let me expand on my comment. What I meant is, the type of functionality you
are adding into distinct, makes it feel less "distinct". to me. If I
read what you are saying
correctly, you want to be also be able to pass a set, so in
addition to filter duplicate elements, it also will filter out
anything in the set.
to me that doesn't feel clojureish. I've always felt that clojure code is about
a lot of small functions that does one thing, but does the one thing really
well, so you can compose the functions together to do what you want.
adding what "feels" like an unrelated capability to a core function seems
"odd" to me. Just my 2 cents.
On Sun, Feb 21, 2010 at 7:25 AM, Eugen Dueck <eu...@dueck.org> wrote:
> Hi,
>
> Clojure is great! The gain in productivity from more low level
> languages like Java, but also more functional languages like Ruby and
> Common LISP etc. amazes me every day. Like how adding a simple "map"
> in front of the count here:
> (count colls)
> changes the code from counting the number of collections to
> enumerating the number of items in those collections. It's all these
> little things that take one minute or 5 in Java, but only 5 seconds in
> Clojure (in case you are a slow typer) that make you so much more
> efficient. With clojure, I spend most of my time thinking about
> problems at a conceptual level, rather than at the implementation
> level (Java: int[] vs. Iterable<Integer> vs. List<Integer> vs.
> Integer[] etc. - arrg!). Thanks Rich!
>
> That said, every now and then I come across a function in clojure.core
> that could be vastly improved in its range of applicability by just
> adding one or more optional parameters, defaulting to the currently
> hard-coded values (like the often implicit 'identity' function). Take
> 'distinct'. I'd like to be able to specify the keyfn, that's an
> important one for me, and while we're at it, I'd like to pass in the
> set that that function builds up incrementally, and normally starts
> with an empty set, so that I can pre-initialize it, say with #{ nil },
> so that it also filters out nils.
>
> This 2nd point is not that important, and I'm not sure if it is that
> great an idea in terms of "orthogonality" of the functions, as we have
> filter. But you'd get rid off one level of indirection. distinct
> basically gives us a filter for free.
>
> But the 1st point I think certainly makes sense, and we have a couple
> of other fns in clojure that have a variant with a keyfn parameter,
> like sort-by etc. I guess they normally get a different name.
>
> I'm not so sure about what the best order of parameters is in clojure,
> and named parameters would make this a no brainer, but this is what I
> currently use, the first parameter being coll, at least in the variant
> with only one parameter that makes it a drop-in replacement candidate
> for distinct:
>
> (defn distinkt
> "Returns a lazy sequence of the elements of coll with duplicates
> removed"
> ([coll] (distinkt coll identity))
> ([coll keyfn] (distinkt coll keyfn #{}))
> ([coll keyfn seen-items]
> (let [step (fn step [xs seen]
> (lazy-seq
> ((fn [[f :as xs] seen]
> (when-let [s (seq xs)]
> (let [key (keyfn f)]
> (if (contains? seen key)
> (recur (rest s) seen)
> (cons f (step (rest s) (conj seen key)))))))
> xs seen)))]
> (step coll seen-items))))
>
> I don't mind writing - or better - copy-and-pasting this code and
> keeping it in my project, but I think it could be useful for others,
> so I wouldn't mind at all if this makes it into clojure.core... :)
>
> Or is the reason for hard coding an (implicit) 'identity' performance?
> Or did I miss some other way to achieve the same goal?
>
> Eugen
>
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