It's already possible, but metadata is the wrong tool. Wrap all return
values that "might" have multiple values in a vector, and return/bind
as many elements as you want. Add macros to make it less ugly, stir
awhile, and you have multiple values.
Using metadata for this purpose would have real problems: the metadata
for multiple-values would bubble up unintentionally from some
functions, and not from others, giving the mistaken impression that
the higher-level functions are returning multiple values. Consider:
(defn values [v & vs]
(vary-meta v assoc ::values vs))
(defmacro mv-bind [names val & body]
`(let [~names (cons ~val (-> ~val meta ::values))]
~@body))
(mv-bind [x y] (values {:age 20} "steve")
(if-not y
x
(assoc x :name y)))
This all looks good (aside from the fact that it won't work for types
that can't have metadata, such as integers), right? But from what I
understand of Common Lisp (and that's not a lot), this is a broken
version of its multiple values. Specifically, a function that's not
concerned at all with multiple values, such as conj, will accidentally
return multiple values if its args were carrying around multiple-value
metadata:
(mv-bind [x y] (conj (values [1 2] "whatever") 3 4)
[x y]) ; returns [[1 2 3 4] "whatever"]!
In contrast, here's a snippet of me trying something similar at an
SBCL repl:
* (multiple-value-bind (x y) (values 1 2) (list x y))
(1 2)
* (multiple-value-bind (x y) (identity (values 1 2)) (list x y))
(1 NIL)
On Mar 25, 9:19 am, Joop Kiefte <iko...@gmail.com> wrote:
> Would it be possible to create something like CL's multiple values this way?
>
> 2011/3/25 James Reeves <jree...@weavejester.com>:
>
>
>
>
>
>
>
> > On 25 March 2011 10:41, msappler <msapp...@web.de> wrote:
> >> Never used metadata until now.
>
> >> I had an insight yesterday that I want to share with you:
>
> >>http://resatori.com/metadata-insight
>
> > In my view, it depends whether your :id key is a surrogate key or a natural
> > key.
>
> > Object metadata should not affect object equality, so if your :id key
> > is just an internal database identifier, you could include it as
> > metadata.
>
> > But if your :id key is exposed to the outside world, then it should
> > not be metadata, because changing the :id would change the external
> > representation of the object, and therefore affect equality.
>
> > I should also point out that if you want information hiding, then
> > object composition might be a better solution. For example:
>
> > (defprotocol Identifiable
> > (id [self] "Return an object's unique id"))
>
> > (deftype DBRef [db-id data]
> > Identifiable
> > (id [_] db-id)
> > clojure.lang.IDeref
> > (deref [_] data))
>
> > (defn db-ref [id data]
> > (DBRef. id data))
>
> > Then we can get information about the reference, and deref it to get
> > its contents:
>
> > (def user
> > (db-ref 193 {:login "fred"}))
>
> > => (id user)
> > 193
> > => @user
> > {:login fred}
>
> > - James
>
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