Why does Clojure's gensym increase by three on each call?

Question

Fairly new to lisps, but in looking into sequential integer generating code, I noticed that repeated calls to (gensym) would increase the number provided after the prefix by 3. I'm curious why that is the case.

user=> (gensym)
G__662
user=> (gensym)
G__665
user=> (gensym)
G__668
user=> (gensym)
G__671
user=> (gensym)
G__674
user=> (gensym)
G__677

I've seen and understand the combined use of atom and inc, but I'm new to the gensym function.

Answer 1

There are a number of correct answers here. One is: it doesn't!

user> (take 5 (repeatedly gensym))
(G__2173 G__2174 G__2175 G__2176 G__2177)

Another is: gensym doesn't make any guarantees as to the form of the symbols it generates, so you really shouldn't care whether they're sequential or not (or even if they contain numbers at all). You certainly shouldn't hijack gensym to produce an integer sequence.

Lastly: why does it increase by three in your example? Because each time you evaluate a form in the repl, the compiler has to create some gensyms of its own. Apparently, for the form (gensym), the number it needs to create is two.

Answer 2

It doesn't!

=> (str (gensym) (gensym))
"G__4027G__4028"

Looking at the source of gensym we can see that it uses clojure.lang.RT/nextID.

(defn gensym
  ([prefix-string] (. clojure.lang.Symbol (intern (str prefix-string (str (. clojure.lang.RT (nextID))))))))

The nextID function is also used in the LispReader. So when you repeatedly evaluate (gensym), the reader is probably using two IDs.

I clearly have something else going on in my process too, as if I wait any time between evaluations, more IDs are consumed and the gensym gaps further than just 3.

https://github.com/clojure/clojure/search?q=nextid