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

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.