Test whether a list contains a specific value in Clojure

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Ah, contains?… supposedly one of the top five FAQs re: Clojure.

It does not check whether a collection contains a value; it checks whether an item could be retrieved with get or, in other words, whether a collection contains a key. This makes sense for sets (which can be thought of as making no distinction between keys and values), maps (so (contains? {:foo 1} :foo) is true) and vectors (but note that (contains? [:foo :bar] 0) is true, because the keys here are indices and the vector in question does “contain” the index 0!).

To add to the confusion, in cases where it doesn’t make sense to call contains?, it simply return false; this is what happens in (contains? :foo 1) and also (contains? '(100 101 102) 101). Update: In Clojure ≥ 1.5 contains? throws when handed an object of a type that doesn’t support the intended “key membership” test.

The correct way to do what you’re trying to do is as follows:

; most of the time this works
(some #{101} '(100 101 102))

When searching for one of a bunch of items, you can use a larger set; when searching for false / nil, you can use false? / nil? — because (#{x} x) returns x, thus (#{nil} nil) is nil; when searching for one of multiple items some of which may be false or nil, you can use

(some (zipmap [...the items...] (repeat true)) the-collection)

(Note that the items can be passed to zipmap in any type of collection.)

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