If we think about what we're doing here is a stateful filter, then maybe we could leverage a few more core Clojure functions:
(defn distinct-by [f coll] (let [seen (atom #{})] (filter (fn [itm] (let [m (f itm)] (when-not (@seen m) (swap! seen conj m)))) coll))) Now it's true, we're using a atom and a bit of mutability where none existed before, but personally I think the intent is a bit clearer in this code. What we're saying is "filter out all results we've seen before". If this was production code I would probably replace the atom with a `volatile!` but that's just for (small) performance reasons. On Sat, Jul 22, 2017 at 7:36 AM, Rob Nikander <rob.nikan...@gmail.com> wrote: > Hi, > > Here's a function and a simple use of it. It works, but the code may not > be good Clojure. I'm wondering how it might be better. > > (defn distinct-by > "Returns a sequence with no repeats of (f item). > A set is built up internally as the sequence is consumed, so don't use > it on > an infinite sequence or you will run out of memory." > ([f coll] > (letfn [(get-more [seen input-seq] > (if (not (empty? input-seq)) > (let [x (first input-seq) > fx (f x)] > (if (contains? seen fx) > (lazy-seq (get-more seen (rest input-seq))) > (lazy-seq (cons x (get-more (conj seen fx) (rest > input-seq))))))))] > (get-more #{} (seq coll))))) > > (def xs (list {:n 1, :a 'a} {:n 2, :a 'b} {:n 3, :a 'a} {:n 4, :a 'c} {:n > 5, :a 'b})) > > (distinct-by :a xs) > => ({:n 1, :a a} {:n 2, :a b} {:n 4, :a c}) > > Rob > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- “One of the main causes of the fall of the Roman Empire was that–lacking zero–they had no way to indicate successful termination of their C programs.” (Robert Firth) -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.