Constants

A constant in Sway is similar to a variable; however, there are a few differences:

• Constants are always evaluated at compile-time
• Constants can be declared both inside of a function and at global / impl scope.
• The mut keyword cannot be used with constants.

Associated Constants

Associated constants are constants associated with a type and can be declared in an impl block or in a trait definition.

Associated constants declared inside a trait definition may omit their initializers to indicate that each implementation of the trait must specify those initializers.

The identifier is the name of the constant used in the path. The type is the type that the definition has to implement.

You can define an associated const directly in the interface surface of a trait:

script;

trait ConstantId {
    const ID: u32 = 0;
}

Alternatively, you can also declare it in the trait, and implement it in the interface of the types implementing the trait.

script;

trait ConstantId {
    const ID: u32;
}

struct Struct {}

impl ConstantId for Struct {
    const ID: u32 = 1;
}

fn main() -> u32 {
    Struct::ID
}

impl self consts

Constants can also be declared inside non-trait impl blocks.

script;

struct Point {
    x: u64,
    y: u64,
}

impl Point {
    const ZERO: Point = Point { x: 0, y: 0 };
}

fn main() -> u64  {
    Point::ZERO.x
}

The following syntax also demonstrates the definition of a struct named S containing a constant ID.

{{#include ../../../code/language/built-ins/constants/src/lib.sw}}