fix
(defn get-if [m pred]
(let [[match & more] (filter (comp pred key) m)]
(if (and match (not more)) (val match))))
Apologies.
On Sunday, October 1, 2017 at 11:44:27 PM UTC+2, Leon Grapenthin wrote:
>
> 1. In general this style of iteration is not idiomatic unless you are
> hunting performance or write sth. that simply can't be composed from core
> sequence library (s. b.). If you have to write this style, look at
> `when-first`, `seq`, `next`. Study implementation of core library.
>
> More idiomatic implementation is
>
> (defn get-if [m pred]
> (let [[match & more] (filter (comp pred key) m)]
> (if-not more (val match))))
>
> 2. Let bindings to prevent repeated evaluation are fine.
> 3. First position is fine, but you should use loop/recur or an inner
> let-bound lambda instead of exposing found in the public API of the function
> 4. Again, look at core functions. Rich does naming very carefully and
> consistent. In (1) I called it pred, because its called pred in functions
> like filter or some.
> 5. See 1
>
>
> On Sunday, October 1, 2017 at 11:11:55 PM UTC+2, 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.
>> 3. In general, when using recursion and multiple arities to get a
>> result, is there an order that is preferred for the extra recursion
>> accumulation values? Here I have [m predicate?] and [found m
>> predicate?] versions of the function, but after looking at it for so
>> long I think it might be more natural to put found as the last argument,
>> as
>> in [m predicate? found], but I'm wondering if there's a standard to
>> follow with things like this.
>> 4. When passing functions as arguments, as I have here with the
>> predicate? function, is there a standard naming convention? I used a
>> question mark here, but would predicate be preferable, or even simply
>> f?
>> 5. Is writing this function even necessary? I didn't see a function
>> that serves the same purpose in the standard libraries, but I'm very new
>> and could easily have missed something!
>>
>> I know that's a lot to ask for such a short segment of code. Feel free to
>> answer as many/few of these questions as you'd like, as any help would be
>> greatly appreciated (though, if you're going to answer one, I think point
>> #2 is the most important) :)
>>
>>
>> Thanks in advance, fellow clojurians!
>>
>
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