Test whether a list contains a specific value in Clojure

Question

What is the best way to test whether a list contains a given value in Clojure?

In particular, the behaviour of contains? is currently confusing me:

Answer 1

Another option:

((set '(100 101 102)) 101)

Use java.util.Collection#contains():

(.contains '(100 101 102) 101)

Answer 2

You can always call java methods with .methodName syntax.

(.contains [100 101 102] 101) => true

Answer 3

(not= -1 (.indexOf '(101 102 103) 102))

Works, but below is better:

(some #(= 102 %) '(101 102 103)) 

Answer 4

For what it is worth, this is my simple implementation of a contains function for lists:

(defn list-contains? [coll value]
  (let [s (seq coll)]
    (if s
      (if (= (first s) value) true (recur (rest s) value))
      false)))

Answer 5

The problem with the 'recommended' solution is it is breaks when the value you are seeking is 'nil'. I prefer this solution:

(defn member?
  "I'm still amazed that Clojure does not provide a simple member function.
   Returns true if `item` is a member of `series`, else nil."
  [item series]
  (and (some #(= item %) series) true))

Answer 6

Here's the classic Lisp solution:

(defn member? [list elt]
    "True if list contains at least one instance of elt"
    (cond 
        (empty? list) false
        (= (first list) elt) true
        true (recur (rest list) elt)))