Control flow

Conditionals

Conditions for if and else if must be parenthesized.

if (score >= 90) {
    println("A");
} else if (score >= 80) {
    println("B");
} else {
    println("C");
}

Write if (score >= 90) { ... }; the unparenthesized form is invalid.

Conditions cannot mutate state

Assignments, compound assignments, ++, and -- are rejected anywhere inside an if, else if, while, or for condition. Use == for comparison, or move the mutation to a statement before the condition.

value = read_value();
if (value == expected) {
    println("matched");
}

Ordinary assignment statements and the increment section of a for loop are still allowed.

while loops

while conditions are also parenthesized.

var index: i32 = 0;
while (index < 10) {
    index += 1;
}

for loops

for uses an initializer, condition, and update expression.

for (var i: i32 = 0; i < 10; i += 1) {
    println("{}", i);
}

The initializer can declare a local with var or evaluate an expression. const and static are top-level declarations and cannot be used as for-loop initializers.

break and continue

var i: i32 = 0;
while (i < 20) {
    i += 1;
    if (i == 5) {
        continue;
    }
    if (i == 10) {
        break;
    }
}

break exits the nearest loop, while continue proceeds to the next iteration.

match

The value matched by match is parenthesized. A pattern can be an integer literal, an enum variant name, or the _ wildcard.

match (status) {
    200 => { println("ok"); }
    404 => { println("not found"); }
    _ => { println("other"); }
}

A match cannot contain duplicate _ wildcard arms. Every arm body is a { ... } block.