Question

What’s the best way to implement keywords as optional flags to a function?
I want to make function calls such as:

(myfunction 5)
(myfunction 6 :do-this)
(myfunction 3 :go-here)
(myfunction 2 :do-this :do-that)

Using defn, I can define a function such as:

(defn myfunction [value & flags] ... )

But the flags becomes a list. I can write my own function to search the list, but such a function isn’t included in the core library, so I assume it’s not idiomatic.

What I’m using now:

(defn flag-set? [list flag] (not (empty? (filter #(= flag %) list))))
(defn flag-add [list flag] (cons flag list))
(defn flag-remove [list flag] (filter #(not= flag %) list))

Answer 1

Lists (as well as vectors and maps) are not a good choice of data structure for value-based lookups (will be linear time), that’s why clojure.core doesn’t have such functions.

Sets do provide fast value-based lookups via “contains?”, so how about

(defn foo [value & flags]
  (let [flags (set flags)]
    (if (contains? flags :add-one)
      (inc value)
      value)))

If there won’t be more than one flag, you can use destructuring like this:

(defn foo [value & [flag]] …)