hi guys, yesterday i was asked an interesting ques...

# announcements

[christoph.pickl]

hi guys, yesterday i was asked an interesting question: how to deal with multiple optional values.
should we go for simple null checks via if, or can we do it via a more sophisticated approach using something like let.

given the following in an external module (to make it even harder to disable smart casting):

data class Person(
        val name: String?,
        val age: Int?
)
fun moduleExternal() = Person("", 1)

and a target function with a signature of:

fun safeCall(x1: String, x2: Int) {}

the following WON'T compile due to not being able to smart cast external module's properties:

fun solution_no_smartcasts() {
    val person = moduleExternal()
    if (person.name != null && person.age != null) {
        safeCall(person.name, person.age) // ERROR!
    }
}

in order to get rid of the smart cast problem first, i came up with the following idea of destructuring:

fun solution_destructure() {
    val (name, age) = moduleExternal()
    if (name != null && age != null) {
        safeCall(name, age)
    }
}

ok, but how about that "low level" if? apple's swift would solve it this way:

if let name = person.name as? String, let age = person.age as? Int {
    // name and age are non-optional in here
}

so i thought about an extended if which "kind of" solves it:

fun <T1, T2> ifNotNull(x1: T1?, x2: T2?, action: (T1, T2) -> Unit) {
    if (x1 != null && x2 != null) {
        action(x1, x2)
    }
}

fun solution_kotlin_lets() {
    val person = moduleExternal()
    ifNotNull(person.name, person.age) { name, age ->
        safeCall(name, age)
    }
}

what are your thoughts on this topic?

[damian]

at the moment we’re using the same construct, just with a return type R? instead of Unit so we can use it like a let for multiple types. (with multiLet(a,b) { a, b -> done } ?: somethingElse

[gildor]

I like this approach

fun solution_destructure() {
    val person = moduleExternal()
    val name = person.name ?: return
    val age = person.age ?: return
    safeCall(name, age)
}

Also it allows easily replace return with error() or some other null handler

[gildor]

But ifNotNull works too, especially for cases where you want to receive result of expression, not just early return or break

[damian]

i’ve used the ?: approach with exceptions to pass the error futher down and handle it correctly / display a message why it didn’t work. i like it for this case - if you need to know which value exactly is null for example

[gildor]

But with empty return it also works, if you don’t need default value or null

[uhe]

We're also using a variant of ifNotNull