Swift 'if let' statement equivalent in Kotlin

Question

In Kotlin is there an equivalent to the Swift code below?

if let a = b.val {

} else {

}

Answer 1

To unwrap multiple variables at once like in the Swift if let syntax you may consider a global utility function along the following lines (example takes 3 parameters, but you can define overloads for any number of parameters):

inline fun <A : Any, B : Any, C : Any> notNull(
    a: A?, b: B?, c: C?, perform: (A, B, C) -> Unit = { _, _, _ -> }
): Boolean {
    if (a != null && b != null && c != null) {
        perform(a, b, c)
        return true
    }
    return false
}

Sample usage:

if (notNull("foo", 1, true) { string, int, boolean ->
    print("The three values were not null and are type-safe: $string, $int, $boolean")
}) else {
    print("At least one of the vales was null")
}

Answer 2

Let's first ensure we understand the semantics of the provided Swift idiom:

if let a = <expr> {
     // then-block
}
else {
     // else-block
}

It means this: "if the <expr> results in a non-nil optional, enter the then-block with the symbol a bound to the unwrapped value. Otherwise enter the else block.

Especially note that a is bound only within the then-block. In Kotlin you can easily get this by calling

<expr>?.also { a ->
    // then-block
}

and you can add an else-block like this:

<expr>?.also { a ->
    // then-block
} ?: run {
    // else-block
}

This results in the same semantics as the Swift idiom.