clojure - lazy sequence internal implementation

Question

I start to read/work on clojure and for that I start to read in parallel 'Programming Clojure' and 'Practical Clojure' books. I saw there one example of how lazy sequence working and for me was very clear in order to understand how lazy-seq work but unfortunately it doesn't work or at least not how I expect.

here is the example:

(defn square[x]
  (do
   (println "[current.elem=" x "]")
   (* x x))
)

(def var-00 (map square '(1 2 3 5 6 4)))

when I call:

var-00

, I expect that no message to print on console(REPL) but I got the follow result:

([current.elem= 1 ][current.elem= 2 ]1 [current.elem= 3 ]4 [current.elem= 5 ]9  [current.elem= 6 ]25 [current.elem= 4 ]36 16)

this mean that the function map was called even I expect to nothing happen since 'var-00' is just a reference to function 'map'; and more awkward from my point of view, if I call:

(nth var-00 2)

I got:

[current.elem= 1 ][current.elem= 2 ][current.elem= 3 ]9

, and if I call again:

(nth var-00 3)

I got:

[current.elem= 1 ][current.elem= 2 ][current.elem= 3 ][current.elem= 5 ]25;

previous elements(1,2,3) was computed again I my opinion those elements should be 'cached' by first call and now only element 5 should be computed. Did I do something wrong or I didn't fully understand how lazy sequence working in clojure ? As a mention I use IntellijIDEA and LaClojure plugin to run the program.

Thanks Sorin.

Leonid Beschastny Leonid Beschastny · Accepted Answer · 2014-03-23T11:31:59

Just checked your coed in Clojure REPL, it works fine for me. Every element got printed only once (when it's evaluated the first time).

I even tried your example in Clojure online REPL:

Clojure online REPL screenshot

But there is one thing that you got wrong. REPL executes each command and then prints its results, so when you type var-00 REPL resolves the symbol and then, in order to print it, executes the whole lazy sequence:

Clojure online REPL screenshot

It have nothing to do with lazy sequences, it's just how REPL works:

Clojure online REPL screenshot

clojure - lazy sequence internal implementation

2 Answers