pop returns a collection with its head element removed, but “head” depends on the collection. For vectors, it removes the last element; for lists and queues, it removes the first-added element. It returns nil when given nil. section 12.6
Use pop with collections implementing IPersistentStack, especially vectors, lists, and queues.
- A vector behaves like a LIFO stack: (pop ["a" "b" "c"]) returns ["a" "b"]. section 12.6
- A queue removes its first-added element, while an empty queue may be popped; empty vectors and lists throw IllegalStateException. section 12.6
- Passing a collection without IPersistentStack support can produce ClassCastException; use butlast for sequence-oriented last-element removal, though it is linear. section 12.6 section 10.1
Checking balanced delimiters with a vector stack
A vector, conj, peek, and pop can implement a LIFO stack while scanning a Clojure form. Matching delimiters pop the opening bracket; mismatches throw an exception. A balanced form leaves [].
Processing a vector from the end
A loop can repeatedly inspect the final element with peek, remove it with pop, and accumulate transformed results. (reverse-mapv str (vec (range 10))) returns ["9" "8" "7" "6" "5" "4" "3" "2" "1" "0"].
