Question

Does emacs lisp have a function that provides a unique object identifier, such as e.g. a memory address? Python has id(), which returns an integer guaranteed to be unique among presently existing objects. What about elisp?

Answer 1

My whole point in asking the question was that I was looking for a way to distinguish between the printed representations of different symbols that have the same name. Thanks to the elisp manual, I’ve discovered the variable print-gensym, which, when non-nil, causes #: to be prepended to uninterned symbols printed. Moreover, if the same call to print prints the same uninterned symbol more than once, it will mark the first one with #N= and subsequent ones with `#N#. This is exactly the kind of functionality I was looking for. For example:

(setq print-gensym t)
  ==> t
(make-symbol "foo")
  ==> #:foo
(setq a (make-symbol "foo"))
  ==> #:foo
(cons a a)
  ==> (#1=#:foo . #1#)
(setq b (make-symbol "foo"))
  ==> #:foo
(cons a b)
  ==> (#:foo . #:foo)

The #: notation works for read as well:

(setq a '#:foo)
  ==> #:foo
(symbol-name a)
  ==> "foo"

Note the ' on '#:foo–the #: notation is a symbol-literal. Without the ', the uninterned symbol is evaluated:

(symbol-name '#:foo)
  ==> "foo"
(symbol-name #:foo)
 ==> (void-variable #:foo)