Add patterns section.

doc/src/patterns/config.md

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+Loadable Configuration
+======================
+
+{{#include ../links.md}}
+
+
+Usage Scenario
+--------------
+
+* A system where settings and configurations are complex and logic-driven.
+
+* Where it is not possible to configure said system via standard configuration file formats such as `TOML` or `YAML`.
+
+* The system configuration is complex enough that it requires a full programming language. Essentially _configuration by code_.
+
+* Yet the configurations must be flexible, late-bound and dynamically loadable, just like a configuration file.
+
+
+Key Concepts
+------------
+
+* Leverage the loadable [modules] of Rhai.  The [`no_module`] feature must not be on.
+
+* Expose the configuration API.  Use separate scripts to configure that API.  Dynamically load scripts via the `import` statement.
+
+* 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`].
+
+
+Implementation
+--------------
+
+### Configuration Type
+
+```rust
+#[derive(Debug, Clone, Default)]
+struct Config {
+    pub id: String;
+    pub some_field: i64;
+    pub some_list: Vec<String>;
+    pub some_map: HashMap<String, bool>;
+}
+```
+
+### Make Shared Object
+
+```rust
+let config: Rc<RefCell<Config>> = Rc::new(RefCell::(Default::default()));
+```
+
+### Register Config API
+
+```rust
+// Notice 'move' is used to move the shared configuration object into the closure.
+let cfg = config.clone();
+engine.register_fn("config_set_id", move |id: String| *cfg.borrow_mut().id = id);
+
+let cfg = config.clone();
+engine.register_fn("config_get_id", move || cfg.borrow().id.clone());
+
+let cfg = config.clone();
+engine.register_fn("config_set", move |value: i64| *cfg.borrow_mut().some_field = value);
+
+// Remember Rhai functions can be overloaded when designing the API.
+
+let cfg = config.clone();
+engine.register_fn("config_add", move |value: String|
+        cfg.borrow_mut().some_list.push(value)
+);
+
+let cfg = config.clone();
+engine.register_fn("config_add", move |values: &mut Array|
+        cfg.borrow_mut().some_list.extend(values.into_iter().map(|v| v.to_string()))
+);
+
+let cfg = config.clone();
+engine.register_fn("config_add", move |key: String, value: bool|
+        cfg.borrow_mut().som_map.insert(key, value)
+);
+```
+
+### Configuration Script
+
+```rust
+------------------
+| my_config.rhai |
+------------------
+
+config_set_id("hello");
+
+config_add("foo");          // add to list
+config_add("bar", true);    // add to map
+config_add("baz", false);   // add to map
+```
+
+### Load the Configuration
+
+```rust
+import "my_config";         // run configuration script without creating a module
+
+let id = config_get_id();
+
+id == "hello";
+```

