1 ;;; New backquote for GNU Emacs.
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 not part of GNU Emacs but is distributed under
9 ;; the same conditions as 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
23 ;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
25 ;; This is a new backquote for GNU Emacs written by
26 ;; Rick Sladkey <jrs@world.std.com>. It has the following
27 ;; features compared to the version 18 backquote:
29 ;; Correctly handles nested backquotes.
30 ;; Correctly handles constants after a splice.
31 ;; Correctly handles top-level atoms and unquotes.
32 ;; Correctly handles unquote after dot.
33 ;; Understands vectors.
34 ;; Minimizes gratuitous consing.
35 ;; Faster operation with simpler semantics.
36 ;; Generates faster run-time expressions.
37 ;; One third fewer calories than our regular beer.
39 ;; This backquote will generate calls to the backquote-list* form.
40 ;; Both a function version and a macro version are included.
41 ;; The macro version is used by default because it is faster
42 ;; and needs no run-time support. It should really be a subr.
48 ;; function and macro versions of backquote-list*
50 (defun backquote-list*-function
(first &rest list
)
51 "Like `list' but the last argument is the tail of the new list.
53 For example (backquote-list* 'a 'b 'c) => (a b . c)"
55 (let* ((rest list
) (newlist (cons first nil
)) (last newlist
))
57 (setcdr last
(cons (car rest
) nil
))
60 (setcdr last
(car rest
))
64 (defmacro backquote-list
*-macro
(first &rest list
)
65 "Like `list' but the last argument is the tail of the new list.
67 For example (backquote-list* 'a 'b 'c) => (a b . c)"
68 (setq list
(reverse (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.
86 (defvar backquote-backquote-symbol
'\
`
87 "*Symbol used to represent a backquote or nested backquote (e.g. `).")
89 (defvar backquote-unquote-symbol
',
90 "*Symbol used to represent an unquote (e.g. `,') inside a backquote.")
92 (defvar backquote-splice-symbol
',@
93 "*Symbol used to represent a splice (e.g. `,@') inside a backquote.")
96 (defmacro backquote
(arg)
97 "Argument STRUCTURE describes a template to build.
99 The whole structure acts as if it were quoted except for certain
100 places where expressions are evaluated and inserted or spliced in.
104 b => (ba bb bc) ; assume b has this value
105 `(a b c) => (a b c) ; backquote acts like quote
106 `(a (, b) c) => (a (ba bb bc) c) ; insert the value of b
107 `(a (,@ b) c) => (a ba bb bc c) ; splice in the value of b
109 Vectors work just like lists. Nested backquotes are permitted."
110 (cdr (backquote-process arg
)))
112 ;; GNU Emacs has no reader macros
115 (defalias '\
` (symbol-function 'backquote
))
117 ;; backquote-process returns a dotted-pair of a tag (0, 1, or 2) and
118 ;; the backquote-processed structure. 0 => the structure is
119 ;; constant, 1 => to be unquoted, 2 => to be spliced in.
120 ;; The top-level backquote macro just discards the tag.
122 (defun backquote-process (s)
125 (let ((n (backquote-process (append s
()))))
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 ;; backquote.el ends here