rhaj/rhai_engine/rhaibook/rust/register-raw.md

Use the Low-Level API to Register a Rust Function

{{#include ../links.md}}

When a native Rust function is registered with an [Engine] using the 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.

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

        let y = args[1].as_int().unwrap();                  // alternatively, use 'as_xxx()'

        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>>

where:

Parameter	Type	Description
T	impl Clone	return type of the function
context	[NativeCallContext]	the current native call context, useful for recursively calling functions on the same [Engine]
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 The First &mut Argument (If Any)

To extract the first &mut argument passed by reference from the args parameter (&mut [&mut Dynamic]), use the following to get a mutable reference to the underlying value:

let value: &mut T = &mut *args[0].write_lock::<T>().unwrap();

*value = ...    // overwrite the existing value of the first `&mut` parameter

When there is a mutable reference to the first &mut argument, there can be no other immutable references to args, otherwise the Rust borrow checker will complain.

Therefore, always extract the mutable reference last, after all other arguments are taken.

Extract Other Pass-By-Value Arguments

To extract an argument passed by value from the args parameter (&mut [&mut Dynamic]), use the following statements.

Argument type	Access statement (n = argument position)	Result	Original value
INT	args[n].as_int().unwrap()	INT	untouched
FLOAT	args[n].as_float().unwrap()	FLOAT	untouched
[Decimal][rust_decimal]	args[n].as_decimal().unwrap()	[Decimal][rust_decimal]	untouched
bool	args[n].as_bool().unwrap()	bool	untouched
char	args[n].as_char().unwrap()	char	untouched
()	args[n].as_unit().unwrap()	()	untouched
[String]	&*args[n].as_immutable_string_ref().unwrap()	[&ImmutableString][ImmutableString]	untouched
[String] (consumed)	args[n].take().cast::<ImmutableString>()	[ImmutableString]	[()]
Others	&*args[n].read_lock::<T>().unwrap()	&T	untouched
Others (consumed)	args[n].take().cast::<T>()	T	[()]

Example – Pass 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.

The example also showcases the use of FnPtr::call_raw, a low-level API which allows binding the this pointer to the function pointer call.

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 = args[1].take().cast::<FnPtr>();            // 2nd argument - function pointer
        let value = args[2].take();                         // 3rd argument - function argument

        // The 1st argument holds the 'this' pointer.
        // This should be done last as it gets a mutable reference to 'args'.
        let this_ptr = args.get_mut(0).unwrap();

        // Use 'FnPtr::call_raw' to call the function pointer with the context
        // while also binding the 'this' pointer!
        fp.call_raw(&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(foo, 1);                  // pass 'foo' as function pointer
    x
"#)?;

In order to access a value argument that is expensive to clone _while_ holding a mutable reference
to the first argument, use one of the following tactics:

1. If it is a [primary type][standard types] other than [string], use `as_xxx()` as above;

2. Directly _consume_ that argument via `arg[i].take()` as above.

3. Use `split_first_mut` to partition the slice:

```rust
// Partition the slice
let (first, rest) = args.split_first_mut().unwrap();

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

// Immutable reference to the second value parameter, of type '&B'
// 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();
```