Ref parameters

Place ref before a parameter to borrow the caller’s variable for mutation:

fun increment(ref value: Int) {
    value = value + 1;
}

The call site also uses ref:

var count = 4;
increment(ref count);
println(toString(count));

This makes the in-place mutation visible when reading the call.

Pass assignable storage

A ref argument must name storage that can be modified:

var score = 10;
increment(ref score);

A temporary expression cannot be passed by mutable reference.

Swap values

fun swap(ref left: Int, ref right: Int) {
    var temporary = left;
    left = right;
    right = temporary;
}

fun main() Int {
    var first = 10;
    var second = 20;
    swap(ref first, ref second);
    return first;
}

Current limitation

ref T is currently a parameter and return contract, not a general variable or field type. A program cannot store an arbitrary reference for later use.

Use an ordinary parameter for a value copy, or a sink parameter when a function accepts ownership.