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.
>
Harmless to the semantics of it being constant, yes, but not the semantics
of it having to be defined just once ( and not having to be "tried" to be
rebound ).
e.g. if the new binding value is dynamic, the program may or may not work
depending on the value ...
I prefer Meikel's approach : the root binding is set once and for all, and
that's the contrat.
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