if-expr.md

r[expr.if]

if expressions

r[expr.if.syntax]

Syntax

^Syntax
IfExpression :
   if IfConditions BlockExpression
   (else ( BlockExpression | IfExpression ) )^?

IfConditions :
   IfCondition ( && IfCondition )*

IfCondition :
      Expression_{except struct expression}
   | let Pattern = Scrutinee

r[expr.if.intro] The syntax of an if expression is a sequence of one or more condition operands separated by &&, followed by a consequent block, any number of else if conditions and blocks, and an optional trailing else block.

r[expr.if.condition-bool] Condition operands must be either an Expression with a boolean type or a conditional let match.

r[expr.if.condition-true] If all of the condition operands evaluate to true and all of the let patterns successfully match their scrutinees, the consequent block is executed and any subsequent else if or else block is skipped.

r[expr.if.else-if] If any condition operand evaluates to false or any let pattern does not match its scrutinee, the consequent block is skipped and any subsequent else if condition is evaluated.

r[expr.if.else] If all if and else if conditions evaluate to false then any else block is executed.

r[expr.if.result] An if expression evaluates to the same value as the executed block, or () if no block is evaluated.

r[expr.if.type] An if expression must have the same type in all situations.

# let x = 3;
if x == 4 {
    println!("x is four");
} else if x == 3 {
    println!("x is three");
} else {
    println!("x is something else");
}

// `if` can be used as an expression.
let y = if 12 * 15 > 150 {
    "Bigger"
} else {
    "Smaller"
};
assert_eq!(y, "Bigger");

r[expr.if.let]

if let patterns

r[expr.if.let.intro] let patterns in an if condition allow binding new variables into scope when the pattern matches successfully.

The following examples illustrate bindings using let patterns:

let dish = ("Ham", "Eggs");

// This body will be skipped because the pattern is refuted.
if let ("Bacon", b) = dish {
    println!("Bacon is served with {}", b);
} else {
    // This block is evaluated instead.
    println!("No bacon will be served");
}

// This body will execute.
if let ("Ham", b) = dish {
    println!("Ham is served with {}", b);
}

if let _ = 5 {
    println!("Irrefutable patterns are always true");
}

r[expr.if.let.or-pattern] Multiple patterns may be specified with the | operator. This has the same semantics as with | in match expressions:

enum E {
    X(u8),
    Y(u8),
    Z(u8),
}
let v = E::Y(12);
if let E::X(n) | E::Y(n) = v {
    assert_eq!(n, 12);
}

r[expr.if.chains]

Chains of conditions

r[expr.if.chains.intro] Multiple condition operands can be separated with &&.

r[expr.if.chains.order] Similar to a && LazyBooleanOperatorExpression, each operand is evaluated from left-to-right until an operand evaluates as false or a let match fails, in which case the subsequent operands are not evaluated.

r[expr.if.chains.bindings] The bindings of each pattern are put into scope to be available for the next condition operand and the consequent block.

The following is an example of chaining multiple expressions, mixing let bindings and boolean expressions, and with expressions able to reference pattern bindings from previous expressions:

fn single() {
    let outer_opt = Some(Some(1i32));

    if let Some(inner_opt) = outer_opt
        && let Some(number) = inner_opt
        && number == 1
    {
        println!("Peek a boo");
    }
}

The above is equivalent to the following without using chains of conditions:

fn nested() {
    let outer_opt = Some(Some(1i32));

    if let Some(inner_opt) = outer_opt {
        if let Some(number) = inner_opt {
            if number == 1 {
                println!("Peek a boo");
            }
        }
    }
}

r[expr.if.chains.or] If any condition operand is a let pattern, then none of the condition operands can be a || lazy boolean operator expression due to ambiguity and precedence with the let scrutinee. If a || expression is needed, then parentheses can be used. For example:

# let foo = Some(123);
# let condition1 = true;
# let condition2 = false;
// Parentheses are required here.
if let Some(x) = foo && (condition1 || condition2) { /*...*/ }