rhai/doc/src/language/eval.md

`eval` Statement
===============

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

Or "How to Shoot Yourself in the Foot even Easier"
------------------------------------------------

Saving the best for last, there is the ever-dreaded... `eval` function!

```rust
let x = 10;

fn foo(x) { x += 12; x }

let script = "let y = x;";      // build a script
script +=    "y += foo(y);";
script +=    "x + y";

let result = eval(script);      // <- look, JavaScript, we can also do this!

print("Answer: " + result);     // prints 42

print("x = " + x);              // prints 10: functions call arguments are passed by value
print("y = " + y);              // prints 32: variables defined in 'eval' persist!

eval("{ let z = y }");          // to keep a variable local, use a statement block

print("z = " + z);              // <- error: variable 'z' not found

"print(42)".eval();             // <- nope... method-call style doesn't work
```

Script segments passed to `eval` execute inside the current [`Scope`], so they can access and modify _everything_,
including all variables that are visible at that position in code! It is almost as if the script segments were
physically pasted in at the position of the `eval` call.


Cannot Define New Functions
--------------------------

New functions cannot be defined within an `eval` call, since functions can only be defined at the _global_ level,
not inside another function call!

```rust
let script = "x += 32";
let x = 10;
eval(script);                   // variable 'x' in the current scope is visible!
print(x);                       // prints 42

// The above is equivalent to:
let script = "x += 32";
let x = 10;
x += 32;
print(x);
```


`eval` is Evil
--------------

For those who subscribe to the (very sensible) motto of ["`eval` is evil"](http://linterrors.com/js/eval-is-evil),
disable `eval` by overloading it, probably with something that throws.

```rust
fn eval(script) { throw "eval is evil! I refuse to run " + script }

let x = eval("40 + 2");         // 'eval' here throws "eval is evil! I refuse to run 40 + 2"
```

Or overload it from Rust:

```rust
fn alt_eval(script: String) -> Result<(), Box<EvalAltResult>> {
    Err(format!("eval is evil! I refuse to run {}", script).into())
}

engine.register_result_fn("eval", alt_eval);
```


`EvalPackage`
-------------

There is even a package named [`EvalPackage`]({{rootUrl}}/rust/packages.md) which implements the disabling override:

```rust
use rhai::Engine;
use rhai::packages::Package             // load the 'Package' trait to use packages
use rhai::packages::EvalPackage;        // the 'eval' package disables 'eval'

let mut engine = Engine::new();
let package = EvalPackage::new();       // create the package

engine.load_package(package.get());     // load the package
```