Conditional if Expressions

An if expression specifies that some code should only be evaluated if a certain condition is true. For example:

if (x > 5) x = x - 5

The condition must be an expression of type bool.

An if expression can optionally include an else clause to specify another expression to evaluate when the condition is false.

if (y <= 10) y = y + 1 else y = 10

Either the "true" branch or the "false" branch will be evaluated, but not both. Either branch can be a single expression or an expression block.

The conditional expressions may produce values so that the if expression has a result.

let z = if (x < 100) x else 100;

The expressions in the true and false branches must have compatible types. For example:

// x and y must be u64 integers
let maximum: u64 = if (x > y) x else y;

// ERROR! branches different types
let z = if (maximum < 10) 10u8 else 100u64;

// ERROR! branches different types, as default false-branch is () not u64
if (maximum >= 10) maximum;

If the else clause is not specified, the false branch defaults to the unit value. The following are equivalent:

if (condition) true_branch // implied default: else ()
if (condition) true_branch else ()

Commonly, if expressions are used in conjunction with expression blocks.

let maximum = if (x > y) x else y;
if (maximum < 10) {
    x = x + 10;
    y = y + 10;
} else if (x >= 10 && y >= 10) {
    x = x - 10;
    y = y - 10;
}

Grammar for Conditionals

if-expression → if ( expression ) expression else-clause_opt > else-clause → else expression