Dynamic Parameters in Rust Functions

It is possible for Rust functions to contain parameters of type Dynamic. Any clonable value can be set into a Dynamic value.

Any parameter in a registered Rust function with a specific type has higher precedence over the Dynamic type, so it is important to understand which version of a function will be used.

For example, the push method of an array is implemented as follows (minus code that protects against over-sized arrays), making the function applicable for all item types.

fn main() {
// 'item: Dynamic' matches all data types
fn push(array: &mut Array, item: Dynamic) {


A Dynamic value has less precedence than a value of a specific type, and parameter matching starts from the left to the right. Candidate functions will be matched in order of parameter types.

Therefore, always leave Dynamic parameters as far to the right as possible.

fn main() {
use rhai::{Engine, Dynamic};

// Different versions of the same function 'foo'
// will be matched in the following order.

fn foo1(x: i64, y: &str, z: bool) { }

fn foo2(x: i64, y: &str, z: Dynamic) { }

fn foo3(x: i64, y: Dynamic, z: bool) { }

fn foo4(x: i64, y: Dynamic, z: Dynamic) { }

fn foo5(x: Dynamic, y: &str, z: bool) { }

fn foo6(x: Dynamic, y: &str, z: Dynamic) { }

fn foo7(x: Dynamic, y: Dynamic, z: bool) { }

fn foo8(x: Dynamic, y: Dynamic, z: Dynamic) { }

let mut engine = Engine::new();

// Register all functions under the same name (order does not matter)

engine.register_fn("foo", foo5)
      .register_fn("foo", foo7)
      .register_fn("foo", foo2)
      .register_fn("foo", foo8)
      .register_fn("foo", foo1)
      .register_fn("foo", foo3)
      .register_fn("foo", foo6)
      .register_fn("foo", foo4);

Warning: Only the Right-Most 16 Parameters Can Be Dynamic

The number of parameter permutations go up exponentially, and therefore there is a realistic limit of 16 parameters allowed to be Dynamic, counting from the right-most side.

For example, Rhai will not find the following function – Oh! and those 16 parameters to the right certainly have nothing to do with it!

fn main() {
// The 'd' parameter counts 17th from the right!
fn weird(a: i64, d: Dynamic, x1: i64, x2: i64, x3: i64, x4: i64,
                             x5: i64, x6: i64, x7: i64, x8: i64,
                             x9: i64, x10: i64, x11: i64, x12: i64,
                             x13: i64, x14: i64, x15: i64, x16: i64) {

    // ... do something unspeakably evil with all those parameters ...