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c0274f38 ER |
1 | ;; backquote.el --- backquoting for Emacs Lisp macros |
2 | ||
e5167999 ER |
3 | ;; Author: Dick King (king@kestrel). |
4 | ;; Last-Modified: 16 Mar 1992 | |
fd7fa35a | 5 | ;; Keywords: extensions |
e5167999 | 6 | |
c440e42b | 7 | ;; Copyright (C) 1985 Free Software Foundation, Inc. |
c440e42b RS |
8 | |
9 | ;; This file is part of GNU Emacs. | |
10 | ||
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 | |
e5167999 | 13 | ;; the Free Software Foundation; either version 2, or (at your option) |
c440e42b RS |
14 | ;; any later version. |
15 | ||
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. | |
20 | ||
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. | |
24 | ||
e5167999 | 25 | ;;; Commentary: |
c440e42b | 26 | |
e5167999 | 27 | ;;; This is a rudimentry backquote package written by D. King, |
c440e42b RS |
28 | ;;; king@kestrel, on 8/31/85. (` x) is a macro |
29 | ;;; that expands to a form that produces x. (` (a b ..)) is | |
30 | ;;; a macro that expands into a form that produces a list of what a b | |
31 | ;;; etc. would have produced. Any element can be of the form | |
32 | ;;; (, <form>) in which case the resulting form evaluates | |
33 | ;;; <form> before putting it into place, or (,@ <form>), in which | |
34 | ;;; case the evaluation of <form> is arranged for and each element | |
35 | ;;; of the result (which must be a (possibly null) list) is inserted. | |
36 | ;;; As an example, the immediately following macro push (v l) could | |
37 | ;;; have been written | |
38 | ;;; (defmacro push (v l) | |
39 | ;;; (` (setq (, l) (cons (,@ (list v l)))))) | |
40 | ;;; although | |
41 | ;;; (defmacro push (v l) | |
42 | ;;; (` (setq (, l) (cons (, v) (, l))))) | |
43 | ;;; is far more natural. The magic atoms , | |
44 | ;;; and ,@ are user-settable and list-valued. We recommend that | |
45 | ;;; things never be removed from this list lest you break something | |
46 | ;;; someone else wrote in the dim past that comes to be recompiled in | |
47 | ;;; the distant future. | |
48 | ||
49 | ;;; LIMITATIONS: tail consing is not handled correctly. Do not say | |
50 | ;;; (` (a . (, b))) - say (` (a (,@ b))) | |
51 | ;;; which works even if b is not list-valued. | |
52 | ;;; No attempt is made to handle vectors. (` [a (, b) c]) doesn't work. | |
53 | ;;; Sorry, you must say things like | |
54 | ;;; (` (a (,@ 'b))) to get (a . b) and | |
55 | ;;; (` ((, ',) c)) to get (, c) - [(` (a , b)) will work but is a bad habit] | |
56 | ;;; I haven't taught it the joys of nconc. | |
57 | ;;; (` atom) dies. (` (, atom)) or anything else is okay. | |
58 | ||
59 | ;;; BEWARE BEWARE BEWARE | |
60 | ;;; inclusion of (,atom) rather than (, atom) or (,@atom) rather than | |
61 | ;;; (,@ atom) will result in errors that will show up very late. | |
62 | ;;; This is so crunchy that I am considering including a check for | |
63 | ;;; this or changing the syntax to ... ,(<form>). RMS: opinion? | |
64 | ||
e5167999 | 65 | ;;; Code: |
c440e42b | 66 | |
c440e42b RS |
67 | ;;; a raft of general-purpose macros follows. See the nearest |
68 | ;;; Commonlisp manual. | |
69 | (defmacro bq-push (v l) | |
70 | "Pushes evaluated first form onto second unevaluated object | |
71 | a list-value atom" | |
72 | (list 'setq l (list 'cons v l))) | |
73 | ||
74 | (defmacro bq-caar (l) | |
75 | (list 'car (list 'car l))) | |
76 | ||
77 | (defmacro bq-cadr (l) | |
78 | (list 'car (list 'cdr l))) | |
79 | ||
80 | (defmacro bq-cdar (l) | |
81 | (list 'cdr (list 'car l))) | |
82 | ||
83 | ||
84 | ;;; These two advertised variables control what characters are used to | |
85 | ;;; unquote things. I have included , and ,@ as the unquote and | |
86 | ;;; splice operators, respectively, to give users of MIT CADR machine | |
87 | ;;; derivitive machines a warm, cosy feeling. | |
88 | ||
89 | (defconst backquote-unquote '(,) | |
