rhai/doc/src/language/throw.md

Throw Exception on Error
=======================

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All of [`Engine`]'s evaluation/consuming methods return `Result<T, Box<rhai::EvalAltResult>>`
with `EvalAltResult` holding error information.

To deliberately return an error during an evaluation, use the `throw` keyword.

```rust
if some_bad_condition_has_happened {
    throw error;    // 'throw' any value as the exception
}

throw;              // defaults to '()'
```

Exceptions thrown via `throw` in the script can be captured in Rust by matching
`Err(Box<EvalAltResult::ErrorRuntime(value, position)>)` with the exception value
captured by `value`.

```rust
let result = engine.eval::<i64>(r#"
    let x = 42;

    if x > 0 {
        throw x;
    }
"#);

println!(result);   // prints "Runtime error: 42 (line 5, position 15)"
```