Constants

A constant is a named, compile-time value.

const WORLD_COUNT: u8 = 10
const DEFAULT_NAME: string = "flower"

const NAME: Type = value

• NAME — a plain identifier (not scoped).
• Type — any type; in practice a primitive.
• value — one of:
  • a literal: an integer, true / false, or a "quoted string" (plain, no interpolation)
  • another constant, by name — WORLD_COUNT, or pkg::mod::DEFAULT for one another schema declares
  • an integer type's bound — u32::MIN, u64::MAX

A value that names a constant or a bound is checked as the value it comes to. It has to be the constant's kind of value (an integer is also a fine float) and, for an integer, fit the declared width:

const LIMIT: u32 = 500
const SMALL: u8 = LIMIT     // error: `LIMIT` is 500, which does not fit in `u8`
const TOP: u64 = u64::MAX   // fine

A name that isn't a constant is an error, and so are constants that name each other in a loop. Generated code holds the value a name comes to, as if the literal had been written.

A constant can carry a docstring:

/// Largest number of worlds a provider may report.
const WORLD_COUNT: u8 = 10

Using one

A constant is referenced by name wherever a value is written: a struct's or an error's field default, a validator property's default, or a validator's argument. It's checked there the same way, against the type it's written for:

struct Person {
    name: char[max 32] = DEFAULT_NAME
    worlds: u8 = WORLD_COUNT
}

Sizes from constants

An array's size can be a constant, in a maximum or an exact length:

const NAME_MAX: u8 = 32
const LIMIT: u32 = 500

struct Customer {
    name: char[max NAME_MAX]
    code: char[NAME_MAX]
    orders: Order[max LIMIT]
    chunk: u8[max u16::MAX]
}

• It names an unsigned integer constant: declared u8, u16, u32 or u64, in this schema or another (pkg::mod::NAME_MAX), or an unsigned type's bound such as u16::MAX. A signed or non-integer constant, or a name that isn't a constant, is a build error saying so.
• It is stored resolved. The stored schema holds the number, so the maximum is the same wherever the constant came from, and changing the constant changes the type, which is a breaking change like any other bound change.
• The same limits apply: at least 1 and at most u32::MAX, so a constant of 0 is an error where it is used as a size.

See Bounds for what a maximum means.