90 | "*A list of all objects that stimulate unquoting in `. Memq test.") | |
91 | ||
92 | ||
93 | (defconst backquote-splice '(,@) | |
94 | "*A list of all objects that stimulate splicing in `. Memq test.") | |
95 | ||
96 | ||
97 | ;;; This is the interface | |
7229064d | 98 | ;;;###autoload |
c440e42b RS |
99 | (defmacro ` (form) |
100 | "(` FORM) is a macro that expands to code to construct FORM. | |
101 | Note that this is very slow in interpreted code, but fast if you compile. | |
102 | FORM is one or more nested lists, which are `almost quoted': | |
103 | They are copied recursively, with non-lists used unchanged in the copy. | |
104 | (` a b) == (list 'a 'b) constructs a new list with two elements, `a' and `b'. | |
105 | (` a (b c)) == (list 'a (list 'b 'c)) constructs two nested new lists. | |
106 | ||
107 | However, certain special lists are not copied. They specify substitution. | |
108 | Lists that look like (, EXP) are evaluated and the result is substituted. | |
109 | (` a (, (+ x 5))) == (list 'a (+ x 5)) | |
110 | ||
111 | Elements of the form (,@ EXP) are evaluated and then all the elements | |
112 | of the result are substituted. This result must be a list; it may | |
113 | be `nil'. | |
114 | ||
115 | As an example, a simple macro `push' could be written: | |
116 | (defmacro push (v l) | |
117 | (` (setq (, l) (cons (,@ (list v l)))))) | |
118 | or as | |
119 | (defmacro push (v l) | |
120 | (` (setq (, l) (cons (, v) (, l))))) | |
121 | ||
122 | LIMITATIONS: \"dotted lists\" are not allowed in FORM. | |
123 | The ultimate cdr of each list scanned by ` must be `nil'. | |
124 | \(This does not apply to constants inside expressions to be substituted.) | |
125 | ||
126 | Substitution elements are not allowed as the cdr | |
127 | of a cons cell. For example, (` (A . (, B))) does not work. | |
128 | Instead, write (` (A (,@ B))). | |
129 | ||
130 | You cannot construct vectors, only lists. Vectors are treated as | |
131 | constants. | |
132 | ||
133 | BEWARE BEWARE BEWARE | |
134 | Inclusion of (,ATOM) rather than (, ATOM) | |
135 | or of (,@ATOM) rather than (,@ ATOM) | |
136 | will result in errors that will show up very late." | |
137 | (bq-make-maker form)) | |
138 | ||
139 | ;;; We develop the method for building the desired list from | |
140 | ;;; the end towards the beginning. The contract is that there be a | |
141 | ;;; variable called state and a list called tailmaker, and that the form | |
142 | ;;; (cons state tailmaker) deliver the goods. Exception - if the | |
143 | ;;; state is quote the tailmaker is the form itself. | |
144 | ;;; This function takes a form and returns what I will call a maker in | |
145 | ;;; what follows. Evaluating the maker would produce the form, | |
146 | ;;; properly evaluated according to , and ,@ rules. | |
147 | ;;; I work backwards - it seemed a lot easier. The reason for this is | |
148 | ;;; if I'm in some sort of a routine building a maker and I switch | |
149 | ;;; gears, it seemed to me easier to jump into some other state and | |
150 | ;;; glue what I've already done to the end, than to to prepare that | |
151 | ;;; something and go back to put things together. | |
152 | (defun bq-make-maker (form) | |
153 | "Given one argument, a `mostly quoted' object, produces a maker. | |
154 | See backquote.el for details" | |
155 | (let ((tailmaker (quote nil)) (qc 0) (ec 0) (state nil)) | |
156 | (mapcar 'bq-iterative-list-builder (reverse form)) | |
157 | (and state | |
158 | (cond ((eq state 'quote) | |
fcb764df | 159 | (list state (if (equal form tailmaker) form tailmaker))) |
c440e42b RS |
160 | ((= (length tailmaker) 1) |
161 | (funcall (bq-cadr (assq state bq-singles)) tailmaker)) | |
162 | (t (cons state tailmaker)))))) | |
163 | ||
164 | ;;; There are exceptions - we wouldn't want to call append of one | |
165 | ;;; argument, for example. | |
166 | (defconst bq-singles '((quote bq-quotecar) | |
167 | (append car) | |
168 | (list bq-make-list) | |
169 | (cons bq-id))) | |
