# Conditionals

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( expression ) expression else-clauseopt else-clauseelse expression