How to perform action for all combinations of elements in lists?

The name of the output you're looking for is a Cartesian Product.

If you don't need all of the combinations up front, and you're just searching for one or more, you could make it a lazily generated sequence:

private fun <A, B> lazyCartesianProduct(
    listA: Iterable<A>,
    listB: Iterable<B>
): Sequence<Pair<A, B>> =
    sequence {
        listA.forEach { a ->
            listB.forEach { b ->
                yield(a to b)
            }
        }
    }

Then you can map, filter etc only what you need:

val names = listOf("John", "Tom")
val days = listOf(1, 2, 3)

lazyCartesianProduct(names, days)
    .map { it.toSomethingElse() }
    .find { it.isWhatImLookingFor() }

The forEach is the closest as it can get:

names.forEach {
    i -> days.forEach { j -> f(i, j)}
}

---

Example:

private fun f(i: String, j: Int) = println("$i,$j")

Result:

John,1
John,2
John,3
Tom,1
Tom,2
Tom,3

What's wrong with a regular for loop?

for (name in names) for (day in days) f(name, day)