13.5 Backquote

Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ‘`’ syntax (usually called backquote).

Backquote allows you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form quote (see Quoting). For example, these two forms yield identical results:

     `(a list of (+ 2 3) elements)
          ⇒ (a list of (+ 2 3) elements)
     '(a list of (+ 2 3) elements)
          ⇒ (a list of (+ 2 3) elements)

The special marker ‘,’ inside of the argument to backquote indicates a value that isn't constant. Backquote evaluates the argument of ‘,’ and puts the value in the list structure:

     (list 'a 'list 'of (+ 2 3) 'elements)
          ⇒ (a list of 5 elements)
     `(a list of ,(+ 2 3) elements)
          ⇒ (a list of 5 elements)

Substitution with ‘,’ is allowed at deeper levels of the list structure also. For example:

     (defmacro t-becomes-nil (variable)
       `(if (eq ,variable t)
            (setq ,variable nil)))
     
     (t-becomes-nil foo)
          == (if (eq foo t) (setq foo nil))

You can also splice an evaluated value into the resulting list, using the special marker ‘,@’. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ‘`’ is often unreadable. Here are some examples:

     (setq some-list '(2 3))
          ⇒ (2 3)
     (cons 1 (append some-list '(4) some-list))
          ⇒ (1 2 3 4 2 3)
     `(1 ,@some-list 4 ,@some-list)
          ⇒ (1 2 3 4 2 3)
     
     (setq list '(hack foo bar))
          ⇒ (hack foo bar)
     (cons 'use
       (cons 'the
         (cons 'words (append (cdr list) '(as elements)))))
          ⇒ (use the words foo bar as elements)
     `(use the words ,@(cdr list) as elements)
          ⇒ (use the words foo bar as elements)