18.5 Backquote

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

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)