# Another Useful Function -- convert-seq-to-comma-separated-string

This is a little note about a function I find myself using frequently in Clojure.

User interface code often needs to display a list of things as a comma-separated list, e.g. “a, b, c, d”.

If all of the things are strings, you can use the built-in string/join function to build such a string. When you have a sequence of things that are not strings, I suppose you could convert each element to a string and then use string/join. Or you could build your own. I like the function presented below, convert-seq-to-comma-separated-string. (Ok, the name needs some work.)

Here’s an example of usage.

(defn convert-seq-to-comma-separated-string
"Return a string containing the members of the sequence separated by commas."
[the-seq]
(let [but-last-comma-mapped (map #(str % ", ") (butlast the-seq))]
(apply str (concat but-last-comma-mapped (str (last the-seq))))))

(defn get-tag-names-for-page
"Returns a case-insensitive sorted-set of tag names associated with the page.
If ther are no such tags (it's nil or an empy seq), returns a sorted set
containing the word 'None'."
([page-id]
(get-tag-names-for-page page-id h2-db))
([page-id db]
(let [tag-ids (get-tag-ids-for-page page-id)]
(if (or (nil? tag-ids) (empty? tag-ids))
(into (sorted-set-by case-insensitive-comparator) ["None"])
(let [tag-ids-as-string (convert-seq-to-comma-separated-string tag-ids)
sql (str "select tag_name from tags where tag_id in ("
tag-ids-as-string ");")
rs (jdbc/query db [sql])]
(reduce #(conj %1 (:tag_name %2))
(sorted-set-by case-insensitive-comparator) rs))))))


In this case, I wanted to build a SQL command that included a list of identifiers, integers in this case. The function has come in very handy for things like this.