In this case the for gives you the destructuring, plus the ability to do
filtering (:when) and mapping (yeilding the value of i) in one expression.
So you have one destructured binding that you can use for both purposes.
If you want to use filter, I think it is probably more idiomatic to follow
it up with a map.
(defn pos2 [needle coll]
(map first
(filter (fn [[_ val]] (= val needle))
(index coll))))
On Sunday, March 25, 2012 5:49:27 PM UTC-5, Dustin Getz wrote:
>
> first a quesiton about idiomatic code:
>
> ;; provided by book Fogus & Houser, Joy of Clojure
> (defn index [coll]
> (cond
> (map? coll) (seq coll)
> (set? coll) (map vector coll coll)
> :else (map vector (iterate inc 0) coll)))
>
> ;; book gives this as idiomatic code
> (defn pos [needle coll]
> (for [[i val] (index coll)
> :when (= needle val)] i))
>
> ;; usage
> (let [coll [:0 :1 :2 :3 :4 :5 :4]]
> (pos :4 coll)) ;-> (4 6)
>
> ;; how can i make this alternate version better, and why provide :when, given
> filter?
> ;; is it possible to get the destructuring elegance above without `for` ?
> ;; is this sequencing property of for (operate on individual values that will
> end up in a seq)
> ;; the reason `for` exists? is not not possible to do have nice syntax
> wtihout for (or seq-monad)?
> (defn pos2 [needle coll]
> (let [pairs (filter (fn [[i val]] (= val needle))
> (index coll))]
> (for [[i val] pairs] i))) ; can't destructure nicely without for, or
> seq-monad, i think
>
>
> meta question: where is the best place to ask noob questions like this,
> and also more long-form discussion questions? stackoverflow frowns on
> questions like this. i could slam in a couple of these a day, that doesn't
> seem appropriate here.
>
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