Elvis operator for computed values

Question

In Kotlin we can use the Elvis operator ?: like this:

val string: String = null ?: "something else"

But what if "something else" is the result of a computation, like

val string: String = null ?: {
    // do some comutations here
    "something else"
}

This won't compile as the right hand side of ?: is a function () => String and not String.

I have a feeling that I need to use one of the function takeIf, takeUnless etc. but I don't get it.

Thanks

Answer 1

Use the run-function. It performs a computation and returns its result.

val string: String = null ?: run {
    // do some computations here
    "something else"
}

Answer 2

You can use run:

val string = something ?: run {
    // you can write multiple lines of code here,
    // to compute a final result
    val x = foo()
    val y = bar(x)
    baz(x, y)
}

Alternatively, if this doesn't need to be inline, just extract a function:

val string = something ?: computeValue()