I was doing some experimenting with deftypes this evening. I
understand that you need to specify clojure.lang.IPersistentMap as an
interface in order to make your type behave like a general-purpose map
(supporting assoc, dissoc, etc.).
After playing around, I think it would be ideal that if you *don't*
specify clojure.lang.IPersistentMap as an interface, you still get an
implementation of assoc that works only with keys already in the type.
In other words, you can't add a new field, and you can't call dissoc
to remove it, but you still have the ability to say, "Give me another
object just like this one, but with such-and-such value changed".
In other words, I'm suggesting that if you do something like:
(deftype Posn [x y])
(def p (Posn 1 2))
You should be able to do (assoc p :x 3), you should get back #:Posn{:x
1, :y 2}, however
(assoc p :z 3) will not work, and (dissoc p :x) would not work.
I'm mainly suggesting this because deftypes seem a bit of a pain to
work with without some way to create an updated object, whereas for
many purposes, the full-blown map capabilities can make your code more
fragile (for example, if you really mean a Posn to contain an :x and
:y, no more, no less).
Thoughts?
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