170 | ||
171 | (defun bq-id (x) x) | |
172 | ||
173 | (defun bq-quotecar (x) (list 'quote (car x))) | |
174 | ||
175 | (defun bq-make-list (x) (cons 'list x)) | |
176 | ||
177 | ;;; fr debugging use only | |
178 | ;(defun funcalll (a b) (funcall a b)) | |
179 | ;(defun funcalll (a b) (debug nil 'enter state tailmaker a b) | |
180 | ; (let ((ans (funcall a b))) (debug nil 'leave state tailmaker) | |
181 | ; ans)) | |
182 | ||
183 | ;;; Given a state/tailmaker pair that already knows how to make a | |
184 | ;;; partial tail of the desired form, this function knows how to add | |
185 | ;;; yet another element to the burgening list. There are four cases; | |
186 | ;;; the next item is an atom (which will certainly be quoted); a | |
187 | ;;; (, xxx), which will be evaluated and put into the list at the top | |
188 | ;;; level; a (,@ xxx), which will be evaluated and spliced in, or | |
189 | ;;; some other list, in which case we first compute the form's maker, | |
190 | ;;; and then we either launch into the quoted case if the maker's | |
191 | ;;; top level function is quote, or into the comma case if it isn't. | |
192 | ;;; The fourth case reduces to one of the other three, so here we have | |
193 | ;;; a choice of three ways to build tailmaker, and cit turns out we | |
194 | ;;; use five possible values of state (although someday I'll add | |
195 | ;;; nconcto the possible values of state). | |
196 | ;;; This maintains the invariant that (cons state tailmaker) is the | |
197 | ;;; maker for the elements of the tail we've eaten so far. | |
198 | (defun bq-iterative-list-builder (form) | |
199 | "Called by `bq-make-maker'. Adds a new item form to tailmaker, | |
200 | changing state if need be, so tailmaker and state constitute a recipe | |
201 | for making the list so far." | |
202 | (cond ((atom form) | |
203 | (funcall (bq-cadr (assq state bq-quotefns)) form)) | |
204 | ((memq (car form) backquote-unquote) | |
205 | (funcall (bq-cadr (assq state bq-evalfns)) (bq-cadr form))) | |
206 | ((memq (car form) backquote-splice) | |
207 | (funcall (bq-cadr (assq state bq-splicefns)) (bq-cadr form))) | |
208 | (t | |
209 | (let ((newform (bq-make-maker form))) | |
210 | (if (and (listp newform) (eq (car newform) 'quote)) | |
211 | (funcall (bq-cadr (assq state bq-quotefns)) (bq-cadr newform)) | |
212 | (funcall (bq-cadr (assq state bq-evalfns)) newform)))) | |
213 | )) | |
214 | ||
215 | ;;; We do a 2-d branch on the form of splicing and the old state. | |
216 | ;;; Here's fifteen functions' names... | |
217 | (defconst bq-splicefns '((nil bq-splicenil) | |
218 | (append bq-spliceappend) | |
219 | (list bq-splicelist) | |
220 | (quote bq-splicequote) | |
221 | (cons bq-splicecons))) | |
222 | ||
223 | (defconst bq-evalfns '((nil bq-evalnil) | |
224 | (append bq-evalappend) | |
225 | (list bq-evallist) | |
226 | (quote bq-evalquote) | |
227 | (cons bq-evalcons))) | |
228 | ||
229 | (defconst bq-quotefns '((nil bq-quotenil) | |
230 | (append bq-quoteappend) | |
231 | (list bq-quotelist) | |
232 | (quote bq-quotequote) | |
233 | (cons bq-quotecons))) | |
234 | ||
235 | ;;; The name of each function is | |
236 | ;;; (concat 'bq- <type-of-element-addition> <old-state>) | |
237 | ;;; I'll comment the non-obvious ones before the definitions... | |
238 | ;;; In what follows, uppercase letters and form will always be | |
239 | ;;; metavariables that don't need commas in backquotes, and I will | |
240 | ;;; assume the existence of something like matches that takes a | |
241 | ;;; backquote-like form and a value, binds metavariables and returns | |
242 | ;;; t if the pattern match is successful, returns nil otherwise. I | |
243 | ;;; will write such a goodie someday. | |
244 | ||
245 | ;;; (setq tailmaker | |
246 | ;;; (if (matches ((quote X) Y) tailmaker) | |
247 | ;;; (` ((quote (form X)) Y)) | |
248 | ;;; (` ((list form (quote X)) Y)))) | |
249 | ;;; (setq state 'append) | |
250 | (defun bq-quotecons (form) | |
