1 ;;; backquote.el --- implement the ` Lisp construct
3 ;; Copyright (C) 1990, 1992, 1994, 2001, 2002, 2003, 2004,
4 ;; 2005 Free Software Foundation, Inc.
6 ;; Author: Rick Sladkey <jrs@world.std.com>
8 ;; Keywords: extensions, internal
10 ;; This file is part of GNU Emacs.
12 ;; GNU Emacs is free software; you can redistribute it and/or modify
13 ;; it under the terms of the GNU General Public License as published by
14 ;; the Free Software Foundation; either version 2, or (at your option)
17 ;; GNU Emacs is distributed in the hope that it will be useful,
18 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
19 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
20 ;; GNU General Public License for more details.
22 ;; You should have received a copy of the GNU General Public License
23 ;; along with GNU Emacs; see the file COPYING. If not, write to the
24 ;; Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
25 ;; Boston, MA 02110-1301, USA.
29 ;; When the Lisp reader sees `(...), it generates (\` (...)).
30 ;; When it sees ,... inside such a backquote form, it generates (\, ...).
31 ;; For ,@... it generates (\,@ ...).
33 ;; This backquote will generate calls to the backquote-list* form.
34 ;; Both a function version and a macro version are included.
35 ;; The macro version is used by default because it is faster
36 ;; and needs no run-time support. It should really be a subr.
42 ;; function and macro versions of backquote-list*
44 (defun backquote-list*-function (first &rest list)
45 "Like `list' but the last argument is the tail of the new list.
47 For example (backquote-list* 'a 'b 'c) => (a b . c)"
48 ;; The recursive solution is much nicer:
49 ;; (if list (cons first (apply 'backquote-list*-function list)) first))
50 ;; but Emacs is not very good at efficiently processing recursion.
52 (let* ((rest list) (newlist (cons first nil)) (last newlist))
54 (setcdr last (cons (car rest) nil))
57 (setcdr last (car rest))
61 (defmacro backquote-list*-macro (first &rest list)
62 "Like `list' but the last argument is the tail of the new list.
64 For example (backquote-list* 'a 'b 'c) => (a b . c)"
65 ;; The recursive solution is much nicer:
66 ;; (if list (list 'cons first (cons 'backquote-list*-macro list)) first))
67 ;; but Emacs is not very good at efficiently processing such things.
68 (setq list (nreverse (cons first list))
72 (let* ((second (car list))
74 (newlist (list 'cons second first)))
76 (setq newlist (list 'cons (car rest) newlist)
81 (defalias 'backquote-list* (symbol-function 'backquote-list*-macro))
83 ;; A few advertised variables that control which symbols are used
84 ;; to represent the backquote, unquote, and splice operations.
85 (defconst backquote-backquote-symbol '\`
86 "Symbol used to represent a backquote or nested backquote.")
88 (defconst backquote-unquote-symbol ',
89 "Symbol used to represent an unquote inside a backquote.")
91 (defconst backquote-splice-symbol ',@
92 "Symbol used to represent a splice inside a backquote.")
95 (defmacro backquote (arg)
96 "Argument STRUCTURE describes a template to build.
98 The whole structure acts as if it were quoted except for certain
99 places where expressions are evaluated and inserted or spliced in.
103 b => (ba bb bc) ; assume b has this value
104 `(a b c) => (a b c) ; backquote acts like quote
105 `(a ,b c) => (a (ba bb bc) c) ; insert the value of b
106 `(a ,@b c) => (a ba bb bc c) ; splice in the value of b
108 Vectors work just like lists. Nested backquotes are permitted."
109 (cdr (backquote-process arg)))
111 ;; GNU Emacs has no reader macros
114 (defalias '\` (symbol-function 'backquote))
116 ;; backquote-process returns a dotted-pair of a tag (0, 1, or 2) and
117 ;; the backquote-processed structure. 0 => the structure is
118 ;; constant, 1 => to be unquoted, 2 => to be spliced in.
119 ;; The top-level backquote macro just discards the tag.
121 (defun backquote-process (s)
124 (let ((n (backquote-process (append s ()))))
128 ((not (listp (cdr n)))
129 (list 'vconcat (cdr n)))
130 ((eq (nth 1 n) 'list)
131 (cons 'vector (nthcdr 2 n)))
132 ((eq (nth 1 n) 'append)
133 (cons 'vconcat (nthcdr 2 n)))
135 (list 'apply '(function vector) (cdr n))))))))
137 (cons 0 (if (or (null s) (eq s t) (not (symbolp s)))
140 ((eq (car s) backquote-unquote-symbol)
142 ((eq (car s) backquote-splice-symbol)
144 ((eq (car s) backquote-backquote-symbol)
145 (backquote-process (cdr (backquote-process (nth 1 s)))))
148 item firstlist list lists expression)
149 ;; Scan this list-level, setting LISTS to a list of forms,
150 ;; each of which produces a list of elements
151 ;; that should go in this level.
152 ;; The order of LISTS is backwards.
153 ;; If there are non-splicing elements (constant or variable)
154 ;; at the beginning, put them in FIRSTLIST,
155 ;; as a list of tagged values (TAG . FORM).
156 ;; If there are any at the end, they go in LIST, likewise.
158 ;; Turn . (, foo) into (,@ foo).
159 (if (eq (car rest) backquote-unquote-symbol)
160 (setq rest (list (list backquote-splice-symbol (nth 1 rest)))))
161 (setq item (backquote-process (car rest)))
164 ;; Put the nonspliced items before the first spliced item
169 ;; Otherwise, put any preceding nonspliced items into LISTS.
171 (setq lists (cons (backquote-listify list '(0 . nil)) lists)))
172 (setq lists (cons (cdr item) lists))
175 (setq list (cons item list))))
176 (setq rest (cdr rest)))
177 ;; Handle nonsplicing final elements, and the tail of the list
178 ;; (which remains in REST).
180 (setq lists (cons (backquote-listify list (backquote-process rest))
182 ;; Turn LISTS into a form that produces the combined list.
185 (eq (car-safe (car lists)) backquote-splice-symbol))
186 (cons 'append (nreverse lists))
188 ;; Tack on any initial elements.
190 (setq expression (backquote-listify firstlist (cons 1 expression))))
191 (if (eq (car-safe expression) 'quote)
192 (cons 0 (list 'quote s))
193 (cons 1 expression))))))
195 ;; backquote-listify takes (tag . structure) pairs from backquote-process
196 ;; and decides between append, list, backquote-list*, and cons depending
197 ;; on which tags are in the list.
199 (defun backquote-listify (list old-tail)
200 (let ((heads nil) (tail (cdr old-tail)) (list-tail list) (item nil))
201 (if (= (car old-tail) 0)
202 (setq tail (eval tail)
204 (while (consp list-tail)
205 (setq item (car list-tail))
206 (setq list-tail (cdr list-tail))
207 (if (or heads old-tail (/= (car item) 0))
208 (setq heads (cons (cdr item) heads))
209 (setq tail (cons (eval (cdr item)) tail))))
213 (setq tail (list 'quote tail)))
215 (let ((use-list* (or (cdr heads)
216 (and (consp (car heads))
217 (eq (car (car heads))
218 backquote-splice-symbol)))))
219 (cons (if use-list* 'backquote-list* 'cons)
220 (append heads (list tail))))
222 (t (cons 'list heads)))))
224 ;;; arch-tag: 1a26206a-6b5e-4c56-8e24-2eef0f7e0e7a
225 ;;; backquote.el ends here