What is idiomatic Clojure to “remove” a single instance from many in a list?

Question

I have a list, which may contain elements that will compare as equal. I would like a similar list, but with one element removed. So from (:a :b :c :b :d) I would like to

Answer 1

I usually solve these problems with a higher-order function like split-with, but someone's already done that. Sometimes it's more readable or more efficient to work at a more primitive level, so here's a better version of your original looping code, using lazy sequences and generalized to take a predicate for removal instead of being constrained to equality checks:

(defn remove-once [pred coll]
  ((fn inner [coll]
     (lazy-seq
      (when-let [[x & xs] (seq coll)]
        (if (pred x)
          xs
          (cons x (inner xs))))))
   coll))


user> (remove-once #{:b} [:a :b :c :b :d])
(:a :c :b :d)