doc/src/patterns/control.md

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+Scriptable Control Layer
+========================
+
+{{#include ../links.md}}
+
+
+Usage Scenario
+--------------
+
+* A system provides core functionalities, but no driving logic.
+
+* The driving logic must be dynamic and hot-loadable.
+
+* A script is used to drive the system and provide control intelligence.
+
+
+Key Concepts
+------------
+
+* Expose a Control API.
+
+* Since Rhai is _sand-boxed_, it cannot mutate the environment.  To perform external actions via an API, the actual system must be wrapped in a `RefCell` (or `RwLock`/`Mutex` for [`sync`]) and shared to the [`Engine`].
+
+
+Implementation
+--------------
+
+### Functional API
+
+Assume that a system provides the following functional API:
+
+```rust
+struct EnergizerBunny;
+
+impl EnergizerBunny {
+    pub fn new () -> Self { ... }
+    pub fn go (&mut self) { ... }
+    pub fn stop (&mut self) { ... }
+    pub fn is_going (&self) { ... }
+    pub fn get_speed (&self) -> i64 { ... }
+    pub fn set_speed (&mut self, speed: i64) { ... }
+}
+```
+
+### Wrap API in Shared Object
+
+```rust
+let bunny: Rc<RefCell<EnergizerBunny>> = Rc::new(RefCell::(EnergizerBunny::new()));
+```
+
+### Register Control API
+
+```rust
+// Notice 'move' is used to move the shared API object into the closure.
+let b = bunny.clone();
+engine.register_fn("bunny_power", move |on: bool| {
+        if on {
+            if b.borrow().is_going() {
+                println!("Still going...");
+            } else {
+                b.borrow_mut().go();
+            }
+        } else {
+            if b.borrow().is_going() {
+                b.borrow_mut().stop();
+            } else {
+                println!("Already out of battery!");
+            }
+        }
+});
+
+let b = bunny.clone();
+engine.register_fn("bunny_is_going", move || b.borrow().is_going());
+
+let b = bunny.clone();
+engine.register_fn("bunny_get_speed", move ||
+        if b.borrow().is_going() { b.borrow().get_speed() } else { 0 }
+);
+
+let b = bunny.clone();
+engine.register_result_fn("bunny_set_speed", move |speed: i64|
+        if speed <= 0 {
+            return Err("Speed must be positive!".into());
+        } else if speed > 100 {
+            return Err("Bunny will be going too fast!".into());
+        }
+
+        if b.borrow().is_going() {
+            b.borrow_mut().set_speed(speed)
+        } else {
+            return Err("Bunny is not yet going!".into());
+        }
+
+        Ok(().into())
+);
+```
+
+### Use the API
+
+```rust
+if !bunny_is_going() { bunny_power(true); }
+
+if bunny_get_speed() > 50 { bunny_set_speed(50); }
+```
+
+
+Caveat
+------
+
+Although this usage pattern appears a perfect fit for _game_ logic, avoid writing the
+_entire game_ in Rhai.  Performance will not be acceptable.
+
+Implement as much functionalities of the game engine in Rust as possible.
+Rhai integrates well with Rust so this is usually not a hinderance.
+
+Lift as much out of Rhai as possible.
+Use Rhai only for the logic that _must_ be dynamic or hot-loadable.

doc/src/patterns/index.md

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+Advanced Patterns
+=================
+
+{{#include ../links.md}}
+
+
+Use Rhai in different scenarios other than simply evaluating a user script.
+
+These patterns are useful when Rhai needs to affect/control the external environment.

doc/src/patterns/oop.md

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+Object-Oriented Programming (OOP)
+================================
+
+{{#include ../links.md}}
+
+Rhai does not have _objects_ per se, but it is possible to _simulate_ object-oriented programming.
+
+
+Use [Object Maps] to Simulate OOP
+--------------------------------
+
+Rhai's [object maps] has [special support for OOP]({{rootUrl}}/language/object-maps-oop.md).
+
+| Rhai concept                                          | Maps to OOP |
+| ----------------------------------------------------- | :---------: |
+| [Object maps]                                         |   objects   |
+| [Object map] properties holding values                | properties  |
+| [Object map] properties that hold [function pointers] |   methods   |
+
+When a property of an [object map] is called like a method function, and if it happens to hold
+a valid [function pointer] (perhaps defined via an [anonymous function]), then the call will be
+dispatched to the actual function with `this` binding to the [object map] itself.
+
+
+Use Anonymous Functions to Define Methods
+----------------------------------------
+
+[Anonymous functions] defined as values for [object map] properties take on a syntactic shape
+that resembles very closely that of class methods in an OOP language.
+
+Anonymous functions can also _capture_ variables from the defining environment, which is a very
+common OOP pattern.  Capturing is accomplished via a feature called _[automatic currying]_ and
+can be turned off via the [`no_closure`] feature.
+
+
+Examples
+--------
+
+```rust
+let factor = 1;
+
+// Define the object
+let obj =
+     #{
+          data: 0,
+          increment: |x| this.data += x,          // 'this' binds to 'obj'
+          update: |x| this.data = x * factor,     // 'this' binds to 'obj', 'factor' is captured
+          action: || print(this.data)             // 'this' binds to 'obj'
+     };
+
+// Use the object
+obj.increment(1);
+obj.action();                                     // prints 1
+
+obj.update(42);
+obj.action();                                     // prints 42
+
+factor = 2;
+
+obj.update(42);
+obj.action();                                     // prints 84
+```

