Expand callback example.

examples/callback.rs

@@ -2,33 +2,41 @@
 
 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`).
+fn main() -> Result<(), Box<EvalAltResult>> {
     let engine = Engine::new();
 
-    let ast = engine.compile(script)?;
+    // This script creates a closure which captures a variable and returns it.
+    let ast = engine.compile(
+        "
+            let x = 18;
+
+            // The following closure captures 'x'
+            return |a, b| {
+                x += 1;         // x is incremented each time
+                (x + a) * b
+            };
+        ",
+    )?;
 
     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)) };
+    // Create a closure by encapsulating the `Engine`, `AST` and `FnPtr`.
+    // In a real application, you'd be handling errors.
+    let func = move |x: i64, y: i64| -> i64 { closure.call(&engine, &ast, (x, y)).unwrap() };
 
     // Now we can call `func` anywhere just like a normal function!
-    let result = func(1, 2)?;
+    let r1 = func(1, 2);
 
-    println!("The Answer: {}", result); // prints 42
+    // Notice that each call to `func` returns a different value
+    // because the captured `x` is always changing!
+    let r2 = func(1, 2);
+    let r3 = func(1, 2);
+
+    println!("The Answers: {}, {}, {}", r1, r2, r3); // prints 40, 42, 44
 
     Ok(())
 }