Register a Fallible Rust Function
================================

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If a function is _fallible_ (i.e. it returns a `Result<_, Error>`), it can be registered with `register_result_fn`
(using the `RegisterResultFn` trait).

The function must return `Result<Dynamic, Box<EvalAltResult>>`.

```rust
use rhai::{Engine, EvalAltResult, Position};
use rhai::RegisterResultFn;                     // use 'RegisterResultFn' trait for 'register_result_fn'

// Function that may fail - the result type must be 'Dynamic'
fn safe_divide(x: i64, y: i64) -> Result<Dynamic, Box<EvalAltResult>> {
    if y == 0 {
        // Return an error if y is zero
        Err("Division by zero!".into())         // shortcut to create Box<EvalAltResult::ErrorRuntime>
    } else {
        Ok((x / y).into())                      // convert result into 'Dynamic'
    }
}

let mut engine = Engine::new();

// Fallible functions that return Result values must use register_result_fn()
engine.register_result_fn("divide", safe_divide);

if let Err(error) = engine.eval::<i64>("divide(40, 0)") {
    println!("Error: {:?}", *error);         // prints ErrorRuntime("Division by zero detected!", (1, 1)")
}
```

Create a `Box<EvalAltResult>`
----------------------------

`Box<EvalAltResult>` implements `From<&str>` and `From<String>` etc.
and the error text gets converted into `Box<EvalAltResult::ErrorRuntime>`.

The error values are `Box`-ed in order to reduce memory footprint of the error path, which should be hit rarely.