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 using
DEFVAR_LISP. Like variables defined in Lisp, these can take on
any value. However, some variables are defined using
DEFVAR_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 values
nil or t. Attempting to assign them any other value
will set them to t:
(let ((display-hourglass 5))
display-hourglass)
⇒ t
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