Lazy Sequences

I like this a lot:

;; define a function with a side effect to show when it is called
(defn square [x]
  (do
    (println (str "Processing: " x))
    (* x x)))

;; define map-result which is result of calling square for every item in the list
(def map-result (map square '( 1 2 3 4 5 6 7 8)))

First time, enough of the lazy sequence is evaluated to generate the result:

practical-clojure.chpt5> (nth map-result 2)
Processing: 1
Processing: 2
Processing: 3
9

The next time, with the same args, the previous calculation was cached, so nothing to evaluate:

practical-clojure.chpt5> (nth map-result 2)
9

The next call it needs to evaluate a couple more of the lazy sequence:

practical-clojure.chpt5> (nth map-result 4)
Processing: 4
Processing: 5
25

And again, this is cached:

practical-clojure.chpt5> (nth map-result 4)
25

This is possible because a sequence looks like:

first --> (rest)
first --> (first --> (rest))
first --> (first --> (first --> rest))

Rest is only evaluated when it is required.