How to name components of a Pair

Another solution is to define an object with meaningful extensions for Pair<A, B> and to bring these extensions into the context using with(...) { ... }.

object PointPairs {
    val <X> Pair<X, *>.x get() = first
    val <Y> Pair<*, Y>.y get() = second
}

And the usage:

val point = Pair(2, 3)

with(PointPairs) {
    println(point.x)
}

This allows you to have several sets of extensions for the same type and to use each where appropriate.

This is not supported. You should write a wrapper (data) class for that purposes or you could use Kotlin destructuring declarations:

val (x, y) = coordinates
println("$x;$y")

See more here.

The definition of Pair in the Kotlin stdlib is as follows:

public data class Pair<out A, out B>(
    public val first: A,
    public val second: B
) : Serializable

So, if you have an instance p of type Pair, you can access the first property only as p.first. However, you can use a Destructuring Declaration like this:

val (x, y) = p

This is made possible because Pair is a data class. Every argument of the primary constructor of a data class gets a componentX() method, where X is the index of the parameter. So the above call can be expressed like this:

val x = p.component1()
val y = p.component2()

This makes it absolutely independent from the actual name of the parameter.

If you, however, want to be able to write p.x, you'll have to go ahead and create your own data class (or use a library):

data class Point(val x : Double, val y : Double)