1 ;;; backquote.el --- implement the ` Lisp construct
3 ;; Copyright (C) 1990, 1992, 1994, 2001-2014 Free Software Foundation,
6 ;; Author: Rick Sladkey <jrs@world.std.com>
7 ;; Maintainer: emacs-devel@gnu.org
8 ;; Keywords: extensions, internal
11 ;; This file is part of GNU Emacs.
13 ;; GNU Emacs is free software: you can redistribute it and/or modify
14 ;; it under the terms of the GNU General Public License as published by
15 ;; the Free Software Foundation, either version 3 of the License, or
16 ;; (at your option) any later version.
18 ;; GNU Emacs is distributed in the hope that it will be useful,
19 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
20 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
21 ;; GNU General Public License for more details.
23 ;; You should have received a copy of the GNU General Public License
24 ;; along with GNU Emacs. If not, see <http://www.gnu.org/licenses/>.
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.")
93 (defmacro backquote (structure)
94 "Argument STRUCTURE describes a template to build.
96 The whole structure acts as if it were quoted except for certain
97 places where expressions are evaluated and inserted or spliced in.
101 b => (ba bb bc) ; assume b has this value
102 `(a b c) => (a b c) ; backquote acts like quote
103 `(a ,b c) => (a (ba bb bc) c) ; insert the value of b
104 `(a ,@b c) => (a ba bb bc c) ; splice in the value of b
106 Vectors work just like lists. Nested backquotes are permitted."
107 (cdr (backquote-process structure)))
109 ;; GNU Emacs has no reader macros
111 (defalias '\` (symbol-function 'backquote))
113 ;; backquote-process returns a dotted-pair of a tag (0, 1, or 2) and
114 ;; the backquote-processed structure. 0 => the structure is
115 ;; constant, 1 => to be unquoted, 2 => to be spliced in.
116 ;; The top-level backquote macro just discards the tag.
118 (defun backquote-delay-process (s level)
119 "Process a (un|back|splice)quote inside a backquote.
120 This simply recurses through the body."
121 (let ((exp (backquote-listify (list (cons 0 (list 'quote (car s))))
122 (backquote-process (cdr s) level))))
123 (if (eq (car-safe exp) 'quote)
124 (cons 0 (list 'quote s))
127 (defun backquote-process (s &optional level)
128 "Process the body of a backquote.
129 S is the body. Returns a cons cell whose cdr is piece of code which
130 is the macro-expansion of S, and whose car is a small integer whose value
131 can either indicate that the code is constant (0), or not (1), or returns
132 a list which should be spliced into its environment (2).
133 LEVEL is only used internally and indicates the nesting level:
134 0 (the default) is for the toplevel nested inside a single backquote."
135 (unless level (setq level 0))
138 (let ((n (backquote-process (append s ()) level)))
142 ((not (listp (cdr n)))
143 (list 'vconcat (cdr n)))
144 ((eq (nth 1 n) 'list)
145 (cons 'vector (nthcdr 2 n)))
146 ((eq (nth 1 n) 'append)
147 (cons 'vconcat (nthcdr 2 n)))
149 (list 'apply '(function vector) (cdr n))))))))
151 (cons 0 (if (or (null s) (eq s t) (not (symbolp s)))
154 ((eq (car s) backquote-unquote-symbol)
157 ;; We could support it with: (cons 2 `(list . ,(cdr s)))
158 ;; But let's not encourage such uses.
159 (error "Multiple args to , are not supported: %S" s)
161 (backquote-delay-process s (1- level))))
162 ((eq (car s) backquote-splice-symbol)
165 ;; (cons 2 `(append . ,(cdr s)))
166 (error "Multiple args to ,@ are not supported: %S" s)
168 (backquote-delay-process s (1- level))))
169 ((eq (car s) backquote-backquote-symbol)
170 (backquote-delay-process s (1+ level)))
173 item firstlist list lists expression)
174 ;; Scan this list-level, setting LISTS to a list of forms,
175 ;; each of which produces a list of elements
176 ;; that should go in this level.
177 ;; The order of LISTS is backwards.
178 ;; If there are non-splicing elements (constant or variable)
179 ;; at the beginning, put them in FIRSTLIST,
180 ;; as a list of tagged values (TAG . FORM).
181 ;; If there are any at the end, they go in LIST, likewise.
182 (while (and (consp rest)
183 ;; Stop if the cdr is an expression inside a backquote or
184 ;; unquote since this needs to go recursively through
185 ;; backquote-process.
186 (not (or (eq (car rest) backquote-unquote-symbol)
187 (eq (car rest) backquote-backquote-symbol))))
188 (setq item (backquote-process (car rest) level))
191 ;; Put the nonspliced items before the first spliced item
196 ;; Otherwise, put any preceding nonspliced items into LISTS.
198 (push (backquote-listify list '(0 . nil)) lists))
199 (push (cdr item) lists)
202 (setq list (cons item list))))
203 (setq rest (cdr rest)))
204 ;; Handle nonsplicing final elements, and the tail of the list
205 ;; (which remains in REST).
207 (push (backquote-listify list (backquote-process rest level))
209 ;; Turn LISTS into a form that produces the combined list.
212 (eq (car-safe (car lists)) backquote-splice-symbol))
213 (cons 'append (nreverse lists))
215 ;; Tack on any initial elements.
217 (setq expression (backquote-listify firstlist (cons 1 expression))))
218 (if (eq (car-safe expression) 'quote)
219 (cons 0 (list 'quote s))
220 (cons 1 expression))))))
222 ;; backquote-listify takes (tag . structure) pairs from backquote-process
223 ;; and decides between append, list, backquote-list*, and cons depending
224 ;; on which tags are in the list.
226 (defun backquote-listify (list old-tail)
227 (let ((heads nil) (tail (cdr old-tail)) (list-tail list) (item nil))
228 (if (= (car old-tail) 0)
229 (setq tail (eval tail)
231 (while (consp list-tail)
232 (setq item (car list-tail))
233 (setq list-tail (cdr list-tail))
234 (if (or heads old-tail (/= (car item) 0))
235 (setq heads (cons (cdr item) heads))
236 (setq tail (cons (eval (cdr item)) tail))))
240 (setq tail (list 'quote tail)))
242 (let ((use-list* (or (cdr heads)
243 (and (consp (car heads))
244 (eq (car (car heads))
245 backquote-splice-symbol)))))
246 (cons (if use-list* 'backquote-list* 'cons)
247 (append heads (list tail))))
249 (t (cons 'list heads)))))
251 ;;; backquote.el ends here