1 ;;; backquote.el -- implement the ` Lisp construct
2 ;;; Copyright (C) 1990, 1992, 1994 Free Software Foundation, Inc.
4 ;; Author: Rick Sladkey <jrs@world.std.com>
6 ;; Keywords: extensions, internal
8 ;; This file is part of GNU Emacs.
10 ;; GNU Emacs is free software; you can redistribute it and/or modify
11 ;; it under the terms of the GNU General Public License as published by
12 ;; the Free Software Foundation; either version 2, or (at your option)
15 ;; GNU Emacs is distributed in the hope that it will be useful,
16 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
17 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
18 ;; GNU General Public License for more details.
20 ;; You should have received a copy of the GNU General Public License
21 ;; along with GNU Emacs; see the file COPYING. If not, write to
22 ;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
26 ;; This backquote will generate calls to the backquote-list* form.
27 ;; Both a function version and a macro version are included.
28 ;; The macro version is used by default because it is faster
29 ;; and needs no run-time support. It should really be a subr.
35 ;; function and macro versions of backquote-list*
37 (defun backquote-list*-function (first &rest list)
38 "Like `list' but the last argument is the tail of the new list.
40 For example (backquote-list* 'a 'b 'c) => (a b . c)"
42 (let* ((rest list) (newlist (cons first nil)) (last newlist))
44 (setcdr last (cons (car rest) nil))
47 (setcdr last (car rest))
51 (defmacro backquote-list*-macro (first &rest list)
52 "Like `list' but the last argument is the tail of the new list.
54 For example (backquote-list* 'a 'b 'c) => (a b . c)"
55 (setq list (reverse (cons first list))
59 (let* ((second (car list))
61 (newlist (list 'cons second first)))
63 (setq newlist (list 'cons (car rest) newlist)
68 (defalias 'backquote-list* (symbol-function 'backquote-list*-macro))
70 ;; A few advertised variables that control which symbols are used
71 ;; to represent the backquote, unquote, and splice operations.
73 (defvar backquote-backquote-symbol '\`
74 "*Symbol used to represent a backquote or nested backquote (e.g. `).")
76 (defvar backquote-unquote-symbol ',
77 "*Symbol used to represent an unquote (e.g. `,') inside a backquote.")
79 (defvar backquote-splice-symbol ',@
80 "*Symbol used to represent a splice (e.g. `,@') inside a backquote.")
83 (defmacro backquote (arg)
84 "Argument STRUCTURE describes a template to build.
86 The whole structure acts as if it were quoted except for certain
87 places where expressions are evaluated and inserted or spliced in.
91 b => (ba bb bc) ; assume b has this value
92 `(a b c) => (a b c) ; backquote acts like quote
93 `(a ,b c) => (a (ba bb bc) c) ; insert the value of b
94 `(a ,@b c) => (a ba bb bc c) ; splice in the value of b
96 Vectors work just like lists. Nested backquotes are permitted."
97 (cdr (backquote-process arg)))
99 ;; GNU Emacs has no reader macros
102 (defalias '\` (symbol-function 'backquote))
104 ;; backquote-process returns a dotted-pair of a tag (0, 1, or 2) and
105 ;; the backquote-processed structure. 0 => the structure is
106 ;; constant, 1 => to be unquoted, 2 => to be spliced in.
107 ;; The top-level backquote macro just discards the tag.
109 (defun backquote-process (s)
112 (let ((n (backquote-process (append s ()))))
116 ((eq (nth 1 n) 'list)
117 (cons 'vector (nthcdr 2 n)))
118 ((eq (nth 1 n) 'append)
119 (cons 'vconcat (nthcdr 2 n)))
121 (list 'apply '(function vector) (cdr n))))))))
123 (cons 0 (if (or (null s) (eq s t) (not (symbolp s)))
126 ((eq (car s) backquote-unquote-symbol)
128 ((eq (car s) backquote-splice-symbol)
130 ((eq (car s) backquote-backquote-symbol)
131 (backquote-process (cdr (backquote-process (nth 1 s)))))
134 item firstlist list lists expression)
135 ;; Scan this list-level, setting LISTS to a list of forms,
136 ;; each of which produces a list of elements
137 ;; that should go in this level.
138 ;; The order of LISTS is backwards.
139 ;; If there are non-splicing elements (constant or variable)
140 ;; at the beginning, put them in FIRSTLIST,
141 ;; as a list of tagged values (TAG . FORM).
142 ;; If there are any at the end, they go in LIST, likewise.
144 ;; Turn . (, foo) into (,@ foo).
145 (if (eq (car rest) backquote-unquote-symbol)
146 (setq rest (list (list backquote-splice-symbol (nth 1 rest)))))
147 (setq item (backquote-process (car rest)))
150 ;; Put the nonspliced items before the first spliced item
155 ;; Otherwise, put any preceding nonspliced items into LISTS.
157 (setq lists (cons (backquote-listify list '(0 . nil)) lists)))
158 (setq lists (cons (cdr item) lists))
161 (setq list (cons item list))))
162 (setq rest (cdr rest)))
163 ;; Handle nonsplicing final elements, and the tail of the list
164 ;; (which remains in REST).
166 (setq lists (cons (backquote-listify list (backquote-process rest))
168 ;; Turn LISTS into a form that produces the combined list.
171 (eq (car-safe (car lists)) backquote-splice-symbol))
172 (cons 'append (nreverse lists))
174 ;; Tack on any initial elements.
176 (setq expression (backquote-listify firstlist (cons 1 expression))))
177 (if (eq (car-safe expression) 'quote)
178 (cons 0 (list 'quote s))
179 (cons 1 expression))))))
181 ;; backquote-listify takes (tag . structure) pairs from backquote-process
182 ;; and decides between append, list, backquote-list*, and cons depending
183 ;; on which tags are in the list.
185 (defun backquote-listify (list old-tail)
186 (let ((heads nil) (tail (cdr old-tail)) (list-tail list) (item nil))
187 (if (= (car old-tail) 0)
188 (setq tail (eval tail)
190 (while (consp list-tail)
191 (setq item (car list-tail))
192 (setq list-tail (cdr list-tail))
193 (if (or heads old-tail (/= (car item) 0))
194 (setq heads (cons (cdr item) heads))
195 (setq tail (cons (eval (cdr item)) tail))))
199 (setq tail (list 'quote tail)))
201 (let ((use-list* (or (cdr heads)
202 (and (consp (car heads))
203 (eq (car (car heads))
204 backquote-splice-symbol)))))
205 (cons (if use-list* 'backquote-list* 'cons)
206 (append heads (list tail))))
208 (t (cons 'list heads)))))
210 ;; backquote.el ends here