(defn list-index-of
  [list element]
  (let [r (reduce
            (fn [n x] (if (= x element) (reduced n) (inc n)))
            0 list)]
    (when (not= r (count list))
      r)))

Checking an index of an element in a list cannot be done using a dedicated function in Clojure. If possible, it may be better to use a vector and its .indexOf method in this case. It still must do a linear search, mind you.

But what if we have a sequence of a “reasonable” size and don’t want to convert it to a vector)? Or, if we are in ClojureScript and we would have to convert it to a JavaScript array and then depend on JavaScript’s strict equality semantics when calling the array’s .indexOf method?

We can use list-index-of. If element is not found it will return nil otherwise it will return a zero-based index.

(list-index-of '(:green :amber :red) :blue)
=> nil

(list-index-of '(:green :amber :red) :amber)
=> 1