From 159b7e4e1e845304bd8eb4398677010b86c7f249 Mon Sep 17 00:00:00 2001
From: Eli Barzilay <eli@barzilay.org>
Date: Tue, 16 Jul 2013 02:08:28 -0400
Subject: [PATCH] Add boxes example.

---
 racket.html.markdown | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/racket.html.markdown b/racket.html.markdown
index d43ac5e2..fbd3f76a 100644
--- a/racket.html.markdown
+++ b/racket.html.markdown
@@ -354,11 +354,17 @@ m ; => '#hash((b . 2) (a . 1) (c . 3))  <-- no `d'
 ;; 6. Mutation
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-;; Use set! to assign a new value to an existing variable
+;; Use `set!' to assign a new value to an existing variable
 (define n 5)
-(set! n 6)
+(set! n (add1 n))
 n ; => 6
 
+;; Use boxes for explicitly mutable values (similar to pointers or
+;; references in other languages)
+(define n* (box 5))
+(set-box! n* (add1 (unbox n*)))
+(unbox n*) ; => 6
+
 ;; Many Racket datatypes can be immutable or mutable
 ;; (Pairs, Lists, Strings, Vectors, Hash Tables, etc...)