@assert

@assert stops compilation with an error if a condition is false.

Syntax

@assert condition
@assert condition, "message"

The message is optional, and must be a string literal.

Example

macro shift_amount(n: int) {
    @assert n >= 0 && n < 32, "shift amount must be 0 to 31"
    @emit n
}

shift_amount 5

5

macro shift_amount(n: int) {
    @assert n >= 0 && n < 32, "shift amount must be 0 to 31"
    @emit n
}

shift_amount 40

shift amount must be 0 to 31

Details

• The condition is true if it’s not 0.
• A passing @assert emits nothing and has no other effect.
• The error points at the @assert that failed.

Asserts, hooks and invariants

An @assert checks something at one point in one macro. For checks that apply more broadly:

• To check the arguments of several macros the same way, put the asserts in one macro and call it from a before hook.
• To check every value of a type, wherever it’s made, use an invariant.