Comment by robto

One of the fun things about Clojure that reinforces this "trivially true" perspective is that maps and sets are functions:

    ;; "maps" the keys to the values
    (map {1 "a" 2 "b"} (take 5 (cycle 1 2))) ;;=> '("a" "b" "a" "b" "a")
    ;; acts as a predicate that tests for membership
    (filter #{"a" "b" "c"} ["a" "b" "c" "d" "e" "f"]) ;;=> '("a" "b" "c")

Once you get used to this idiom you naturally start thing of other functions (or applicative functors) the same way. The syntax sugar makes for some very concise and expressive code too.