Optional Features
================

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By default, Rhai includes all the standard functionalities in a small, tight package.

Most features are here to opt-**out** of certain functionalities that are not needed.
Notice that this deviates from Rust norm where features are _additive_.

Excluding unneeded functionalities can result in smaller, faster builds as well as
more control over what a script can (or cannot) do.

| Feature             | Additive? | Description                                                                                                                                                                                                 |
| ------------------- | :-------: | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `unchecked`         |    no     | disables arithmetic checking (such as over-flows and division by zero), call stack depth limit, operations count limit and modules loading limit.<br/>Beware that a bad script may panic the entire system! |
| `sync`              |    no     | restricts all values types to those that are `Send + Sync`. Under this feature, all Rhai types, including [`Engine`], [`Scope`] and [`AST`], are all `Send + Sync`                                          |
| `no_optimize`       |    no     | disables [script optimization]                                                                                                                                                                              |
| `no_float`          |    no     | disables floating-point numbers and math                                                                                                                                                                    |
| `f32_float`         |    no     | sets the system floating-point type to `f32` instead of `f64`                                                                                                                                               |
| `only_i32`          |    no     | sets the system integer type to `i32` and disable all other integer types. `INT` is set to `i32`                                                                                                            |
| `only_i64`          |    no     | sets the system integer type to `i64` and disable all other integer types. `INT` is set to `i64`                                                                                                            |
| `no_index`          |    no     | disables [arrays] and indexing features                                                                                                                                                                     |
| `no_object`         |    no     | disables support for [custom types] and [object maps]                                                                                                                                                       |
| `no_function`       |    no     | disables script-defined [functions]                                                                                                                                                                         |
| `no_module`         |    no     | disables loading external [modules]                                                                                                                                                                         |
| `no_closure`        |    no     | disables [capturing][automatic currying] external variables in [anonymous functions] to simulate _closures_, or [capturing the calling scope]({{rootUrl}}/language/fn-capture.md) in function calls         |
| `no_std`            |    no     | builds for `no-std` (implies `no_closure`). Notice that additional dependencies will be pulled in to replace `std` features                                                                                 |
| `serde`             |    yes    | enables serialization/deserialization via `serde`. Notice that the [`serde`](https://crates.io/crates/serde) crate will be pulled in together with its dependencies                                         |
| `unicode-xid-ident` |    no     | allows [Unicode Standard Annex #31](http://www.unicode.org/reports/tr31/) as identifiers                                                                                                                    |
| `internals`         |    yes    | exposes internal data structures (e.g. [`AST`] nodes). Beware that Rhai internals are volatile and may change from version to version                                                                       |


Example
-------

The `Cargo.toml` configuration below turns on these six features:

* `sync` (everything `Send + Sync`)
* `unchecked` (disable all checking - should not be used with untrusted user scripts)
* `only_i32` (only 32-bit signed integers)
* `no_float` (no floating point numbers)
* `no_module` (no loading external [modules])
* `no_function` (no defining [functions])

```toml
[dependencies]
rhai = { version = "{{version}}", features = [ "sync", "unchecked", "only_i32", "no_float", "no_module", "no_function" ] }
```

The resulting scripting engine supports only the `i32` integer numeral type (and no others like `u32`, `i16` or `i64`),
no floating-point, is `Send + Sync` (so it can be safely used across threads), and does not support defining [functions]
nor loading external [modules].

This configuration is perfect for an expression parser in a 32-bit embedded system without floating-point hardware.


Caveat - Features Are Not Additive
---------------------------------

Most Rhai features are not strictly _additive_ - i.e. they do not only add optional functionalities.

In fact, most features are _subtractive_ - i.e. they _remove_ functionalities.

There is a reason for this design, because the _lack_ of a language feature by itself is a feature.

See [here]({{rootUrl}}/patterns/multiple.md) for more details.