gnu.org (original) (raw)

12.16 Variables with Restricted Values

Ordinary Lisp variables can be assigned any value that is a valid Lisp object. However, certain Lisp variables are not defined in Lisp, but in C. Most of these variables are defined in the C code usingDEFVAR_LISP. Like variables defined in Lisp, these can take on any value. However, some variables are defined usingDEFVAR_INT or DEFVAR_BOOL. See Writing Emacs Primitives, in particular the description of functions of the type syms_of_filename, for a brief discussion of the C implementation.

Variables of type DEFVAR_BOOL can only take on the valuesnil or t. Attempting to assign them any other value will set them to t:

(let ((display-hourglass 5)) display-hourglass) ⇒ t

Variable: byte-boolean-vars ¶

This variable holds a list of all variables of type DEFVAR_BOOL.

Variables of type DEFVAR_INT can take on only integer values. Attempting to assign them any other value will result in an error:

(setq undo-limit 1000.0) error→ Wrong type argument: integerp, 1000.0