Add dynamic dispatch.

doc/src/language/fn-ptr.md

@@ -7,8 +7,8 @@ In fact, internally a function pointer simply stores the _name_ of the function
 Call a function pointer using the `call` method, which needs to be called in method-call style.
 
 
-Built-in Functions
-------------------
+Built-in methods
+----------------
 
 The following standard methods (mostly defined in the [`BasicFnPackage`]({{rootUrl}}/rust/packages.md) but excluded if
 using a [raw `Engine`]) operate on [strings]:
@@ -54,3 +54,54 @@ let hello = Fn(fn_name + "_world");
 
 hello.call(0);              // error: function not found - "hello_world (i64)"
 ```
+
+
+Dynamic Dispatch
+----------------
+
+The purpose of function pointers is to enable rudimentary _dynamic dispatch_, meaning to determine,
+at runtime, which function to call among a group.
+
+Although it is possible to simulate dynamic dispatch via a number and a large `if-then-else-if` statement,
+using function pointers significantly simplifies the code.
+
+```rust
+let x = some_calculation();
+
+// These are the functions to call depending on the value of 'x'
+fn method1(x) { ... }
+fn method2(x) { ... }
+fn method3(x) { ... }
+
+// Traditional - using decision variable
+let func = sign(x);
+
+// Dispatch with if-statement
+if func == -1 {
+    method1(42);
+} else if func == 0 {
+    method2(42);
+} else if func == 1 {
+    method3(42);
+}
+
+// Using pure function pointer
+let func = if x < 0 {
+    Fn("method1")
+} else if x == 0 {
+    Fn("method2")
+} else if x > 0 {
+    Fn("method3")
+}
+
+// Dynamic dispatch
+func.call(42);
+
+// Using functions map
+let map = [ Fn("method1"), Fn("method2"), Fn("method3") ];
+
+let func = sign(x) + 1;
+
+// Dynamic dispatch
+map[func].call(42);
+```