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Global/Local Storage Rules

Wave clearly differentiates storage lifetime and mutability at the keyword level.

Summary​

  • Global Constant: const
  • Global Storage Variable: static
  • Local Variable: var, let, let mut

Thus, only const and static are declared at the top-level, while only the var and let family are declared inside functions/blocks.

Global Constant: const​

const is treated as a compile-time constant and cannot be reassigned.

const PAGE_SIZE: i32 = 4096;
const MAGIC: i32 = 0x1BADB002;

Global Storage Variable: static​

static is a variable with global storage. It can be reassigned and is initialized to the zero value of the type if no initial value is provided.

static COUNTER: i32 = 0;
static VGA_BUFFER: ptr<char> = 0xb8000 as ptr<char>;

Local Variable: var / let​

Only local variable keywords are used inside functions or blocks.

fun main() -> i32 {
    var x: i32 = 10;
    let y: i32 = 20;
    let mut z: i32 = 30;

    x = x + 1;
    z = z + 1;
    return x + y + z;
}

Constraints​

  • var, let cannot be used at the top-level.
  • const, static cannot be used inside functions/blocks.
  • let is immutable and cannot be reassigned.