151 lines
3.7 KiB
Markdown
151 lines
3.7 KiB
Markdown
Loadable Configuration
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======================
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{{#include ../links.md}}
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Usage Scenario
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--------------
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* A system where settings and configurations are complex and logic-driven.
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* Where said system is too complex to configure via standard configuration file formats such as `JSON`, `TOML` or `YAML`.
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* The system is complex enough to require a full programming language to configure. Essentially _configuration by code_.
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* Yet the configuration must be flexible, late-bound and dynamically loadable, just like a configuration file.
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Key Concepts
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------------
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* Leverage the loadable [modules] of Rhai. The [`no_module`] feature must not be on.
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* Expose the configuration API. Use separate scripts to configure that API. Dynamically load scripts via the `import` statement.
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* Leverage [function overloading] to simplify the API design.
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* Since Rhai is _sand-boxed_, it cannot mutate the environment. To modify the external configuration object via an API, it must be wrapped in a `RefCell` (or `RwLock`/`Mutex` for [`sync`]) and shared to the [`Engine`].
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Implementation
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--------------
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### Configuration Type
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```rust
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#[derive(Debug, Clone, Default)]
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struct Config {
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pub id: String;
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pub some_field: i64;
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pub some_list: Vec<String>;
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pub some_map: HashMap<String, bool>;
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}
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```
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### Make Shared Object
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```rust
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let config: Rc<RefCell<Config>> = Rc::new(RefCell::new(Default::default()));
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```
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### Register Config API
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```rust
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// Notice 'move' is used to move the shared configuration object into the closure.
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let cfg = config.clone();
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engine.register_fn("config_set_id", move |id: String| *cfg.borrow_mut().id = id);
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let cfg = config.clone();
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engine.register_fn("config_get_id", move || cfg.borrow().id.clone());
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let cfg = config.clone();
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engine.register_fn("config_set", move |value: i64| *cfg.borrow_mut().some_field = value);
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// Remember Rhai functions can be overloaded when designing the API.
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let cfg = config.clone();
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engine.register_fn("config_add", move |value: String|
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cfg.borrow_mut().some_list.push(value)
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);
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let cfg = config.clone();
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engine.register_fn("config_add", move |values: &mut Array|
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cfg.borrow_mut().some_list.extend(values.into_iter().map(|v| v.to_string()))
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);
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let cfg = config.clone();
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engine.register_fn("config_add", move |key: String, value: bool|
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cfg.borrow_mut().some_map.insert(key, value)
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);
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let cfg = config.clone();
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engine.register_fn("config_contains", move |value: String|
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cfg.borrow().some_list.contains(&value)
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);
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let cfg = config.clone();
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engine.register_fn("config_is_set", move |value: String|
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cfg.borrow().some_map.get(&value).cloned().unwrap_or(false)
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);
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```
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### Configuration Script
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```rust
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------------------
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| my_config.rhai |
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------------------
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config_set_id("hello");
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config_add("foo"); // add to list
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config_add("bar", true); // add to map
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if config_contains("hey") || config_is_set("hey") {
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config_add("baz", false); // add to map
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}
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```
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### Load the Configuration
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```rust
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import "my_config"; // run configuration script without creating a module
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let id = config_get_id();
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id == "hello";
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```
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Consider a Custom Syntax
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------------------------
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This is probably one of the few scenarios where a [custom syntax] can be recommended.
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A properly-designed [custom syntax] can make the configuration file clean, simple to write,
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easy to understand and quick to modify.
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For example, the above configuration example may be expressed by this custom syntax:
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```rust
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------------------
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| my_config.rhai |
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------------------
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// Configure ID
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id "hello";
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// Add to list
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list +"foo"
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// Add to map
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map "bar" => true;
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if config contains "hey" || config is_set "hey" {
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map "baz" => false;
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}
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```
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Notice that `contains` and `is_set` may be implemented as a [custom operator].
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