Use the Low-Level API to Register a Rust Function

When a native Rust function is registered with an Engine using the Engine::register_XXX API, Rhai transparently converts all function arguments from Dynamic into the correct types before calling the function.

For more power and flexibility, there is a low-level API to work directly with Dynamic values without the conversions.

Raw Function Registration

The Engine::register_raw_fn method is marked volatile, meaning that it may be changed without warning.

If this is acceptable, then using this method to register a Rust function opens up more opportunities.

In particular, a the current native call context (in form of the NativeCallContext type) is passed as an argument. NativeCallContext exposes the current Engine, among others, so the Rust function can also use Engine facilities (such as evaluating a script).


#![allow(unused)]
fn main() {
engine.register_raw_fn(
    "increment_by",                                         // function name
    &[                                                      // a slice containing parameter types
        std::any::TypeId::of::<i64>(),                      // type of first parameter
        std::any::TypeId::of::<i64>()                       // type of second parameter
    ],
    |context, args| {                                       // fixed function signature
        // Arguments are guaranteed to be correct in number and of the correct types.

        // But remember this is Rust, so you can keep only one mutable reference at any one time!
        // Therefore, get a '&mut' reference to the first argument _last_.
        // Alternatively, use `args.split_first_mut()` etc. to split the slice first.

        let y = *args[1].read_lock::<i64>().unwrap();       // get a reference to the second argument
                                                            // then copy it because it is a primary type

        let y = std::mem::take(args[1]).cast::<i64>();      // alternatively, directly 'consume' it

        let x = args[0].write_lock::<i64>().unwrap();       // get a '&mut' reference to the first argument

        *x += y;                                            // perform the action

        Ok(Dynamic::UNIT)                                       // must be 'Result<Dynamic, Box<EvalAltResult>>'
    }
);

// The above is the same as (in fact, internally they are equivalent):

engine.register_fn("increment_by", |x: &mut i64, y: i64| *x += y);
}

Function Signature

The function signature passed to Engine::register_raw_fn takes the following form:

Fn(context: NativeCallContext, args: &mut [&mut Dynamic])
-> Result<T, Box<EvalAltResult>> + 'static

where:

ParameterTypeDescription
Timpl Clonereturn type of the function
contextNativeCallContextthe current native call context
engine()&Enginethe current Engine, with all configurations and settings.
This is sometimes useful for calling a script-defined function within the same evaluation context using Engine::call_fn, or calling a function pointer.
fn_name()&strname of the function called (useful when the same Rust function is mapped to multiple Rhai-callable function names)
source()Option<&str>reference to the current source, if any
iter_imports()impl Iterator<Item = (&str, &Module)>iterator of the current stack of modules imported via import statements
imports()&Importsreference to the current stack of modules imported via import statements; requires the internals feature
iter_namespaces()impl Iterator<Item = &Module>iterator of the namespaces (as modules) containing all script-defined functions
namespaces()&[&Module]reference to the namespaces (as modules) containing all script-defined functions; requires the internals feature
args&mut [&mut Dynamic]a slice containing &mut references to Dynamic values.
The slice is guaranteed to contain enough arguments of the correct types.

Return value

The return value is the result of the function call.

Remember, in Rhai, all arguments except the first one are always passed by value (i.e. cloned). Therefore, it is unnecessary to ever mutate any argument except the first one, as all mutations will be on the cloned copy.

Extract Arguments

To extract an argument from the args parameter (&mut [&mut Dynamic]), use the following:

Argument typeAccess (n = argument position)Result
Primary typeargs[n].clone().cast::<T>()copy of value
Custom typeargs[n].read_lock::<T>().unwrap()immutable reference to value
Custom type (consumed)std::mem::take(args[n]).cast::<T>()the consumed value; the original value becomes ()
this objectargs[0].write_lock::<T>().unwrap()mutable reference to value

When there is a mutable reference to the this object (i.e. the first argument), there can be no other immutable references to args, otherwise the Rust borrow checker will complain.

Example – Passing a Callback to a Rust Function

The low-level API is useful when there is a need to interact with the scripting Engine within a function.

The following example registers a function that takes a function pointer as an argument, then calls it within the same Engine. This way, a callback function can be provided to a native Rust function.


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

let mut engine = Engine::new();

// Register a Rust function
engine.register_raw_fn(
    "bar",
    &[
        std::any::TypeId::of::<i64>(),                      // parameter types
        std::any::TypeId::of::<FnPtr>(),
        std::any::TypeId::of::<i64>(),
    ],
    |context, args| {
        // 'args' is guaranteed to contain enough arguments of the correct types

        let fp = std::mem::take(args[1]).cast::<FnPtr>();   // 2nd argument - function pointer
        let value = args[2].clone();                        // 3rd argument - function argument
        let this_ptr = args.get_mut(0).unwrap();            // 1st argument - this pointer

        // Use 'FnPtr::call_dynamic' to call the function pointer.
        // Beware, private script-defined functions will not be found.
        fp.call_dynamic(context, Some(this_ptr), [value])
    },
);

let result = engine.eval::<i64>(
          r#"
                fn foo(x) { this += x; }    // script-defined function 'foo'

                let x = 41;                 // object
                x.bar(Fn("foo"), 1);        // pass 'foo' as function pointer
                x
          "#)?;
}

TL;DR – Why read_lock and write_lock

The Dynamic API that casts it to a reference to a particular data type is read_lock (for an immutable reference) and write_lock (for a mutable reference).

As the naming shows, something is locked in order to allow this access, and that something is a shared value created by capturing variables from closures.

Shared values are implemented as Rc<RefCell<Dynamic>> (Arc<RwLock<Dynamic>> under sync).

If the value is not a shared value, or if running under no_closure where there is no capturing, this API de-sugars to a simple Dynamic::downcast_ref and Dynamic::downcast_mut. In other words, there is no locking and reference counting overhead for the vast majority of non-shared values.

If the value is a shared value, then it is first locked and the returned lock guard then allows access to the underlying value in the specified type.

Hold Multiple References

In order to access a value argument that is expensive to clone while holding a mutable reference to the first argument, either consume that argument via mem::take as above, or use args.split_first to partition the slice:


#![allow(unused)]
fn main() {
// Partition the slice
let (first, rest) = args.split_first_mut().unwrap();

// Mutable reference to the first parameter
let this_ptr = &mut *first.write_lock::<A>().unwrap();

// Immutable reference to the second value parameter
// This can be mutable but there is no point because the parameter is passed by value
let value_ref = &*rest[0].read_lock::<B>().unwrap();
}