On Sat, 13 Nov 2010 06:31:03 -0800, garf wrote:
> If I have a struct whose creation will require some function calls that
> references some of its members, I am unsure how to go about it in a
> clean way. For example:
> (defstruct tz :a :b :c)
>
> (def tz1 1 2 (+ (:a tz1) (:b tz1))
>
Could you give a more realistic example of how you plan to use this?
If the only restriction is that you only know the values for :a and :b at
the time you want to create the structure, then this would work:
(defn create-tz [a-val b-val]
(struct-map tz :a a-val :b b-val :c (some-fn a-val b-val)))
> will not work, but reflects the problem. Ideally at the end of this I
> could do (:c tz1) and get 3 back. Any suggestions? Originally I had
> used assocs, but that runs into the immutability problem, for example
>
> (defstruct tz :a :b)
> (def tz1 1 2)
> (assoc tz1 :c (+ (:a tz1) (:b tz1))
>
> does not actually update tz1
>
Correct. Values are immutable, and in Clojure, even collections are
values!
If you want to have a variable hold a reference to some changing values
then what you want is to use an atom or a ref. Atom is more easy to use,
and unless you have several mutable values that need concurrent updating,
is sufficient, so here's an example:
> (def tz1 (atom (struct-map tz :a 1 :b 2)))
#'user/tz1
> @tz1
{:a 1, :b 2, :c nil}
> (swap! tz1 #(assoc % :c (+ (:a %) (:b %))))
{:a 1, :b 2, :c 3}
> @tz1
{:a 1, :b 2, :c 3}
Hope that helps,
--
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