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*gensym-counter*

∗gensym-counter∗ Variable

Value Type:

a non-negative integer .

Initial Value:

implementation-dependent.

Description:

A number which will be used in constructing the name of the next symbol generated by the function gensym.

*gensym-counter* can be either assigned or bound at any time, but its value must always be a non-negative integer .

Affected By:

gensym.

See Also:

gensym

Notes:

The ability to pass a numeric argument to gensym has been deprecated; explicitly binding *gensym-counter* is now stylistically preferred.

Expanded Reference: *gensym-counter*​

Basic usage​

*gensym-counter* holds the next suffix number that gensym will use. It is incremented each time gensym is called without an explicit integer argument.

(let ((*gensym-counter* 0))
  (values (gensym) (gensym) (gensym)))
;; => #:G0
;; => #:G1
;; => #:G2

Reading the current counter value​

You can inspect the counter to see what number the next gensym will use.

(let ((*gensym-counter* 42))
  (values *gensym-counter*
          (gensym)
          *gensym-counter*))
;; => 42
;; => #:G42
;; => 43

Setting the counter for deterministic names​

Binding *gensym-counter* is the recommended way to control gensym suffixes, replacing the deprecated integer argument to gensym.

(let ((*gensym-counter* 1000))
  (list (gensym "VAR") (gensym "VAR") (gensym "VAR")))
;; => (#:VAR1000 #:VAR1001 #:VAR1002)

The counter must be a non-negative integer​

*gensym-counter* must always hold a non-negative integer. Setting it to a negative value or a non-integer has undefined consequences.

(let ((*gensym-counter* 0))
  (dotimes (i 5) (gensym))
  *gensym-counter*)
=> 5

Independent of gentemp counter​

*gensym-counter* is used only by gensym. The deprecated gentemp function uses its own internal counter.

(let ((*gensym-counter* 500))
  (values (symbol-name (gensym))
          *gensym-counter*))
=> "G500"
=> 501