Custom Operators

For use as a DSL (Domain-Specific Languages), it is sometimes more convenient to augment Rhai with customized operators performing specific logic.

Engine::register_custom_operator registers a keyword as a custom operator, giving it a particular precedence (which cannot be zero).

Example

#![allow(unused)]
fn main() {
use rhai::Engine;

let mut engine = Engine::new();

// Register a custom operator named 'foo' and give it a precedence of 160
// (i.e. between +|- and *|/)
// Also register the implementation of the customer operator as a function
engine.register_custom_operator("foo", 160)?
      .register_fn("foo", |x: i64, y: i64| (x * y) - (x + y));

// The custom operator can be used in expressions
let result = engine.eval_expression::<i64>("1 + 2 * 3 foo 4 - 5 / 6")?;
//                                                    ^ custom operator

// The above is equivalent to: 1 + ((2 * 3) foo 4) - (5 / 6)
result == 15;
}

Alternatives to a Custom Operator

Custom operators are merely syntactic sugar. They map directly to registered functions.

Therefore, the following are equivalent (assuming foo has been registered as a custom operator):

#![allow(unused)]
fn main() {
1 + 2 * 3 foo 4 - 5 / 6     // use custom operator

1 + foo(2 * 3, 4) - 5 / 6   // use function call
}

A script using custom operators can always be pre-processed, via a pre-processor application, into a syntax that uses the corresponding function calls.

Using Engine::register_custom_operator merely enables a convenient shortcut.

Must be a Valid Identifier or Reserved Symbol

All custom operators must be identifiers that follow the same naming rules as variables.

Alternatively, they can also be reserved symbols, disabled operators or keywords.

#![allow(unused)]
fn main() {
engine.register_custom_operator("foo", 20);     // 'foo' is a valid custom operator

engine.register_custom_operator("#", 20);       // the reserved symbol '#' is also
                                                // a valid custom operator

engine.register_custom_operator("+", 30);       // <- error: '+' is an active operator

engine.register_custom_operator("=>", 30);      // <- error: '=>' is an active symbol
}

Binary Operators Only

All custom operators must be binary (i.e. they take two operands). Unary custom operators are not supported.

#![allow(unused)]
fn main() {
engine.register_custom_operator("foo", 160)?
      .register_fn("foo", |x: i64| x * x);

engine.eval::<i64>("1 + 2 * 3 foo 4 - 5 / 6")?; // error: function 'foo (i64, i64)' not found
}

Operator Precedence

All operators in Rhai has a precedence indicating how tightly they bind.

A higher precedence binds more tightly than a lower precedence, so * and / binds before + and - etc.

When registering a custom operator, the operator’s precedence must also be provided.

The following precedence table shows the built-in precedence of standard Rhai operators: