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Variables with Restricted Values (GNU Emacs Lisp Reference Manual)

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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

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

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