

[David Kubecka]

Ah, I hate those early returns 🙂 Especially from lambdas

[ephemient]

well you could write

if (result != null) it to result else null

or the equivalent ?.let if you wanted, I just prefer the early return to the additional nested scope

[David Kubecka]

Well, I probably prefer your second solution, although I would just use the infix to (have to invent an alias for the inner it, though). Btw it didn't occur to me that it can be used this way in its prefix variant but of course it makes sense.