Function Namespaces
===================

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


Each Function is a Separate Compilation Unit
--------------------------------------------

[Functions] in Rhai are _pure_ and they form individual _compilation units_.

This means that individual [functions] can be separated, exported, re-grouped, imported, and
generally mix-'n-matched with other completely unrelated scripts.

For example, the `AST::merge` and `AST::combine` methods (or the equivalent `+` and `+=` operators)
allow combining all [functions] in one [`AST`] into another, forming a new, unified, group of [functions].


Namespace Types
---------------

In general, there are two main types of _namespaces_ where [functions] are looked up:

| Namespace | Quantity | Source                                                                                                                                                                                                                                                    | Lookup                   | Sub-modules? | Variables? |
| --------- | :------: | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------ | :----------: | :--------: |
| Global    |   one    | <ol><li>[`AST`] being evaluated</li><li>`Engine::register_XXX` API</li><li>global registered [modules]</li><li>[functions] in [imported][`import`] [modules] marked _global_</li><li>[functions] in registered static [modules] marked _global_</li></ol> | simple name              |   ignored    |  ignored   |
| Module    |   many   | <ol><li>[Module] registered via `Engine::register_static_module`</li><li>[Module] loaded via [`import`] statement</li></ol>                                                                                                                               | namespace-qualified name |     yes      |    yes     |

### Module Namespaces

There can be multiple [module] namespaces at any time during a script evaluation, usually loaded via
the [`import`] statement.

_Static_ [module] namespaces can also be registered into an [`Engine`] via `Engine::register_static_module`.

[Functions] and [variables] in module namespaces are isolated and encapsulated within their own environments.

They must be called or accessed in a _namespace-qualified_ manner.

```js
import "my_module" as m;        // new module namespace 'm' created via 'import'

let x = m::calc_result();       // namespace-qualified function call

let y = m::MY_NUMBER;           // namespace-qualified variable/constant access

let z = calc_result();          // <- error: function 'calc_result' not found
                                //    in global namespace!
```

### Global Namespace

There is one _global_ namespace for every [`Engine`], which includes (in the following search order):

* all [functions] defined in the [`AST`] currently being evaluated,

* all native Rust functions and iterators registered via the `Engine::register_XXX` API,

* all functions and iterators defined in global [modules] that are registered into the [`Engine`]
  via `register_global_module`,

* functions defined in [modules] registered into the [`Engine`] via `register_static_module` that
  are specifically marked for exposure to the global namespace (e.g. via the `#[rhai(global)]`
  attribute in a [plugin module]).

* [functions] defined in [imported][`import`] [modules] that are specifically marked for exposure to
  the global namespace (e.g. via the `#[rhai(global)]` attribute in a [plugin module]).

Anywhere in a Rhai script, when a function call is made, the function is searched within the
global namespace, in the above search order.

Therefore, function calls in Rhai are _late_ bound &ndash; meaning that the function called cannot be
determined or guaranteed; there is no way to _lock down_ the function being called.
This aspect is very similar to JavaScript before ES6 modules.

```rust
// Compile a script into AST
let ast1 = engine.compile(
r#"
    fn get_message() {
        "Hello!"                // greeting message
    }

    fn say_hello() {
        print(get_message());   // prints message
    }

    say_hello();
"#)?;

// Compile another script with an overriding function
let ast2 = engine.compile(r#"fn get_message() { "Boo!" }"#)?;

// Combine the two AST's
ast1 += ast2;                   // 'message' will be overwritten

engine.run_ast(&ast1)?;         // prints 'Boo!'
```

Therefore, care must be taken when _cross-calling_ [functions] to make sure that the correct
[function] is called.

The only practical way to ensure that a [function] is a correct one is to use [modules] &ndash;
i.e. define the [function] in a separate [module] and then [`import`] it:

```js
┌──────────────┐
│ message.rhai │
└──────────────┘

fn get_message() { "Hello!" }


┌─────────────┐
│ script.rhai │
└─────────────┘

import "message" as msg;

fn say_hello() {
    print(msg::get_message());
}
say_hello();
```