Does `count` realize a lazy sequence in Clojure?

Question

Let's say I have a LazySeq

(def s (take 10 (iterate + 0)))

Does (count s) realize the sequence?

Answer 1

If you are asking about lazy sequences, Yes.

user> (def s (map #(do (println "doing work") %) (range 4)))
#'user/s
user> (count s)
doing work
doing work
doing work                       
doing work
4  

Some of the data structures can give you answers in constant time, though lazy sequences do not have a stored count, and counting always realizes them.