On Sunday, October 1, 2017 at 5:11:55 PM UTC-4, Scott Barrett wrote:
>
> Clojure noob, here. I'm very excited to be learning about this language
> and becoming a part of this community :) I'm writing a function that works
> well, but seems just a bit wrong to me, stylistically speaking. I was
> hoping I could get some guidance from you all.
>
> Here's the code:
>
> (defn get-if
> "Gets the value of a map if exactly one key matches a predicate,
> otherwise nil"
> ([m predicate?] (get-if nil m predicate?))
> ([found m predicate?]
> (if-let [e (first m)]
> (let [pred (predicate? (key e))]
> (if (not (and pred found))
> (recur (if pred (val e) found) (rest m) predicate?))
> found))))
>
> This has gone through a few revisions to get it as concise as possible,
> but here are my questions/remarks:
>
> 1. Is it idiomatic to use if-let to move through a collection the way
> I have? In my experience with lispy languages, recursion over sequences
> tend to take the form (if (null item) accumlated-value
> (recur-over-rest)). This if-let form turns that on its head, which
> looks a little backwards at first to me, but it saves a level of
> indentation which is generally preferable in my experience.
> 2. The main part of this code that's bugging me is the let form, which
> is a total hack to keep from testing (predicate? (key e)) twice. Even
> still, I have to test the truthiness of pred twice; once in the (not
> (and ...)) form and once again in the if of the recur form. I feel
> like a clever use of (and ...) or (or ...) would save me here, but I
> haven't come upon a solution using those forms yet.
>
> I like to use multiple recur forms anyway to make clearer the different
cases, and I like to use COND to flatten nested IFs:
(defn get-if
"Gets the value of a map if exactly one key matches a predicate, otherwise
nil"
([m predicate?]
(loop [[e & rest] m
found nil]
(cond
(nil? e) found
(predicate? (key e)) (when-not found
(recur rest (val e)))
:default (recur rest found)))))
Now you can test end-of-data up front, and by using LOOP for recursion we
avoid a lot of noise, including repeatedly passing the predicate.
hth, hk
>
>
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