Add callback example.

examples/callback.rs

@@ -0,0 +1,34 @@
+//! This example stores a Rhai closure for later use as a callback.
+
+use rhai::{Engine, EvalAltResult, FnPtr};
+
+fn main() -> Result<(), Box<EvalAltResult>> {
+    // This script creates a closure which may capture variables.
+    let script = "
+        let x = 20;
+
+        // The following closure captures 'x'
+        return |a, b| (x + a) * b;
+    ";
+
+    // To call a Rhai closure at a later time, you'd need three things:
+    // 1) an `Engine` (with all needed functions registered),
+    // 2) a compiled `AST`,
+    // 3) the closure (of type `FnPtr`).
+    let engine = Engine::new();
+
+    let ast = engine.compile(script)?;
+
+    let closure = engine.eval_ast::<FnPtr>(&ast)?;
+
+    // Create a closure that we can call any time, encapsulating the
+    // `Engine`, `AST` and `FnPtr`.
+    let func = move |x: i64, y: i64| -> Result<i64, _> { closure.call(&engine, &ast, (x, y)) };
+
+    // Now we can call `func` anywhere just like a normal function!
+    let result = func(1, 2)?;
+
+    println!("The Answer: {}", result); // prints 42
+
+    Ok(())
+}