Function parameters

Immutable reference

fun add_25(Array[Int] numbers) {
    numbers.push(25)    // Error: `numbers` is immutable
}

When using a regular type as a parameter, only it's immutable properties can be used

fun count(Array[Int] numbers) -> Int {
    val items_count = numbers.size()    // Ok, `size` is pure

    items_count
}

To use immutable properties you must use a mutable reference.

fun push_25(mut Array[Int] numbers) {
    numbers.push(25)    // Ok, will also mutate the original array
}

Placing a mut before the type makes the parameter a mutable reference. Mutable methods can be used, and the original data can be mutated.

The caller must also use mut.

val numbers = Array(0, 1, 2, 3)

push_25(mut numbers)    // Pass `numbers` as reference.

print(numbers(4))   // `Some(25)`

fun add_25(clone Array[Int] numbers) {
    numbers.push(25)    // Ok, the original array is unaffected
}

Using the clone keyword before the type creates a mutable copy of the parameter (CoW). The original data will not be mutated.

val numbers = Array(1, 2, 3, 4)

add_25(clone numbers)    // Pass `numbers` as clone.

print(numbers(4))   // None