Export a Rust Function to Rhai
=============================

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


Sometimes only a few ad hoc functions are required and it is simpler to register
individual functions instead of a full-blown [plugin module].


Macros
------

| Macro                   | Signature                                                          | Description                                                     |
| ----------------------- | ------------------------------------------------------------------ | --------------------------------------------------------------- |
| `#[export_fn]`          | apply to rust function defined in a Rust module                    | exports the function                                            |
| `register_exported_fn!` | `register_exported_fn!(&mut `_engine_`, "`_name_`", `_function_`)` | registers the function into an [`Engine`] under a specific name |
| `set_exported_fn!`      | `set_exported_fn!(&mut `_module_`, "`_name_`", `_function_`)`      | registers the function into a [`Module`] under a specific name  |


`#[export_fn]` and `register_exported_fn!`
-----------------------------------------

Apply `#[export_fn]` onto a function defined at _module level_ to convert it into a Rhai plugin function.

The function cannot be nested inside another function - it can only be defined directly under a module.

To register the plugin function, simply call `register_exported_fn!`.  The name of the function can be
any text string, so it is possible to register _overloaded_ functions as well as operators.

```rust
use rhai::plugin::*;        // import macros

#[export_fn]
fn increment(num: &mut i64) {
    *num += 1;
}

fn main() {
    let mut engine = Engine::new();

    // 'register_exported_fn!' registers the function as 'inc' with the Engine.
    register_exported_fn!(engine, "inc", increment);
}
```


Fallible Functions
------------------

To register [fallible functions] (i.e. functions that may return errors), apply the
`#[rhai_fn(return_raw)]` attribute on plugin functions that return `Result<Dynamic, Box<EvalAltResult>>`.

A syntax error is generated if the function with `#[rhai_fn(return_raw)]` does not
have the appropriate return type.

```rust
use rhai::plugin::*;        // a "prelude" import for macros

#[export_fn]
#[rhai_fn(return_raw)]
pub fn double_and_divide(x: i64, y: i64) -> Result<Dynamic, Box<EvalAltResult>> {
    if y == 0 {
        Err("Division by zero!".into())
    } else {
        let result = (x * 2) / y;
        Ok(result.into())
    }
}

fn main() {
    let mut engine = Engine::new();

    // Overloads the operator '+' with the Engine.
    register_exported_fn!(engine, "+", double_and_divide);
}
```


`NativeCallContext` Parameter
----------------------------

If the _first_ parameter of a function is of type `rhai::NativeCallContext`, then it is treated
specially by the plugins system.

`NativeCallContext` is a type that encapsulates the current _native call context_ and exposes the following:

| Field               |              Type               | Description                                                                                                                                                                                                                                |
| ------------------- | :-----------------------------: | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `engine()`          |            `&Engine`            | the current [`Engine`], with all configurations and settings.<br/>This is sometimes useful for calling a script-defined function within the same evaluation context using [`Engine::call_fn`][`call_fn`], or calling a [function pointer]. |
| `imports()`         |       `Option<&Imports>`        | reference to the current stack of [modules] imported via `import` statements (if any)                                                                                                                                                      |
| `iter_namespaces()` | `impl Iterator<Item = &Module>` | iterator of the namespaces (as [modules]) containing all script-defined functions                                                                                                                                                          |

This first parameter, if exists, will be stripped before all other processing.  It is _virtual_.
Most importantly, it does _not_ count as a parameter to the function and there is no need to provide
this argument when calling the function in Rhai.

The native call context can be used to call a [function pointer] or [closure] that has been passed
as a parameter to the function, thereby implementing a _callback_:

```rust
use rhai::{Dynamic, FnPtr, NativeCallContext, EvalAltResult};
use rhai::plugin::*;        // a "prelude" import for macros

#[export_fn]
#[rhai_fn(return_raw)]
pub fn greet(context: NativeCallContext, callback: FnPtr)
                            -> Result<Dynamic, Box<EvalAltResult>>
{
    // Call the callback closure with the current context
    // to obtain the name to greet!
    let name = callback.call_dynamic(context, None, [])?;
    Ok(format!("hello, {}!", name).into())
}
```

The native call context is also useful in another scenario: protecting a function from malicious scripts.

```rust
use rhai::{Dynamic, Array, NativeCallContext, EvalAltResult, NO_POS};
use rhai::plugin::*;        // a "prelude" import for macros

// This function builds an array of arbitrary size, but is protected
// against attacks by first checking with the allowed limit set
// into the 'Engine'.
#[export_fn]
#[rhai_fn(return_raw)]
pub fn grow(context: NativeCallContext, size: i64)
                            -> Result<Dynamic, Box<EvalAltResult>>
{
    // Make sure the function does not generate a
    // data structure larger than the allowed limit
    // for the Engine!
    if size as usize > context.engine().max_array_size()
    {
        return EvalAltResult::ErrorDataTooLarge(
            "Size to grow".to_string(),
            context.engine().max_array_size(),
            size as usize,
            NO_POS,
        ).into();
    }

    let array = Array::new();

    for x in 0..size {
        array.push(x.into());
    }

    OK(array.into())
}
```