Re: Possible to merge destructuring :or defaults with :as?
Hi Stephen,
Thanks for the reply!
Such behavior would be quite surprising; to see why, desugar :keys:
I may well be missing something, but I'm not sure I see your problem here.
(defn foo [ {x :x y :y :as args :or {x x-default y y-default}}]
args)
This is checking input args for an :x keyword. If found, it's binding the
associated value to 'x. Else it's binding the default value to 'x.
Either way, it's also binding all the args as originally given to 'args.
So with (foo :y boo!) we have at this point:
* All args as originally given, bound to 'args. So '(:y boo!)
* A map of symbols to default values. So {x x-default y y-default}
* Bindings as a result of the destructuring. So [x x-default y boo!]
What I'm suggesting might be useful, is destructuring sugar to bind one
additional map: {:x x-default :y boo!}.
The clojure.core/destructure code is pretty hairy, so I may be missing some
implementation-specific reason why this would be difficult to do. But I
don't see anything logically inconsistent or surprising about wanting to do
it. It'd be just another binding, and so not outside the scope of what
destructure is tasked with doing.
Indeed, I find real-world situations coming up quite often where this would
be a useful convenience.
When you want this,
(let [{blah} (merge my-defaults kwargs)]
Unfortunately (again, unless I'm missing something) this fails on two
counts. One, it loses the transparency of the defaults to callers (the
original motivation I described). Two, it loses the convenience that :or
gives of actually having the symbols automatically bound as well.
I.e. to really match the functionality I'm talking about, you'd need to get
more verbose:
(defn foo [ {:keys [x y] :as args}]
(let [merged-args (merge {:x x-default} args)
{:keys [x y]} merged-args]
merged-args))
And, indeed, the repetition in this form makes it clear (I think) why this
would ideally be the domain of the destructuring itself.
Does that make sense?
--
*Peter Taoussanis*
ptaoussa...@gmail.com
twitter.com/ptaoussanis
+66 (0)82 597 2595 (Thailand)
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Re: Possible to merge destructuring :or defaults with :as?
In case you are primarily interested in clojure functions with keyword arguments (or optional arguments), you might check if clojure.options (https://github.com/guv/clojure.options/) suits you. -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
Re: Possible to merge destructuring :or defaults with :as?
On Fri, 2012-08-31 at 03:40 -0700, Peter Taoussanis wrote:
(foo) = {:x x-default :y y-default}
I.e. to merge the :or defaults over the :args binding?
Such behavior would be quite surprising; to see why, desugar :keys:
{x (expr-that-computes :x),
y (expr-that-computes :y),
z (expr-that-computes :z),
:or {x x-default y y-default} :merge-as args}
which treats bindings as having bidirectional flow; we bind a name to
the value of a keyword, rewriting in the other direction if the value
isn't present.
Instead,
Rationale: one of the nice things of setting defaults via :or is that
they're visible to callers via :arglists. This'd help make the process
more convenient in the (relatively common) case where you'd like to
preserve arbitrary args, but apply some defaults in a transparent way.
When you want this,
(let [{blah} (merge my-defaults kwargs)]
--
Stephen Compall
^aCollection allSatisfy: [:each | aCondition]: less is better than
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Possible to merge destructuring :or defaults with :as?
Hey all,
As currently implemented:
(defn foo
[ {:keys [x y z] :or {x x-default y y-default} *:as* args}]
args)
(foo) = *nil*
Is there a way anyone can think of (macro, etc.) that'd allow for something
like this:
(defn foo
[ {:keys [x y z] :or {x x-default y y-default} *:merge-as* args}]
args)
(foo) =* {:x x-default :y y-default**}*
I.e. to merge the :or defaults over the :args binding?
Rationale: one of the nice things of setting defaults via :or is that
they're visible to callers via :arglists. This'd help make the process more
convenient in the (relatively common) case where you'd like to preserve
arbitrary args, but apply some defaults in a transparent way.
Thanks!
- Peter Taoussanis
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