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_archive/rhai_engine/rhaibook/engine/var.md

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+Variable Resolver
+=================
+
+{{#include ../links.md}}
+
+[`Engine::on_var`]: https://docs.rs/rhai/{{version}}/rhai/struct.Engine.html#method.on_var
+
+
+By default, Rhai looks up access to [variables] from the enclosing block scope, working its way
+outwards until it reaches the top (global) level, then it searches the [`Scope`] that is passed into
+the `Engine::eval` call.
+
+There is a built-in facility for advanced users to _hook_ into the [variable] resolution service and
+to override its default behavior.
+
+To do so, provide a closure to the [`Engine`] via [`Engine::on_var`].
+
+```rust
+let mut engine = Engine::new();
+
+// Register a variable resolver.
+engine.on_var(|name, index, context| {
+    match name {
+        "MYSTIC_NUMBER" => Ok(Some(42_i64.into())),
+        // Override a variable - make it not found even if it exists!
+        "DO_NOT_USE" => Err(EvalAltResult::ErrorVariableNotFound(name.to_string(), Position::NONE).into()),
+        // Silently maps 'chameleon' into 'innocent'.
+        "chameleon" => context.scope().get_value("innocent").map(Some).ok_or_else(|| 
+            EvalAltResult::ErrorVariableNotFound(name.to_string(), Position::NONE).into()
+        ),
+        // Return Ok(None) to continue with the normal variable resolution process.
+        _ => Ok(None)
+    }
+});
+```
+
+```admonish info.small "Benefits of using a variable resolver"
+
+1. Avoid having to maintain a custom [`Scope`] with all [variables] regardless of need
+   (because a script may not use them all).
+
+2. _Short-circuit_ [variable] access, essentially overriding standard behavior.
+
+3. _Lazy-load_ [variables] when they are accessed, not up-front.
+   This benefits when the number of [variables] is very large, when they are timing-dependent,
+   or when they are expensive to load.
+
+4. Rename system [variables] on a script-by-script basis without having to construct different [`Scope`]'s.
+```
+
+```admonish warning.small "Returned values are constants"
+
+[Variable] values returned by a variable resolver are treated as _[constants]_.
+
+This is to avoid needing a mutable reference to the underlying data provider which may not be possible to obtain.
+
+To change these [variables], better push them into a custom [`Scope`] instead of
+using a variable resolver.
+```
+
+```admonish tip.small "Tip: Returning shared values"
+
+It is possible to return a _shared_ value from a variable resolver.
+
+This is one way to implement [Mutable Global State]({{rootUrl}}/patterns/global-mutable-state.md).
+```
+
+
+Function Signature
+------------------
+
+The function signature passed to [`Engine::on_var`] takes the following form.
+
+> ```rust
+> Fn(name: &str, index: usize, context: EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
+> ```
+
+where:
+
+| Parameter |      Type       | Description                                                                                                                                                                                                                                                                                                                                                                                          |
+| --------- | :-------------: | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `name`    |     `&str`      | [variable] name                                                                                                                                                                                                                                                                                                                                                                                      |
+| `index`   |     `usize`     | an offset from the bottom of the current [`Scope`] that the [variable] is supposed to reside.<br/>Offsets start from 1, with 1 meaning the last [variable] in the current [`Scope`].  Essentially the correct [variable] is at position `scope.len() - index`.<br/>If `index` is zero, then there is no pre-calculated offset position and a search through the current [`Scope`] must be performed. |
+| `context` | [`EvalContext`] | mutable reference to the current _evaluation context_                                                                                                                                                                                                                                                                                                                                                |
+
+and [`EvalContext`] is a type that encapsulates the current _evaluation context_.
+
+### Return value
+
+The return value is `Result<Option<Dynamic>, Box<EvalAltResult>>` where:
+
+| Value                     | Description                                                                                                                                                                        |
+| ------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `Ok(None)`                | normal [variable] resolution process should continue, i.e. continue searching through the [`Scope`]                                                                                |
+| `Ok(Some(value))`         | value (a [`Dynamic`]) of the [variable], treated as a [constant]                                                                                                                   |
+| `Err(Box<EvalAltResult>)` | error that is reflected back to the [`Engine`], normally `EvalAltResult::ErrorVariableNotFound` to indicate that the [variable] does not exist, but it can be any `EvalAltResult`. |