Assertions

Use assert to check conditions that must hold if the program is correct. If the condition is false, the program prints the location and aborts.

int divide(int a, int b) {
    assert(b != 0);
    return a / b;
}

void main() {
    println(divide(7, 2)); // prints 3
}

assert takes an optional second argument, a string literal used as the failure message.

void main() {
    var x = 2 + 2;
    assert(x == 4, "math is broken");
    println("math works"); // prints math works
}

assert(false) marks unreachable code: the compiler knows execution cannot continue past it, so no return is needed after it. To abort unconditionally with a message, call abort:

void fail(string message) {
    abort("fatal error: ", message);
}

void main() {
    var ok = true;
    if !ok {
        fail("something went wrong");
    }
    println("continuing");
}

Asserts run in debug and release-safe builds. Code that must abort in release builds should call abort explicitly.