Q5.1.5: What is the recommended use of setq?

• Global variables

  You will typically defvar your global variable to a default value, and use setq to set it later.

  It is never a good practice to setq user variables (like case-fold-search, etc.), as it ignores the user’s choice unconditionally. Note that defvar doesn’t change the value of a variable if it was bound previously. If you wish to change a user-variable temporarily, use let:

  (let ((case-fold-search nil))
    ...                                   ; code with searches that must be case-sensitive
    ...)
  

  You will notice the user-variables by their docstrings beginning with an asterisk (a convention).

• Local variables

  Bind them with let, which will unbind them (or restore their previous value, if they were bound) after exiting from the let form. Change the value of local variables with setq or whatever you like (e.g. incf, setf and such). The let form can even return one of its local variables.

  Typical usage:

  ;; iterate through the elements of the list returned by
  ;; `hairy-function-that-returns-list'
  (let ((l (hairy-function-that-returns-list)))
    (while l
      ... do something with (car l) ...
      (setq l (cdr l))))
  

  Another typical usage includes building a value simply to work with it.

  ;; Build the mode keymap out of the key-translation-alist
  (let ((inbox (file-truename (expand-file-name box)))
        (i 0))
    ... code dealing with inbox ...
    inbox)
  

  This piece of code uses the local variable inbox, which becomes unbound (or regains old value) after exiting the form. The form also returns the value of inbox, which can be reused, for instance:

  (setq foo-processed-inbox
        (let .....))