switch Expression

The switch expression allows matching on literal values, and it mostly follows Rust’s match syntax.

switch calc_secret_value(x) {
    1 => print("It's one!"),
    2 => {
        print("It's two!");
        print("Again!");
    }
    3 => print("Go!"),
    // _ is the default when no cases match. It must be the last case.
    _ => print(`Oops! Something's wrong: ${x}`)
}

The default case (i.e. when no other cases match), however, must be the last case in the statement.

switch wrong_default {
    1 => 2,
    _ => 9,     // <- syntax error: default case not the last
    2 => 3,
    3 => 4,     // <- ending with extra comma is OK
}

Expression, Not Statement

switch is not a statement, but an expression. This means that a switch expression can appear anywhere a regular expression can, e.g. as function call arguments.

let x = switch foo { 1 => true, _ => false };

func(switch foo {
    "hello" => 42,
    "world" => 123,
    _ => 0
});

// The above is somewhat equivalent to:

let x = if foo == 1 { true } else { false };

if foo == "hello" {
    func(42);
} else if foo == "world" {
    func(123);
} else {
    func(0);
}

Array and Object Map Literals Also Work

The switch expression can match against any literal, including array and object map literals.

// Match on arrays
switch [foo, bar, baz] {
    ["hello", 42, true] => { ... }
    ["hello", 123, false] => { ... }
    ["world", 1, true] => { ... }
    _ => { ... }
}

// Match on object maps
switch map {
    #{ a: 1, b: 2, c: true } => { ... }
    #{ a: 42, d: "hello" } => { ... }
    _ => { ... }
}

Switching on arrays is very useful when working with Rust enums (see this chapter for more details).

Case Conditions

Similar to Rust, each case (except the default case at the end) can provide an optional condition that must evaluate to true in order for the case to match.

let result = switch calc_secret_value(x) {
    1 if some_external_condition(x, y, z) => 100,

    2 if x < foo => 200,
    2 if bar() => 999,      // <- syntax error: still cannot have duplicated cases

    3 => if CONDITION {     // <- put condition inside statement block for
        123                 //    duplicated cases
    } else {
        0
    }

    _ if CONDITION => 8888  // <- syntax error: default case cannot have condition
};

Difference From if-else if Chain

Although a switch expression looks almost the same as an if-else if chain, there are subtle differences between the two.

Look-up Table vs x == y

A switch expression matches through hashing via a look-up table. Therefore, matching is very fast. Walking down an if-else if chain is much slower.

On the other hand, operators can be overloaded in Rhai, meaning that it is possible to override the == operator for integers such that x == y returns a different result from the built-in default.

switch expressions do not use the == operator for comparison; instead, they hash the data values and jump directly to the correct statements via a pre-compiled look-up table. This makes matching extremely efficient, but it also means that overloading the == operator will have no effect.

Therefore, in environments where it is desirable to overload the == operator for standard types – though it is difficult to think of valid scenarios where you’d want 1 == 1 to return something other than true – avoid using the switch expression.

Efficiency

Because the switch expression works through a look-up table, it is very efficient even for large number of cases; in fact, switching is an O(1) operation regardless of the size of the data and number of cases to match.

A long if-else if chain becomes increasingly slower with each additional case because essentially an O(n) linear scan is performed.