251 | (if (and (listp (car tailmaker)) | |
252 | (eq (bq-caar tailmaker) 'quote)) | |
253 | (setq tailmaker | |
254 | (list (list 'quote (list form (bq-cadr (car tailmaker)))) | |
255 | (bq-cadr tailmaker))) | |
256 | (setq tailmaker | |
257 | (list (list 'list | |
258 | (list 'quote form) | |
259 | (car tailmaker)) | |
260 | (bq-cadr tailmaker)))) | |
261 | (setq state 'append)) | |
262 | ||
263 | (defun bq-quotequote (form) | |
264 | (bq-push form tailmaker)) | |
265 | ||
266 | ;;; Could be improved to convert (list 'a 'b 'c .. 'w x) | |
267 | ;;; to (append '(a b c .. w) x) | |
268 | ;;; when there are enough elements | |
269 | (defun bq-quotelist (form) | |
270 | (bq-push (list 'quote form) tailmaker)) | |
271 | ||
272 | ;;; (setq tailmaker | |
273 | ;;; (if (matches ((quote X) (,@ Y))) | |
274 | ;;; (` ((quote (, (cons form X))) (,@ Y))))) | |
275 | (defun bq-quoteappend (form) | |
276 | (cond ((and (listp tailmaker) | |
277 | (listp (car tailmaker)) | |
278 | (eq (bq-caar tailmaker) 'quote)) | |
279 | (rplaca (bq-cdar tailmaker) | |
280 | (cons form (car (bq-cdar tailmaker))))) | |
281 | (t (bq-push (list 'quote (list form)) tailmaker)))) | |
282 | ||
283 | (defun bq-quotenil (form) | |
284 | (setq tailmaker (list form)) | |
285 | (setq state 'quote)) | |
286 | ||
287 | ;;; (if (matches (X Y) tailmaker) ; it must | |
288 | ;;; (` ((list form X) Y))) | |
289 | (defun bq-evalcons (form) | |
290 | (setq tailmaker | |
291 | (list (list 'list form (car tailmaker)) | |
292 | (bq-cadr tailmaker))) | |
293 | (setq state 'append)) | |
294 | ||
295 | ;;; (if (matches (X Y Z (,@ W))) | |
296 | ;;; (progn (setq state 'append) | |
297 | ;;; (` ((list form) (quote (X Y Z (,@ W)))))) | |
298 | ;;; (progn (setq state 'list) | |
299 | ;;; (list form 'X 'Y .. ))) ; quote each one there is | |
300 | (defun bq-evalquote (form) | |
301 | (cond ((< (length tailmaker) 3) | |
302 | (setq tailmaker | |
303 | (cons form | |
304 | (mapcar (function (lambda (x) | |
305 | (list 'quote x))) | |
306 | tailmaker))) | |
307 | (setq state 'list)) | |
308 | (t | |
309 | (setq tailmaker | |
310 | (list (list 'list form) | |
311 | (list 'quote tailmaker))) | |
312 | (setq state 'append)))) | |
313 | ||
314 | (defun bq-evallist (form) | |
315 | (bq-push form tailmaker)) | |
316 | ||
317 | ;;; (cond ((matches ((list (,@ X)) (,@ Y))) | |
318 | ;;; (` ((list form (,@ X)) (,@ Y)))) | |
319 | ;;; ((matches (X)) | |
320 | ;;; (` (form (,@ X))) (setq state 'cons)) | |
321 | ;;; ((matches ((,@ X))) | |
322 | ;;; (` (form (,@ X))))) | |
323 | (defun bq-evalappend (form) | |
324 | (cond ((and (listp tailmaker) | |
325 | (listp (car tailmaker)) | |
326 | (eq (bq-caar tailmaker) 'list)) | |
327 | (rplacd (car tailmaker) | |
328 | (cons form (bq-cdar tailmaker)))) | |
329 | ((= (length tailmaker) 1) | |
330 | (setq tailmaker (cons form tailmaker) | |
331 | state 'cons)) | |
332 | (t (bq-push (list 'list form) tailmaker)))) | |
333 | ||
334 | (defun bq-evalnil (form) | |
335 | (setq tailmaker (list form) | |
336 | state 'list)) | |
337 | ||
338 | ;;; (if (matches (X Y)) ; it must | |
339 | ;;; (progn (setq state 'append) | |
340 | ;;; (` (form (cons X Y))))) ; couldn't think of anything clever | |
341 | (defun bq-splicecons (form) | |
342 | (setq tailmaker | |
343 | (list form | |
344 | (list 'cons (car tailmaker) (bq-cadr tailmaker))) | |
345 | state 'append)) | |
346 | ||
347 | (defun bq-splicequote (form) | |
348 | (setq tailmaker (list form (list 'quote tailmaker)) | |
349 | state 'append)) | |
350 | ||
351 | (defun bq-splicelist (form) | |
352 | (setq tailmaker (list form (cons 'list tailmaker)) | |
353 | state 'append)) | |
354 | ||
355 | (defun bq-spliceappend (form) | |
356 | (bq-push form tailmaker)) | |
357 | ||
358 | (defun bq-splicenil (form) | |
359 | (setq state 'append | |
360 | tailmaker (list form))) | |
49116ac0 JB |
361 | |
362 | (provide 'backquote) | |
363 | ||
c0274f38 | 364 | ;;; backquote.el ends here |