Rhai Engine Integration for Loco

This crate adds Rhai script support to Loco.

Why Include a Scripting Engine

Although a system based upon Loco is usually compiled for maximum performance, there are times where user requirements are dynamic and need to be adapted to, preferably without recompilation.

Scripts are tremendously useful in the following cases:

• Complex custom configuration or custom business logic per installation at different sites without recompilation. In a different programming language, DLL's or dynamically-linked libraries may be used.

• Rapidly adapt to changing environments (e.g. handle new data formats, input changes, or novel user errors etc.) without hard-coding the rules (which may soon change again).

• Trial testing new features or business logic with fast iteration (without recompilation). The final version, once stable, can be converted into native Rust code for performance.

• Develop Tera filters in script so they can be iterated quickly. Useful ones can then be converted into Rust native filters. This can normally be achieved via Tera macros, but the Rhai scripting language is more powerful and expressive than Tera expressions, allowing more complex logic to be implemented.

Usage

Import rhai-loco inside Cargo.toml:

[dependencies]
rhai-loco = "0.13.1"

Configuration

The Loco config section of initializers can be used to set options for the Rhai engine.

# Initializers configuration
initializers:
  # Scripting engine configuration
  scripting:
    # Directory holding scripts
    scripts_path: assets/scripts
    # Directory holding Tera filter scripts
    filters_path: assets/scripts/tera/filters

Enable Scripted Tera Filters

Modify the ViewEngineInitializer under src/initializers/view_engine.rs:

┌─────────────────────────────────┐
│ src/initializers/view_engine.rs │
└─────────────────────────────────┘

// Within this method...
async fn after_routes(&self, router: AxumRouter, ctx: &AppContext) -> Result<AxumRouter> {
    let mut tera_engine = engines::TeraView::build()?;

            :
            :

    ///////////////////////////////////////////////////////////////////////////////////
    // Add the following to enable scripted Tera filters

    // Get scripting engine configuration object
    let config = ctx.config.initializers.as_ref()
        .and_then(|m| m.get(rhai_loco::ScriptingEngineInitializer::NAME))
        .cloned().unwrap_or_default();

    let config: rhai_loco::ScriptingEngineInitializerConfig = serde_json::from_value(config)?;

    if config.filters_path.is_dir() {
        // This code is duplicated from the original code
        // to expose the i18n `t` function to Rhai scripts
        let i18n = if Path::new(I18N_DIR).is_dir() {
            let arc = ArcLoader::builder(I18N_DIR, unic_langid::langid!("de-DE"))
                .shared_resources(Some(&[I18N_SHARED.into()]))
                .customize(|bundle| bundle.set_use_isolating(false))
                .build()
                .map_err(|e| Error::string(&e.to_string()))?;
            Some(FluentLoader::new(arc))
        } else {
            None
        };
        rhai_loco::RhaiScript::register_tera_filters(
            &mut tera_engine,
            config.filters_path,
            |_engine| {},   // custom configuration of the Rhai Engine, if any
            i18n,
        )?;
        info!("Filter scripts loaded");
    }

    // End addition
    ///////////////////////////////////////////////////////////////////////////////////

    Ok(router.layer(Extension(ViewEngine::from(tera_engine))))
}

Each Rhai script file (extension .rhai) can contain multiple filters. Sub-directories are ignored.

Each function inside the Rhai script file constitutes one filter, unless marked as private. The name of the function is the name of the filter.

Function Signature

Each filter function must take exactly one parameter, which is an object-map containing all the variables in the filter call.

In addition, variables in the filter call can also be accessed as stand-alone variables.

The original data value is mapped to this.

Example

For a filter call:

┌───────────────┐
│ Tera template │
└───────────────┘

{{ "hello" | super_duper(a = "world", b = 42, c = true) }}

The filter function super_duper can be defined as follows in a Rhai script file:

┌─────────────┐
│ Rhai script │
└─────────────┘

// This private function is ignored
private fn do_something(x) {
    ...
}

// This function has the wrong number of parameters and is ignored
fn do_other_things(x, y, z) {
    ...
}

// Filter 'super_duper'
fn super_duper(vars) {
    // 'this' maps to "hello"
    // 'vars' contains 'a', 'b' and 'c'
    // The stand-alone variables 'a', 'b' and 'c' can also be accessed

    let name = if vars.b > 0 {  // access 'b' under 'vars'
        ...
    } else if c {               // access 'c'
        ...
    } else !a.is_empty() {      // access 'a'
        ...
    } else {
        ...
    }

    // 'this' can be modified
    this[0].to_upper();

    // Return new value
    `${this}, ${name}!`
}

Scripted filters as conversion/formatting tool

Scripted filters can be very flexible for ad-hoc conversion/formatting purposes because they enable rapid iterations and changes without recompiling.

