2009/10/7 Stephen C. Gilardi <squee...@mac.com>
> On Oct 2, 2009, at 10:29 AM, Mark wrote:
>
> Is there a way to make a declaration in Clojure that cannot be rebound
>> later? Ideally, I'd like something that fails if I try to do this:
>>
>> (def myname "mark")
>> ; ...more code, elided...
>> (def myname "Mark")
>>
>
> Along these lines, I was thinking of adding defconst to
> clojure.contrib.def. It's like Common Lisp's defconst, but the fact that its
> value isn't allowed to change is enforced:
>
> (defmacro defconst
> "Defines a var with a constant value and optional doc string. Any attempt
> to redefine, bind, or set! the const to a different value will throw an
> exception."
> ([name init]
> `(do (set-validator! (defvar ~name ~init) #{~init}) (var ~name)))
> ([name init doc]
> `(defconst ~(with-meta name (assoc (meta name) :doc doc)) ~init)))
>
> It does allow a redefine, binding, or set! to an equal value mainly as an
> artifact of its implementation, but also because such a "change" would be
> harmless to the semantics of it being constant.
>
> user=> (defconst pi (* 4 (Math/atan 1)))
> #'user/pi
> user=> pi
> 3.141592653589793
> user=> (binding [pi 3] (prn pi))
> java.lang.IllegalStateException: Invalid reference state (NO_SOURCE_FILE:0)
> user=> (binding [pi pi] (prn pi))
> 3.141592653589793
> nil
> user=> (binding [pi pi] (set! pi 3))
> java.lang.IllegalStateException: Invalid reference state (NO_SOURCE_FILE:0)
> user=> (defconst pi 3)
> java.lang.IllegalStateException: Invalid reference state (NO_SOURCE_FILE:7)
> user=> (def pi 3)
> java.lang.IllegalStateException: Invalid reference state
> (NO_SOURCE_FILE:10)
> user=> (defconst pi (* 4 (Math/atan 1)))
> #'user/pi
>
> I'd appreciate hearing any suggestions for improvement or other feedback.
>
Hello,
~init twice in the 2 args arity version of the macro
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