Change Option<usize> to usize for variable resolver index.

doc/src/SUMMARY.md

@@ -121,6 +121,7 @@ The Rhai Scripting Language
    5. [Multi-Layer Functions](patterns/multi-layer.md)
    6. [One Engine Instance Per Call](patterns/parallel.md)
    7. [Scriptable Event Handler with State](patterns/events.md)
+   8. [Dynamic Constants Provider](patterns/dynamic-const.md)
 9. [Advanced Topics](advanced.md)
    1. [Capture Scope for Function Call](language/fn-capture.md)
    2. [Low-Level API](rust/register-raw.md)

doc/src/advanced.md

@@ -3,14 +3,4 @@ Advanced Topics
 
 {{#include links.md}}
 
-This section covers advanced features such as:
+This section covers advanced features of the Rhai [`Engine`].
-
-* [Advanced patterns]({{rootUrl}}/patterns/index.md) in using Rhai.
-
-* [Capture the calling scope]({{rootUrl}}/language/fn-capture.md) in a function call.
-
-* [`serde`] integration.
-
-* Low-level [function registration API]({{rootUrl}}/rust/register-raw.md)
-
-* [Domain-Specific Languages][DSL].

doc/src/engine/custom-syntax.md

@@ -122,7 +122,7 @@ where:
 
 * `context: &mut EvalContext` - mutable reference to the current evaluation _context_ (**do not touch**) which exposes the following fields:
   * `context.engine(): &Engine` - reference to the current [`Engine`].
-  * `context.namespace(): &Module` - reference to the current _global namespace_ containing all script-defined functions.
+  * `context.namespace(): &Module` - reference to the current _global namespace_ (as a [module]) containing all script-defined functions.
   * `context.call_level(): usize` - the current nesting level of function calls.
 
 * `inputs: &[Expression]` - a list of input expression trees.

doc/src/engine/var.md

@@ -16,7 +16,7 @@ To do so, provide a closure to the [`Engine`] via the [`Engine::on_var`] method:
 let mut engine = Engine::new();
 
 // Register a variable resolver.
-engine.on_var(|name, index, engine, scope, lib| {
+engine.on_var(|name, index, scope, context| {
     match name {
         "MYSTIC_NUMBER" => Ok(Some((42 as INT).into())),
         // Override a variable - make it not found even if it exists!
@@ -50,26 +50,23 @@ Function Signature
 
 The function signature passed to `Engine::on_var` takes the following form:
 
-> `Fn(name: &str, index: Option<usize>, scope: &Scope, context: &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>> + 'static`
+> `Fn(name: &str, index: usize, scope: &Scope, context: &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>> + 'static`
 
 where:
 
 * `name: &str` - variable name.
 
-* `index: Option<usize>` - an offset from the bottom of the current [`Scope`] that the variable is supposed to reside.
+* `index: usize` - an offset from the bottom of the current [`Scope`] that the variable is supposed to reside.
   Offsets start from 1, with 1 meaning the last variable in the current [`Scope`].  Essentially the correct variable is at position `scope.len() - index`.
-  Notice that, if there was an [`eval`] statement before the current statement, new variables may have been introduced and this index may be incorrect.
-  Therefore, this index is for reference only.  It should not be relied upon.
 
-  If `index` is `None`, then there is no pre-calculated offset position and a search through the current [`Scope`] must be performed.
+  If `index` is zero, then there is no pre-calculated offset position and a search through the current [`Scope`] must be performed.
 
 * `scope : &Scope` - reference to the current [`Scope`] containing all variables up to the current evaluation position.
 
 * `context: &EvalContext` - reference to the current evaluation _context_, which exposes the following fields:
   * `context.engine(): &Engine` - reference to the current [`Engine`].
-  * `context.namespace(): &Module` - reference to the current _global namespace_ containing all script-defined functions.
+  * `context.namespace(): &Module` - reference to the current _global namespace_ (as a [module]) containing all script-defined functions.
   * `context.call_level(): usize` - the current nesting level of function calls.
 
 The return value is `Result<Option<Dynamic>, Box<EvalAltResult>>` where `Ok(None)` indicates that the normal
 variable resolution process should continue.
-

doc/src/patterns/dynamic-const.md

@@ -0,0 +1,56 @@
+Dynamic Constants Provider
+=========================
+
+{{#include ../links.md}}
+
+
+Usage Scenario
+--------------
+
+* A system has a _large_ number of constants, but only a minor set will be used by any script.
+
+* The system constants are expensive to load.
+
+* The system constants set is too massive to push into a custom [`Scope`].
+
+* The values of system constants are volatile and call-dependent.
+
+
+Key Concepts
+------------
+
+* Use a [variable resolver] to intercept variable access.
+
+* Only load a variable when it is being used.
+
+* Perform a lookup based on variable name, and provide the correct data value.
+
+* May even perform back-end network access or look up the latest value from a database.
+
+
+Implementation
+--------------
+
+```rust
+let mut engine = Engine::new();
+
+// Create shared data provider.
+// Assume that Provider::get(&str) -> Option<value> gets a system constant.
+let provider: Arc<Provider> = get_data_provider();
+
+// Clone the shared provider
+let db = provider.clone();
+
+// Register a variable resolver.
+// Move the shared provider into the closure.
+engine.on_var(move |name, _, _, _| Ok(db.get(name).map(Dynamic::from)));
+```
+
+
+Values are Constants
+--------------------
+
+All values provided by a [variable resolver] are _constants_ due to their dynamic nature.
+They cannot be assigned to.
+
+In order to change values in an external system, register a dedicated API for that purpose.

src/api.rs

@@ -1721,12 +1721,7 @@ impl Engine {
     #[inline(always)]
     pub fn on_var(
         &mut self,
-        callback: impl Fn(
+        callback: impl Fn(&str, usize, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
-                &str,
-                Option<usize>,
-                &Scope,
-                &EvalContext,
-            ) -> Result<Option<Dynamic>, Box<EvalAltResult>>
             + SendSync
             + 'static,
     ) -> &mut Self {

src/engine.rs

@@ -745,7 +745,7 @@ impl Engine {
                 this_ptr,
                 level: 0,
             };
-            if let Some(result) = resolve_var(name, index.map(|v| v.get()), scope, &context)
+            if let Some(result) = resolve_var(name, index.map_or(0, |v| v.get()), scope, &context)
                 .map_err(|err| err.fill_position(*pos))?
             {
                 return Ok((result.into(), name, ScopeEntryType::Constant, *pos));

src/fn_native.rs

@@ -224,13 +224,13 @@ pub type Callback<T, R> = Box<dyn Fn(&T) -> R + Send + Sync + 'static>;
 /// A standard callback function.
 #[cfg(not(feature = "sync"))]
 pub type OnVarCallback = Box<
-    dyn Fn(&str, Option<usize>, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
+    dyn Fn(&str, usize, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
         + 'static,
 >;
 /// A standard callback function.
 #[cfg(feature = "sync")]
 pub type OnVarCallback = Box<
-    dyn Fn(&str, Option<usize>, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
+    dyn Fn(&str, usize, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
         + Send
         + Sync
         + 'static,