┌────────────────────┐
│ Rhai filter script │
└────────────────────┘

/// Say we have in-house status codes that we need to convert into text
/// for display with i18n support...
fn status(vars) {
    switch this {
        case "P" => t("Pending", lang),
        case "A" => t("Active", lang),
        case "C" => t("Cancelled", lang),
        case "X" => t("Deleted", lang),
    }
}

/// Use script to inject HTML also!
/// The input value is used to select from the list of options
fn all_status(vars) {`
    <option value="P" ${if this == "P" { "selected" }}>t("Pending", lang)</option>
    <option value="A" ${if this == "A" { "selected" }}>t("Active", lang)</option>
    <option value="C" ${if this == "C" { "selected" }}>t("Cancelled", lang)</option>
    <option value="X" ${if this == "X" { "selected" }}>t("Deleted", lang)</option>
`}

/// Say we have CSS classes that we need to add based on certain data values
fn count_css(vars) {
    if this.count > 1 {
        "error more-than-one"
    } else if this.count == 0 {
        "error missing-value"
    } else {
        "success"
    }
}

┌───────────────┐
│ Tera template │
└───────────────┘

<!-- use script to determine the CSS class -->
<div id="record" class="{{ value | count_css }}">
    <!-- use script to map the status display -->
    <span>{{ value.status | status(lang="de-DE") }} : {{ value.count }}</span>
</div>

<!-- use script to inject HTML directly -->
<select>
    <option value="">t("All", "de-DE")</option>
    <!-- avoid escaping as text via the `safe` filter -->
    {{ "A" | all_status(lang="de-DE") | safe }}
</select>

The above is equivalent to the following Tera template.

Technically speaking, you either maintain such ad-hoc behavior in script or inside the Tera template itself, but doing so in script allows for reuse and a cleaner template.

┌───────────────┐
│ Tera template │
└───────────────┘

<div id="record" class="{% if value.count > 1 %}
                            error more-than-one
                        {% elif value.count == 0 %}
                            error missing-value
                        {% else %}
                            success
                        {% endif %}">

    <span>
        {% if value.status == "P" %}
            t(key = "Pending", lang = "de-DE")
        {% elif value.status == "A" %}
            t(key = "Active", lang = "de-DE")
        {% elif value.status == "C" %}
            t(key = "Cancelled", lang = "de-DE")
        {% elif value.status == "D" %}
            t(key = "Deleted", lang = "de-DE")
        {% endif %}
        : {{ value.count }}
    </span>
</div>

Run a Rhai script in Loco Request

The scripting engine is first injected into Loco via the ScriptingEngineInitializer:

┌────────────┐
│ src/app.rs │
└────────────┘

async fn initializers(_ctx: &AppContext) -> Result<Vec<Box<dyn Initializer>>> {
    Ok(vec![
        // Add the scripting engine initializer
        Box::new(rhai_loco::ScriptingEngineInitializer),
        Box::new(initializers::view_engine::ViewEngineInitializer),
    ])
}

The scripting engine can then be extracted in requests using ScriptingEngine.

For example, the following adds custom scripting support to the login authentication process:

┌─────────────────────────┐
│ src/controllers/auth.rs │
└─────────────────────────┘

// Import the scripting engine types
use rhai_loco::{RhaiScript, ScriptingEngine};

pub async fn login(
    State(ctx): State<AppContext>,
    // Extract the scripting engine
    ScriptingEngine(script): ScriptingEngine<RhaiScript>,
    Json(mut params): Json<LoginParams>,
) -> Result<Json<LoginResponse>> {
    // Use `run_script_if_exists` to run a function `login` from a script
    // `on_login.rhai` if it exists under `assets/scripts/`.
    //
    // Use `run_script` if the script is required to exist or an error is returned.
    let result = script
        .run_script_if_exists("on_login", &mut params, "login", ())
        //                    ^ script file            ^ function name
        //                                ^ data mapped to `this` in script
        //                                                      ^^ function arguments
        .or_else(|err| script.convert_runtime_error(err, |msg| unauthorized(&msg)))?;
        //                                               ^^^^^^^^^^^^^^^^^^^^^^^^
        //                      turn any runtime error into an unauthorized response

                :
                :
}

This calls a function named login within the script file on_login.rhai if it exists:

┌──────────────────────────────┐
│ assets/scripts/on_login.rhai │
└──────────────────────────────┘

// Function for custom login logic
fn login() {
    // Can import other Rhai modules!
    import "super/secure/vault" as vault;

    debug(`Trying to login with user = ${this.user} and password = ${this.password}`);

    let security_context = vault.extensive_checking(this.user, this.password);

    if security_context.passed {
        // Data values can be changed!
        this.user = security_context.masked_user;
        this.password = security_context.masked_password;
        return security_context.id;
    } else {
        vault::black_list(this.user);
        throw `The user ${this.user} has been black-listed!`;
    }
}

Custom Engine Setup

In order to customize the Rhai scripting engine, for example to add custom functions or custom types support, it is easy to perform custom setup on the Rhai engine via ScriptingEngineInitializerWithSetup:

┌────────────┐
│ src/app.rs │
└────────────┘

async fn initializers(_ctx: &AppContext) -> Result<Vec<Box<dyn Initializer>>> {
    Ok(vec![
        // Add the scripting engine initializer
        Box::new(rhai_loco::ScriptingEngineInitializerWithSetup::new_with_setup(|engine| {
                        :
            // ... do custom setup of Rhai engine here ...
                        :
        })),
        Box::new(initializers::view_engine::ViewEngineInitializer),
    ])
}