confusion regarding "you can use a list as a stack to get the first element" koan. about clojure-koans

[trptcolin]

For lists, it's definitely a stack - "last in first out" refers not to the ordering as you look at the data structure, but to the order in which elements are inserted and removed. So the last element to be inserted would be the first one to come back out.

Here's a lein repl session with similar examples to that koan:

user=> (cons :a '(:b :c :d :e)) ;; adding :a to the *front* of the list
(:a :b :c :d :e) 
user=> (conj '(:a :b :c :d) :e) ;; adding :e to the front using conj
(:e :a :b :c :d)
user=> (peek '(:a :b :c :d :e)) ;; "peeking" at the last element inserted, :a
:a
user=> (pop '(:a :b :c :d :e)) ;; removing the last element added, :a
(:b :c :d :e)

Basically, it's efficient to add elements to the front of a list, and it's efficient to remove elements from the front of a list. And adding & removing from the same end is what a stack does. With a queue ("first in first out"), we'd expect to add on one end, and remove from the other end.

"Insert" (or "add") and "remove" take on a little different meaning due to the immutability of the data structures, but hopefully this sheds some more light on why the list gets used like a stack!