if

if runs a block when a condition is true. else if tests further conditions in turn, and a final else runs when none of them held. The first branch whose condition is true runs, and the rest are skipped.

if-statement = "if" condition block
               { "else" "if" condition block }
               [ "else" block ]

if x > 0 {
    PrintLine("positive");
} else if x < 0 {
    PrintLine("negative");
} else {
    PrintLine("zero");
}

The condition

The condition is a bool, written without parentheses, and a structure literal inside it is wrapped in parentheses — the rules every condition follows, set out in Statements. Combine tests with the logical operators:

if age >= 18 && hasTicket {
    Admit();
}

A type test is a bool too, so if shape is Circle { … } reads naturally.

Bodies

Every branch is a block, even for a single statement, and the braces are what end the condition. A binding declared in a branch lasts until that branch's closing brace.

if is not an expression

if chooses between statements; it does not produce a value, so let s = if ready { 1 } else { 2 }; does not parse. To choose a value, use the conditional operator or a match expression:

let label = count == 1 ? "item" : "items";
let size = match count {
    0 => "none",
    1 => "one",
    else => "many"
};

Or declare a var and assign it in each branch; the compiler checks that every branch does — see Initialization.

if and when

when has the same shape as if but is decided by the compiler, which keeps only the branch it selects — see Conditional compilation. A chain keeps the keyword it starts with:

error: expected 'if' after 'else' in a run-time 'if' chain; 'when' is the compile-time conditional

See also

• Statements — the rules every condition follows
• Conditional — choosing between two values
• match — choosing by the shape of a value
• Learn: If, Else if