If you have assertions enabled, you can
declare
(defn foo [a b & {:keys [c d]}]
{:pre [c d]}
1)
to make sure that c and d are not nil
(defn foo [a b & {:keys [c d] :as m}]
{:pre [(every? m [:c :d]]}
1)
to make sure that c and d are contained and
(defn foo [a b & {:keys [c d] :as m}]
{:pre [(zero? (count (dissoc m :c :d)))]}
1)
to make sure that m contains only :c and :d
It will only complain at run-time but at least it will complain.
On Sunday, August 23, 2015 at 1:34:11 AM UTC+2, Dave Tenny wrote:
>
> I sure wish Clojure would generate "IllegalArgumentException" in the
> following sample case:
>
> (defn foo [a b & {:keys [c d]}] 1)
> (foo 1 2 :e 5) ; blithely ignores :e 5
>
> I understand that Clojure's destructuring things are very nice, and more
> powerful than Common Lisp's, and I like
> that when I need it.
>
> However I can't tell you how many times I've been bitten by this. Some
> simple typo or other other parameter name error on keyword arguments
> with untended and way-too-long-to-debug consequences.
>
> In my mind, on this subject, Common Lisp lambda lists got this right and
> Clojure gets a poor grade.
> Something about being doomed to repeat history. In Common Lisp, if you
> really wanted to allow other (arbitrary) keywords you'd
> just use &allow-other-keys.
>
> Maybe clojure should only allow the above case to go un-complained-about
> if :as was specified for the map.
>
> If there's some automatic enforcement I'm missing that comes with 'defn'
> please let me know, I'm still learning the language.
>
> I've thought more that once about making a common lisp DEFUN statement
> that maps to DEFN but implements
> lambda list semantics (including 'supplied-p' parameters). I've just been
> too lazy to do it.
>
> It would also likely perform poorly after injecting the additional
> checks/rearrangements into the function on top of what Clojure has already
> done,
> so I suppose it would have to be taken a step further so that it didn't
> generate DEFN expressions at all but was implemented at DEFN's level as
> a seperately named form.
>
> Tips welcome. Just thinking aloud.
>
> - Dave
>
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