Thanks Sean!
I've come up with this variation:
(def ascending compare)
(def descending #(compare %2 %1))
(defn compare-by [& key-cmp-pairs]
(fn [x y]
(loop [[k cmp & more] key-cmp-pairs]
{:pre [(keyword? k), (fn? cmp), (even? (count more))]}
(let [result (cmp (k x) (k y))]
(if (and (zero? result) more)
(recur more)
result)))))
I'm not really sure what I think of preconditions yet, but I'm enjoying
playing with the idea.
Alex
On Tue, Nov 30, 2010 at 10:07 AM, Sean Devlin <[email protected]>wrote:
> If you like assert args, you should check out pre & post conditions
> for your functions. They're built in to Clojure since 1.1
>
> The official docs:
>
> http://clojure.org/special_forms#Special%20Forms--%28fn%20name?%20%28[params*%20]%20condition-map?%20exprs*%29+%29
>
> Video Tutorial:
> http://vimeo.com/8399758
>
> On Nov 29, 10:35 pm, Alex Baranosky <[email protected]>
> wrote:
> > I had some fun with this and added assert-args:
> >
> > (defmacro assert-args [fnname pred msg & pred-msg-pairs]
> > `(when-not ~pred
> > (throw (IllegalArgumentException. ~(str fnname " requires " msg))))
> > (when (seq pred-msg-pairs)
> > (list* `assert-args fnname pred-msg-pairs)))
> >
> > (def ascending compare)
> > (def descending #(compare %2 %1))
> >
> > (defn compare-by [& key-cmp-pairs]
> > (assert-args compare-by
> > (even? (count key-cmp-pairs)) "even number of args (keyword,
> > comparator)"
> > (every? #(or (keyword? %) (fn? %)) key-cmp-pairs) "all args to be
> > keywords or functions")
> > (fn [x y]
> > (loop [[k cmp & more] key-cmp-pairs]
> > (let [result (cmp (k x) (k y))]
> > (if (and (zero? result) more)
> > (recur more)
> > result)))))
>
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