rhai/doc/src/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 `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 reference to the current `Engine` instance is passed as an argument so the Rust function
can also use `Engine` facilities (like evaluating a script).

```rust
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
    ],
    |engine: &Engine, lib: &Module, args: &mut [&mut Dynamic]| {    // 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_at_mut(1)` etc. to split the slice first.

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

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

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

        *x += y;                                            // perform the action

        Ok(().into())                                       // 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(engine: &Engine, lib: &Module, args: &mut [&mut Dynamic]) -> Result<T, Box<EvalAltResult>> + 'static`

where:

* `T : Variant + Clone` - return type of the function.

* `engine : &Engine` - the current [`Engine`], with all configurations and settings.

* `lib : &Module` - the current global library of script-defined functions, as a [`Module`].
  This is sometimes useful for calling a script-defined function within the same evaluation context using [`Engine::call_fn`][`call_fn`].

* `args : &mut [&mut Dynamic]` - a slice containing `&mut` references to [`Dynamic`] values.
  The slice is guaranteed to contain enough arguments _of the correct types_.

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 type                  | Access (`n` = argument position)       | Result                                                     |
| ------------------------------ | -------------------------------------- | ---------------------------------------------------------- |
| [Primary type][standard types] | `args[n].clone().cast::<T>()`          | Copy of value.                                             |
| Custom type                    | `args[n].downcast_ref::<T>().unwrap()` | Immutable reference to value.                              |
| Custom type (consumed)         | `std::mem::take(args[n]).cast::<T>()`  | The _consumed_ value.<br/>The original value becomes `()`. |
| `this` object                  | `args[0].downcast_mut::<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 Function Pointer 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.

```rust
use rhai::{Engine, Module, Dynamic, 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>(),
    ],
    move |engine: &Engine, lib: &Module, args: &mut [&mut Dynamic]| {
        // '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, only script-defined functions are supported by 'FnPtr::call_dynamic'.
        // If it is a native Rust function, directly call it here in Rust instead!
        fp.call_dynamic(engine, lib, 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
"#)?;
```


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_at`
to partition the slice:

```rust
// Partition the slice
let (first, rest) = args.split_at_mut(1);

// Mutable reference to the first parameter
let this_ptr = first[0].downcast_mut::<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 = rest[0].downcast_ref::<B>().unwrap();
```