Idiomatic way to convert a List to a Pair in Kotlin

For a more general solution you could use the extension function zipWithNext* which

Returns a list of pairs of each two adjacent elements in this collection.

The example in the documentation explains it better:

val letters = ('a'..'f').toList()
val pairs = letters.zipWithNext()

println(letters) // [a, b, c, d, e, f]
println(pairs) // [(a, b), (b, c), (c, d), (d, e), (e, f)]

*note that this function is available since v1.2 of Kotlin.

You can make this extension for yourself:

fun <T> List<T>.toPair(): Pair<T, T> {
    if (this.size != 2) {
        throw IllegalArgumentException("List is not of length 2!")
    }
    return Pair(this[0], this[1])
}

The error handling is up to you, you could also return a Pair<T, T>? with null being returned in the case where the List is not the correct length. You could also only check that it has at least 2 elements, and not require it to have exactly 2.

Usage is as you've described:

listOf(a, b).toPair()

Here's a variation on @zsmb13's solution that avoids creating the exception explicitly and dereferencing the list by index:

fun <T> List<T>.toPair(): Pair<T, T> {
    require (this.size == 2) { "List is not of length 2!" }
    val (a, b) = this
    return Pair(a, b)
}