What does gensym do in Lisp?

Question

contextualization: I've been doing a university project in which I have to write a parser for regular expressions and build the corresponding epsilon-NFA. I have to do this in Prolog and Lisp. I don't know if questions like this are allowed, if not I apologize.

I heard some of my classmates talking about how they used the function gensym for that, I asked them what it did and even checked up online but I literally can't understand what this function does neither why or when is best to use it.

In particular, I'm more intrested in what it does in Lisp. Thank you all.

Answer 1

GENSYM creates unique symbols. Each call creates a new symbol. The symbol usually has a name which includes a number, which is counted up. The name is also unique (the symbol itself is already unique) with a number, so that a human reader can identify different uninterned symbols in the source code.

CL-USER 39 > (gensym)
#:G1083

CL-USER 40 > (gensym)
#:G1084

CL-USER 41 > (gensym)
#:G1085

CL-USER 42 > (gensym)
#:G1086

gensym is often used in Lisp macros for code generation, when the macro needs to create new identifiers, which then don't clash with existing identifiers.

Example: we are going to double the result of a Lisp form and we are making sure that the Lisp form itself will be computed only once. We do that by saving the value in a local variable. The identifier for the local variable will be computed by gensym.

CL-USER 43 > (defmacro double-it (it)
               (let ((new-identifier (gensym)))
                 `(let ((,new-identifier ,it))
                    (+ ,new-identifier ,new-identifier))))
DOUBLE-IT

CL-USER 44 > (macroexpand-1 '(double-it (cos 1.4)))
(LET ((#:G1091 (COS 1.4)))
  (+ #:G1091 #:G1091))
T

CL-USER 45 > (double-it (cos 1.4))
0.33993432