Impl blocks

Impl blocks attach methods to a struct type.

Syntax

impl TypeName {
  fn method_name(self, arg) {
    // body -- self refers to the struct instance
  }
}

The first parameter of each method must be self, which receives the struct instance the method is called on.

Method calls

Methods are called using dot syntax on struct instances:

struct Point {
  x: int
  y: int
}

impl Point {
  fn distance(self) {
    return sqrt(self.x * self.x + self.y * self.y)
  }
  fn translate(self, dx, dy) {
    return Point { x: self.x + dx, y: self.y + dy }
  }
}

const p = Point { x: 3, y: 4 }
log(p.distance())           // 5.0
const p2 = p.translate(10, 20)
log(p2.x)                   // 13

When instance.method(args) is called, the VM looks up methods registered by the impl block for the instance's struct type. The instance is automatically passed as the self argument.