Function parameters

Immutable reference

fun add_25(Array[Int] numbers) {
╰╴No statement matched
  numbers.push(25)    // Error: `numbers` is immutable
}
thp

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

fun count(Array[Int] numbers) -> Int {
╰╴No statement matched
  val items_count = numbers.size()    // Ok, `size` is pure

  items_count

}
thp

To use immutable properties you must use a mutable reference.

Mutable reference

fun push_25(mut Array[Int] numbers) {
╰╴No statement matched
  numbers.push(25)    // Ok, will also mutate the original array
}
thp

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 define the value as var, and when calling must use mut.

var numbers = Expected an expression here, found a DatatypeArray(0, 1, 2, 3)
              ╰╴Invalid variable declaration

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

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

Clone

fun add_25(clone Array[Int] numbers) {
╰╴No statement matched
  numbers.push(25)    // Ok, the original array is unaffected
}
thp

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

The caller must also use clone when calling the function.

val numbers = Expected an expression here, found a DatatypeArray(1, 2, 3, 4)
              ╰╴Invalid variable declaration

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

print(numbers(4)) // None
thp