1 ;;; backquote.el --- implement the ` Lisp construct
3 ;; Copyright (C) 1990, 92, 1994, 2001, 2004 Free Software Foundation, Inc.
5 ;; Author: Rick Sladkey <jrs@world.std.com>
7 ;; Keywords: extensions, internal
9 ;; This file is part of GNU Emacs.
11 ;; GNU Emacs is free software; you can redistribute it and/or modify
12 ;; it under the terms of the GNU General Public License as published by
13 ;; the Free Software Foundation; either version 2, or (at your option)
16 ;; GNU Emacs is distributed in the hope that it will be useful,
17 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
18 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
19 ;; GNU General Public License for more details.
21 ;; You should have received a copy of the GNU General Public License
22 ;; along with GNU Emacs; see the file COPYING. If not, write to the
23 ;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
24 ;; Boston, MA 02111-1307, USA.
28 ;; When the Lisp reader sees `(...), it generates (\` (...)).
29 ;; When it sees ,... inside such a backquote form, it generates (\, ...).
30 ;; For ,@... it generates (\,@ ...).
32 ;; This backquote will generate calls to the backquote-list* form.
33 ;; Both a function version and a macro version are included.
34 ;; The macro version is used by default because it is faster
35 ;; and needs no run-time support. It should really be a subr.
41 ;; function and macro versions of backquote-list*
43 (defun backquote-list*-function (first &rest list)
44 "Like `list' but the last argument is the tail of the new list.
46 For example (backquote-list* 'a 'b 'c) => (a b . c)"
47 ;; The recursive solution is much nicer:
48 ;; (if list (cons first (apply 'backquote-list*-function list)) first))
49 ;; but Emacs is not very good at efficiently processing recursion.
51 (let* ((rest list) (newlist (cons first nil)) (last newlist))
53 (setcdr last (cons (car rest) nil))
56 (setcdr last (car rest))
60 (defmacro backquote-list*-macro (first &rest list)
61 "Like `list' but the last argument is the tail of the new list.
63 For example (backquote-list* 'a 'b 'c) => (a b . c)"
64 ;; The recursive solution is much nicer:
65 ;; (if list (list 'cons first (cons 'backquote-list*-macro list)) first))
66 ;; but Emacs is not very good at efficiently processing such things.
67 (setq list (nreverse (cons first list))
71 (let* ((second (car list))
73 (newlist (list 'cons second first)))
75 (setq newlist (list 'cons (car rest) newlist)
80 (defalias 'backquote-list* (symbol-function 'backquote-list*-macro))
82 ;; A few advertised variables that control which symbols are used
83 ;; to represent the backquote, unquote, and splice operations.
84 (defconst backquote-backquote-symbol '\`
85 "Symbol used to represent a backquote or nested backquote.")
87 (defconst backquote-unquote-symbol ',
88 "Symbol used to represent an unquote inside a backquote.")
90 (defconst backquote-splice-symbol ',@
91 "Symbol used to represent a splice inside a backquote.")
94 (defmacro backquote (arg)
95 "Argument STRUCTURE describes a template to build.
97 The whole structure acts as if it were quoted except for certain
98 places where expressions are evaluated and inserted or spliced in.
102 b => (ba bb bc) ; assume b has this value
103 `(a b c) => (a b c) ; backquote acts like quote
104 `(a ,b c) => (a (ba bb bc) c) ; insert the value of b
105 `(a ,@b c) => (a ba bb bc c) ; splice in the value of b
107 Vectors work just like lists. Nested backquotes are permitted."
108 (cdr (backquote-process arg)))
110 ;; GNU Emacs has no reader macros
113 (defalias '\` (symbol-function 'backquote))
115 ;; backquote-process returns a dotted-pair of a tag (0, 1, or 2) and
116 ;; the backquote-processed structure. 0 => the structure is
117 ;; constant, 1 => to be unquoted, 2 => to be spliced in.
118 ;; The top-level backquote macro just discards the tag.
120 (defun backquote-process (s)
123 (let ((n (backquote-process (append s ()))))
127 ((not (listp (cdr n)))
128 (list 'vconcat (cdr n)))
129 ((eq (nth 1 n) 'list)
130 (cons 'vector (nthcdr 2 n)))
131 ((eq (nth 1 n) 'append)
132 (cons 'vconcat (nthcdr 2 n)))
134 (list 'apply '(function vector) (cdr n))))))))
136 (cons 0 (if (or (null s) (eq s t) (not (symbolp s)))
139 ((eq (car s) backquote-unquote-symbol)
141 ((eq (car s) backquote-splice-symbol)
143 ((eq (car s) backquote-backquote-symbol)
144 (backquote-process (cdr (backquote-process (nth 1 s)))))
147 item firstlist list lists expression)
148 ;; Scan this list-level, setting LISTS to a list of forms,
149 ;; each of which produces a list of elements
150 ;; that should go in this level.
151 ;; The order of LISTS is backwards.
152 ;; If there are non-splicing elements (constant or variable)
153 ;; at the beginning, put them in FIRSTLIST,
154 ;; as a list of tagged values (TAG . FORM).
155 ;; If there are any at the end, they go in LIST, likewise.
157 ;; Turn . (, foo) into (,@ foo).
158 (if (eq (car rest) backquote-unquote-symbol)
159 (setq rest (list (list backquote-splice-symbol (nth 1 rest)))))
160 (setq item (backquote-process (car rest)))
163 ;; Put the nonspliced items before the first spliced item
168 ;; Otherwise, put any preceding nonspliced items into LISTS.
170 (setq lists (cons (backquote-listify list '(0 . nil)) lists)))
171 (setq lists (cons (cdr item) lists))
174 (setq list (cons item list))))
175 (setq rest (cdr rest)))
176 ;; Handle nonsplicing final elements, and the tail of the list
177 ;; (which remains in REST).
179 (setq lists (cons (backquote-listify list (backquote-process rest))
181 ;; Turn LISTS into a form that produces the combined list.
184 (eq (car-safe (car lists)) backquote-splice-symbol))
185 (cons 'append (nreverse lists))
187 ;; Tack on any initial elements.
189 (setq expression (backquote-listify firstlist (cons 1 expression))))
190 (if (eq (car-safe expression) 'quote)
191 (cons 0 (list 'quote s))
192 (cons 1 expression))))))
194 ;; backquote-listify takes (tag . structure) pairs from backquote-process
195 ;; and decides between append, list, backquote-list*, and cons depending
196 ;; on which tags are in the list.
198 (defun backquote-listify (list old-tail)
199 (let ((heads nil) (tail (cdr old-tail)) (list-tail list) (item nil))
200 (if (= (car old-tail) 0)
201 (setq tail (eval tail)
203 (while (consp list-tail)
204 (setq item (car list-tail))
205 (setq list-tail (cdr list-tail))
206 (if (or heads old-tail (/= (car item) 0))
207 (setq heads (cons (cdr item) heads))
208 (setq tail (cons (eval (cdr item)) tail))))
212 (setq tail (list 'quote tail)))
214 (let ((use-list* (or (cdr heads)
215 (and (consp (car heads))
216 (eq (car (car heads))
217 backquote-splice-symbol)))))
218 (cons (if use-list* 'backquote-list* 'cons)
219 (append heads (list tail))))
221 (t (cons 'list heads)))))
223 ;;; arch-tag: 1a26206a-6b5e-4c56-8e24-2eef0f7e0e7a
224 ;;; backquote.el ends here