1 ;;; backquote.el --- implement the ` Lisp construct
3 ;;; Copyright (C) 1990, 1992, 1994 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 ;; This backquote will generate calls to the backquote-list* form.
29 ;; Both a function version and a macro version are included.
30 ;; The macro version is used by default because it is faster
31 ;; and needs no run-time support. It should really be a subr.
37 ;; function and macro versions of backquote-list*
39 (defun backquote-list*-function (first &rest list)
40 "Like `list' but the last argument is the tail of the new list.
42 For example (backquote-list* 'a 'b 'c) => (a b . c)"
44 (let* ((rest list) (newlist (cons first nil)) (last newlist))
46 (setcdr last (cons (car rest) nil))
49 (setcdr last (car rest))
53 (defmacro backquote-list*-macro (first &rest list)
54 "Like `list' but the last argument is the tail of the new list.
56 For example (backquote-list* 'a 'b 'c) => (a b . c)"
57 (setq list (reverse (cons first list))
61 (let* ((second (car list))
63 (newlist (list 'cons second first)))
65 (setq newlist (list 'cons (car rest) newlist)
70 (defalias 'backquote-list* (symbol-function 'backquote-list*-macro))
72 ;; A few advertised variables that control which symbols are used
73 ;; to represent the backquote, unquote, and splice operations.
75 (defvar backquote-backquote-symbol '\`
76 "*Symbol used to represent a backquote or nested backquote (e.g. `).")
78 (defvar backquote-unquote-symbol ',
79 "*Symbol used to represent an unquote (e.g. `,') inside a backquote.")
81 (defvar backquote-splice-symbol ',@
82 "*Symbol used to represent a splice (e.g. `,@') inside a backquote.")
85 (defmacro backquote (arg)
86 "Argument STRUCTURE describes a template to build.
88 The whole structure acts as if it were quoted except for certain
89 places where expressions are evaluated and inserted or spliced in.
93 b => (ba bb bc) ; assume b has this value
94 `(a b c) => (a b c) ; backquote acts like quote
95 `(a ,b c) => (a (ba bb bc) c) ; insert the value of b
96 `(a ,@b c) => (a ba bb bc c) ; splice in the value of b
98 Vectors work just like lists. Nested backquotes are permitted."
99 (cdr (backquote-process arg)))
101 ;; GNU Emacs has no reader macros
104 (defalias '\` (symbol-function 'backquote))
106 ;; backquote-process returns a dotted-pair of a tag (0, 1, or 2) and
107 ;; the backquote-processed structure. 0 => the structure is
108 ;; constant, 1 => to be unquoted, 2 => to be spliced in.
109 ;; The top-level backquote macro just discards the tag.
111 (defun backquote-process (s)
114 (let ((n (backquote-process (append s ()))))
118 ((eq (nth 1 n) 'list)
119 (cons 'vector (nthcdr 2 n)))
120 ((eq (nth 1 n) 'append)
121 (cons 'vconcat (nthcdr 2 n)))
123 (list 'apply '(function vector) (cdr n))))))))
125 (cons 0 (if (or (null s) (eq s t) (not (symbolp s)))
128 ((eq (car s) backquote-unquote-symbol)
130 ((eq (car s) backquote-splice-symbol)
132 ((eq (car s) backquote-backquote-symbol)
133 (backquote-process (cdr (backquote-process (nth 1 s)))))
136 item firstlist list lists expression)
137 ;; Scan this list-level, setting LISTS to a list of forms,
138 ;; each of which produces a list of elements
139 ;; that should go in this level.
140 ;; The order of LISTS is backwards.
141 ;; If there are non-splicing elements (constant or variable)
142 ;; at the beginning, put them in FIRSTLIST,
143 ;; as a list of tagged values (TAG . FORM).
144 ;; If there are any at the end, they go in LIST, likewise.
146 ;; Turn . (, foo) into (,@ foo).
147 (if (eq (car rest) backquote-unquote-symbol)
148 (setq rest (list (list backquote-splice-symbol (nth 1 rest)))))
149 (setq item (backquote-process (car rest)))
152 ;; Put the nonspliced items before the first spliced item
157 ;; Otherwise, put any preceding nonspliced items into LISTS.
159 (setq lists (cons (backquote-listify list '(0 . nil)) lists)))
160 (setq lists (cons (cdr item) lists))
163 (setq list (cons item list))))
164 (setq rest (cdr rest)))
165 ;; Handle nonsplicing final elements, and the tail of the list
166 ;; (which remains in REST).
168 (setq lists (cons (backquote-listify list (backquote-process rest))
170 ;; Turn LISTS into a form that produces the combined list.
173 (eq (car-safe (car lists)) backquote-splice-symbol))
174 (cons 'append (nreverse lists))
176 ;; Tack on any initial elements.
178 (setq expression (backquote-listify firstlist (cons 1 expression))))
179 (if (eq (car-safe expression) 'quote)
180 (cons 0 (list 'quote s))
181 (cons 1 expression))))))
183 ;; backquote-listify takes (tag . structure) pairs from backquote-process
184 ;; and decides between append, list, backquote-list*, and cons depending
185 ;; on which tags are in the list.
187 (defun backquote-listify (list old-tail)
188 (let ((heads nil) (tail (cdr old-tail)) (list-tail list) (item nil))
189 (if (= (car old-tail) 0)
190 (setq tail (eval tail)
192 (while (consp list-tail)
193 (setq item (car list-tail))
194 (setq list-tail (cdr list-tail))
195 (if (or heads old-tail (/= (car item) 0))
196 (setq heads (cons (cdr item) heads))
197 (setq tail (cons (eval (cdr item)) tail))))
201 (setq tail (list 'quote tail)))
203 (let ((use-list* (or (cdr heads)
204 (and (consp (car heads))
205 (eq (car (car heads))
206 backquote-splice-symbol)))))
207 (cons (if use-list* 'backquote-list* 'cons)
208 (append heads (list tail))))
210 (t (cons 'list heads)))))
212 ;; backquote.el ends here