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(elisp)Backquote

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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' (Note: 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)

   In old Emacs versions, before version 19.29, ``' used a different
syntax which required an extra level of parentheses around the entire
backquote construct.  Likewise, each `,' or `,@' substitution required
an extra level of parentheses surrounding both the `,' or `,@' and the
following expression.  The old syntax required whitespace between the
``', `,' or `,@' and the following expression.

   This syntax is still accepted, for compatibility with old Emacs
versions, but we recommend not using it in new programs.