doc/src/patterns/singleton.md

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+Singleton Command Objects
+========================
+
+{{#include ../links.md}}
+
+
+Usage Scenario
+--------------
+
+* A system provides core functionalities, but no driving logic.
+
+* The driving logic must be dynamic and hot-loadable.
+
+* A script is used to drive the system and provide control intelligence.
+
+* The API is multiplexed, meaning that it can act on multiple system-provided entities, or
+
+* The API lends itself readily to an object-oriented (OO) representation.
+
+
+Key Concepts
+------------
+
+* Expose a Command type with an API.
+
+* Since Rhai is _sand-boxed_, it cannot mutate the environment.  To perform external actions via an API, the command object type must be wrapped in a `RefCell` (or `RwLock`/`Mutex` for [`sync`]) and shared to the [`Engine`].
+
+* Load each command object into a custom [`Scope`] as constant variables.
+
+* Control each command object in script via the constants.
+
+
+Implementation
+--------------
+
+### Functional API
+
+Assume the following command object type:
+
+```rust
+struct EnergizerBunny { ... }
+
+impl EnergizerBunny {
+    pub fn new () -> Self { ... }
+    pub fn go (&mut self) { ... }
+    pub fn stop (&mut self) { ... }
+    pub fn is_going (&self) { ... }
+    pub fn get_speed (&self) -> i64 { ... }
+    pub fn set_speed (&mut self, speed: i64) { ... }
+    pub fn turn (&mut self, left_turn: bool) { ... }
+}
+```
+
+### Wrap Command Object Type as Shared
+
+```rust
+let SharedBunnyType = Rc<RefCell<EnergizerBunny>>;
+```
+
+### Register the Custom Type
+
+```rust
+engine.register_type_with_name::<SharedBunnyType>("EnergizerBunny");
+```
+
+### Register Methods and Getters/Setters
+
+```rust
+engine
+    .register_get_set("power",
+        |bunny: &mut SharedBunnyType| bunny.borrow().is_going(),
+        |bunny: &mut SharedBunnyType, on: bool| {
+            if on {
+                if bunny.borrow().is_going() {
+                    println!("Still going...");
+                } else {
+                    bunny.borrow_mut().go();
+                }
+            } else {
+                if bunny.borrow().is_going() {
+                    bunny.borrow_mut().stop();
+                } else {
+                    println!("Already out of battery!");
+                }
+            }
+        }
+    ).register_get("speed", |bunny: &mut SharedBunnyType| {
+        if bunny.borrow().is_going() {
+            bunny.borrow().get_speed()
+        } else {
+            0
+        }
+    }).register_set_result("speed", |bunny: &mut SharedBunnyType, speed: i64| {
+        if speed <= 0 {
+            Err("Speed must be positive!".into())
+        } else if speed > 100 {
+            Err("Bunny will be going too fast!".into())
+        } else if !bunny.borrow().is_going() {
+            Err("Bunny is not yet going!".into())
+        } else {
+            b.borrow_mut().set_speed(speed);
+            Ok(().into())
+        }
+    }).register_fn("turn_left", |bunny: &mut SharedBunnyType| {
+        if bunny.borrow().is_going() {
+            bunny.borrow_mut().turn(true);
+        }
+    }).register_fn("turn_right", |bunny: &mut SharedBunnyType| {
+        if bunny.borrow().is_going() {
+            bunny.borrow_mut().turn(false);
+        }
+    });
+```
+
+### Push Constant Command Object into Custom Scope
+
+```rust
+let bunny: SharedBunnyType = Rc::new(RefCell::(EnergizerBunny::new()));
+
+let mut scope = Scope::new();
+scope.push_constant("BUNNY", bunny.clone());
+
+engine.consume_with_scope(&mut scope, script)?;
+```
+
+### Use the Command API in Script
+
+```rust
+// Access the command object via constant variable 'BUNNY'.
+
+if !BUNNY.power { BUNNY.power = true; }
+
+if BUNNY.speed > 50 { BUNNY.speed = 50; }
+
+BUNNY.turn_left();
+```