Re: immutable defs?

[Stephen C. Gilardi]

On Oct 7, 2009, at 12:47 AM, Mark Tomko wrote:

This is pretty much what I'd had in mind.

Thanks for the comments and suggestions all.

I don't see how the text of
the Exception is set by the macro, but it'd be really spectacular if
the message were more clear.  Is that message coming from the defvar
form?

The text of the exception was the default one when a validator (http://clojure.org/api#toc518 ) returns logical false. It's customizable by throwing a RuntimeException rather than returning logical false.

Here's a new version which addresses this and the other feedback so far:

(defn const-validator
  "Returns a validator that ensures the value of a const never changes"
  [const]
  (fn [x]
    (or (= x @const)
        (throw
         (UnsupportedOperationException.
          (str "the value of const " const " cannot be modified"))))))

(defmacro defconst

  ([name value]
     `(do
        (defvar ~name ~value)
        (set-validator! (var ~name) (const-validator (var ~name)))
        (var ~name)))
  ([name value doc]

With examples:

user=> (defconst e (Math/exp 1))
#'user/e
user=> e
2.7182818284590455
user=> (def e 4)

user=> (binding [e 4] (prn e))

user=> (binding [e e] (prn e))
2.7182818284590455
nil
user=> (binding [e e] (set! e 4))

user=> (defconst my-false false)
#'user/my-false
user=> my-false
false
user=> (def my-false 3)

user=> (defconst my-nil nil)
#'user/my-nil
user=> my-nil
nil
user=> (defn my-nil [x] (+ 3 x))

user=>

--Steve

"Defines a const: a var whose value is constant. Any attempt to redefine, bind, or set! the const to a different value will throw an exception." `(defconst ~(with-meta name (assoc (meta name) :doc doc)) ~value))) java.lang.UnsupportedOperationException: the value of const #'user/e cannot be modified (NO_SOURCE_FILE:4) java.lang.UnsupportedOperationException: the value of const #'user/e cannot be modified (NO_SOURCE_FILE:0) java.lang.UnsupportedOperationException: the value of const #'user/e cannot be modified (NO_SOURCE_FILE:0) java.lang.UnsupportedOperationException: the value of const #'user/my- false cannot be modified (NO_SOURCE_FILE:10) java.lang.UnsupportedOperationException: the value of const #'user/my- nil cannot be modified (NO_SOURCE_FILE:13)

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