92 lines
3.2 KiB
Markdown
92 lines
3.2 KiB
Markdown
Function Namespaces
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==================
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{{#include ../links.md}}
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Each Function is a Separate Compilation Unit
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-------------------------------------------
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[Functions] in Rhai are _pure_ and they form individual _compilation units_.
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This means that individual functions can be separated, exported, re-grouped, imported,
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and generally mix-'n-match-ed with other completely unrelated scripts.
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For example, the `AST::merge` and `AST::combine` methods (or the equivalent `+` and `+=` operators)
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allow combining all functions in one [`AST`] into another, forming a new, unified, group of functions.
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In general, there are two types of _namespaces_ where functions are looked up:
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| Namespace | Source | Lookup method | Sub-modules? | Variables? |
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| --------- | ------------------------------------------------------------------------------------- | ------------------------------ | :----------: | :--------: |
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| Global | 1) `Engine::register_XXX` API<br/>2) [`AST`] being evaluated<br/>3) [packages] loaded | simple function name | ignored | ignored |
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| Module | [`Module`] | module-qualified function name | yes | yes |
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Global Namespace
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----------------
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There is one _global_ namespace for every [`Engine`], which includes:
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* All the native Rust functions registered via the `Engine::register_XXX` API.
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* All the Rust functions defined in [packages] that are loaded into the [`Engine`].
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In addition, during evaluation of an [`AST`], all script-defined functions bundled together within
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the [`AST`] are added to the global namespace and override any existing registered functions of
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the same names and number of parameters.
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Anywhere in a Rhai script, when a function call is made, it is searched within the global namespace.
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Therefore, function calls in Rhai are _late_ bound - meaning that the function called cannot be
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determined or guaranteed and there is no way to _lock down_ the function being called.
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This aspect is very similar to JavaScript before ES6 modules.
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```rust
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// Compile a script into AST
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let ast1 = engine.compile(
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r#"
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fn get_message() {
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"Hello!" // greeting message
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}
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fn say_hello() {
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print(get_message()); // prints message
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}
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say_hello();
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"#
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)?;
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// Compile another script with an overriding function
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let ast2 = engine.compile(r#"fn get_message() { "Boo!" }"#)?;
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// Combine the two AST's
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ast1 += ast2; // 'message' will be overwritten
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engine.consume_ast(&ast1)?; // prints 'Boo!'
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```
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Therefore, care must be taken when _cross-calling_ functions to make sure that the correct
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functions are called.
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The only practical way to ensure that a function is a correct one is to use [modules] -
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i.e. define the function in a separate module and then [`import`] it:
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```rust
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----------------
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| message.rhai |
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----------------
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fn get_message() { "Hello!" }
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---------------
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| script.rhai |
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---------------
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import "message" as msg;
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fn say_hello() {
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print(msg::get_message());
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}
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say_hello();
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```
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