Value and pointer receivers

A native struct makes copying visible. This recipe performs one update on a receiver copy and one through a shared pointer.

Project tree

struct-receivers/
└── main.km

Source

import go fmt from "fmt";

struct Counter {
  public value: int;

  public function added(delta: int): Counter {
    let copy = this;
    copy.value += delta;
    return copy;
  }

  public pointer function add(delta: int): void {
    this.value += delta;
  }
}

function main(): void {
  let counter = Counter { value: 10 };
  const copied = counter.added(5);
  counter.add(2);
  fmt.Println(counter.value, copied.value);
}

Run

keika check main.km
keika run main.km

Expected output:

12 15

added(5) changes and returns a copy whose value is 15; it does not change counter. add(2) is a pointer receiver, so selecting it from the addressable local mutates counter to 12.

Method values

Forming a method value follows Go behavior: a value receiver is copied when the method value is formed, while a pointer receiver retains shared storage. A nil pointer receiver can be called, but dereferencing it retains ordinary Go panic behavior.

The same receiver methods can be organized outside the struct with function name(this: Counter, ...) or function name(this: *Counter, ...) when the receiver type is declared in the same module.