Tutorial 2: structs and impl

This tutorial covers:

• defining a struct (data layout)
• attaching behavior with impl blocks
• calling methods with explicit receivers (self: &Self, mut self: &Self)

Reference: Structs, impls, layout (see the sidebar under “Language”).

1) Define a struct#

struct Packet {
  sequence: u32,
  size: u16,
  is_urgent: u8,
}

Structs define shape: fields and layout. Behavior lives in impl blocks.

2) Add behavior with an impl block#

impl Packet {
  public fn mark_urgent (mut self: &Self) -> void {
    self.is_urgent = 1;
  }

  public fn is_large (self: &Self) -> bool {
    return self.size > 1024;
  }
}

Two important ideas:

• Methods are functions with an explicit receiver parameter (self).
• mut self: &Self means the method needs to mutate the value.

3) Use it (a complete program)#

Create packet.slk:

import { println } from "std/io";

struct Packet {
  sequence: u32,
  size: u16,
  is_urgent: u8,
}

impl Packet {
  public fn mark_urgent (mut self: &Self) -> void {
    self.is_urgent = 1;
  }

  public fn is_large (self: &Self) -> bool {
    return self.size > 1024;
  }
}

fn main () -> int {
  let mut p = Packet{ sequence: 1, size: 2048, is_urgent: 0 };

  if p.is_large() {
    (mut p).mark_urgent();
  }

  println("seq={d} urgent={d}", p.sequence as int, p.is_urgent as int);
  return 0;
}

Then:

silk check packet.slk
silk build packet.slk -o build/packet
./build/packet

Optional: constructor overloads and new#

Silk supports a conventional pattern for heap allocation: new Type(...) selects a constructor overload in an impl block.

impl Packet {
  fn constructor (mut self: &Self, seq: u32, size: u16) -> void {
    self.sequence = seq;
    self.size = size;
    self.is_urgent = 0;
  }
}

This keeps construction explicit (you can read the initializer logic) without inventing a second “class system”.

Next#

• Tutorial 3: Arrays and slices