mutable read-only example

Author: AlexKnauth

typed-racket-doc/typed-racket/scribblings/reference/types.scrbl

@@ -361,7 +361,12 @@ is a subtype of @racket[(Read-Only-Boxof b)] if @racket[a] is a subtype of @rack
 
 @ex[(box-immutable "hello world")
     (code:comment "though this does not necessarily imply immutability:")
-    (if (zero? (random 2)) (box "hello") (box 'hello))]
+    (define b : (Box Integer) (box 3))
+    (define b-read-only : (Read-Only-Boxof Integer) b)
+    (set-box! b-read-only 5)
+    (unbox b-read-only)
+    (set-box! b 5)
+    (unbox b-read-only)]
 
 @defidform[BoxTop]{is the type of a @rtech{box} with an unknown element
   type and is the supertype of all box types. Only read-only box operations