Clojure “same value” allocation

Question

Say I have the following code

(let [a (some-function 3) b (some-function (+ 1 2)) ca] (= ab))

Suppose some-function returns a very big data structure (say one million sized array)

First question: how much memory will clojure allocate? Will it allocate memory for each of these 3 vectors or they will share the same data structure?

Second question (closely related): how fast would it be to compare them? Will = iterate over each element or not?

This simplified example may look dumb but there are similar real life situations where this matters a lot like

(map some-function [1 23 1 32 1 44 1 5 1 1 1 1])

EDIT

in my specific case, some-function returns a conj of two sets which may be very big

Answer 1

1. Whether a and b share the structure depends entirely on what some-function does. It's impossible to answer without any details. c and a will be bound to the same value, so no additional memory allocation for c .

2. In general, semantics and behavior of = depend on what values you compare. If you compare collections, then yes, = will iterate through each element until it either exhausts them, or finds the first pair of unequal ones.

Additionally:

= will first check if the objects being compared are the same (reference equality,) so comparison of a and c will be instant.

While it's impossible to say for certain if different some-function invocation return the same or different structure, chances are without memoization they will be different. Just wrap your some-function using memoize to be certain there is no double allocation (of course assuming your some-function is pure.) As simple as

(def some-function (memoize some-function))