Clojure: cons (seq) vs. conj (list)

Question

I know that cons returns a seq and conj returns a collection. I also know that conj \"adds\" the item to the optimal end of the colle

Answer 1

Another difference is that because conj takes a sequence as the first argument, it plays nicely with alter when updating a ref to some sequence:

(dosync (alter a-sequence-ref conj an-item))

This basically does (conj a-sequence-ref an-item) in a thread-safe manner. This wouldn't work with cons. See the chapter on Concurrency in Programming Clojure by Stu Halloway for more info.