3 ;; JSCL is free software: you can redistribute it and/or
4 ;; modify it under the terms of the GNU General Public License as
5 ;; published by the Free Software Foundation, either version 3 of the
6 ;; License, or (at your option) any later version.
8 ;; JSCL is distributed in the hope that it will be useful, but
9 ;; WITHOUT ANY WARRANTY; without even the implied warranty of
10 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
11 ;; General Public License for more details.
13 ;; You should have received a copy of the GNU General Public License
14 ;; along with JSCL. If not, see <http://www.gnu.org/licenses/>.
16 ;;; Backquote implementation.
18 ;;; Author: Guy L. Steele Jr. Date: 27 December 1985
19 ;;; Tested under Symbolics Common Lisp and Lucid Common Lisp.
20 ;;; This software is in the public domain.
22 ;;; The following are unique tokens used during processing.
23 ;;; They need not be symbols; they need not even be atoms.
24 (defvar *comma* 'unquote)
25 (defvar *comma-atsign* 'unquote-splicing)
27 (defvar *bq-list* (make-symbol "BQ-LIST"))
28 (defvar *bq-append* (make-symbol "BQ-APPEND"))
29 (defvar *bq-list** (make-symbol "BQ-LIST*"))
30 (defvar *bq-nconc* (make-symbol "BQ-NCONC"))
31 (defvar *bq-clobberable* (make-symbol "BQ-CLOBBERABLE"))
32 (defvar *bq-quote* (make-symbol "BQ-QUOTE"))
33 (defvar *bq-quote-nil* (list *bq-quote* nil))
35 ;;; BACKQUOTE is an ordinary macro (not a read-macro) that processes
36 ;;; the expression foo, looking for occurrences of #:COMMA,
37 ;;; #:COMMA-ATSIGN, and #:COMMA-DOT. It constructs code in strict
38 ;;; accordance with the rules on pages 349-350 of the first edition
39 ;;; (pages 528-529 of this second edition). It then optionally
40 ;;; applies a code simplifier.
42 ;;; If the value of *BQ-SIMPLIFY* is non-NIL, then BACKQUOTE
43 ;;; processing applies the code simplifier. If the value is NIL,
44 ;;; then the code resulting from BACKQUOTE is exactly that
45 ;;; specified by the official rules.
46 (defparameter *bq-simplify* t)
48 (defmacro backquote (x)
49 (bq-completely-process x))
51 ;;; Backquote processing proceeds in three stages:
53 ;;; (1) BQ-PROCESS applies the rules to remove occurrences of
54 ;;; #:COMMA, #:COMMA-ATSIGN, and #:COMMA-DOT corresponding to
55 ;;; this level of BACKQUOTE. (It also causes embedded calls to
56 ;;; BACKQUOTE to be expanded so that nesting is properly handled.)
57 ;;; Code is produced that is expressed in terms of functions
58 ;;; #:BQ-LIST, #:BQ-APPEND, and #:BQ-CLOBBERABLE. This is done
59 ;;; so that the simplifier will simplify only list construction
60 ;;; functions actually generated by BACKQUOTE and will not involve
61 ;;; any user code in the simplification. #:BQ-LIST means LIST,
62 ;;; #:BQ-APPEND means APPEND, and #:BQ-CLOBBERABLE means IDENTITY
63 ;;; but indicates places where "%." was used and where NCONC may
64 ;;; therefore be introduced by the simplifier for efficiency.
66 ;;; (2) BQ-SIMPLIFY, if used, rewrites the code produced by
67 ;;; BQ-PROCESS to produce equivalent but faster code. The
68 ;;; additional functions #:BQ-LIST* and #:BQ-NCONC may be
69 ;;; introduced into the code.
71 ;;; (3) BQ-REMOVE-TOKENS goes through the code and replaces
72 ;;; #:BQ-LIST with LIST, #:BQ-APPEND with APPEND, and so on.
73 ;;; #:BQ-CLOBBERABLE is simply eliminated (a call to it being
74 ;;; replaced by its argument). #:BQ-LIST* is replaced by either
75 ;;; LIST* or CONS (the latter is used in the two-argument case,
76 ;;; purely to make the resulting code a tad more readable).
78 (defun bq-completely-process (x)
79 (let ((raw-result (bq-process x)))
80 (bq-remove-tokens (if *bq-simplify*
81 (bq-simplify raw-result)
87 ((eq (car x) 'backquote)
88 (bq-process (bq-completely-process (cadr x))))
89 ((eq (car x) *comma*) (cadr x))
90 ((eq (car x) *comma-atsign*)
91 (error ",@~S after `" (cadr x)))
92 ;; ((eq (car x) *comma-dot*)
93 ;; ;; (error ",.~S after `" (cadr x))
94 ;; (error "ill-formed"))
96 (q '() (cons (bracket (car p)) q)))
99 (nreconc q (list (list *bq-quote* p)))))
100 (when (eq (car p) *comma*)
101 (unless (null (cddr p))
102 (error "Malformed ,~S" p))
103 (return (cons *bq-append*
104 (nreconc q (list (cadr p))))))
105 (when (eq (car p) *comma-atsign*)
106 (error "Dotted ,@~S" p))
107 ;; (when (eq (car p) *comma-dot*)
108 ;; ;; (error "Dotted ,.~S" p)
112 ;;; This implements the bracket operator of the formal rules.
115 (list *bq-list* (bq-process x)))
116 ((eq (car x) *comma*)
117 (list *bq-list* (cadr x)))
118 ((eq (car x) *comma-atsign*)
120 ;; ((eq (car x) *comma-dot*)
121 ;; (list *bq-clobberable* (cadr x)))
122 (t (list *bq-list* (bq-process x)))))
124 ;;; This auxiliary function is like MAPCAR but has two extra
125 ;;; purposes: (1) it handles dotted lists; (2) it tries to make
126 ;;; the result share with the argument x as much as possible.
127 (defun maptree (fn x)
130 (let ((a (funcall fn (car x)))
131 (d (maptree fn (cdr x))))
132 (if (and (eql a (car x)) (eql d (cdr x)))
136 ;;; This predicate is true of a form that when read looked
137 ;;; like %@foo or %.foo.
138 (defun bq-splicing-frob (x)
140 (or (eq (car x) *comma-atsign*)
141 ;; (eq (car x) *comma-dot*)
144 ;;; This predicate is true of a form that when read
145 ;;; looked like %@foo or %.foo or just plain %foo.
148 (or (eq (car x) *comma*)
149 (eq (car x) *comma-atsign*)
150 ;; (eq (car x) *comma-dot*)
153 ;;; The simplifier essentially looks for calls to #:BQ-APPEND and
154 ;;; tries to simplify them. The arguments to #:BQ-APPEND are
155 ;;; processed from right to left, building up a replacement form.
156 ;;; At each step a number of special cases are handled that,
157 ;;; loosely speaking, look like this:
159 ;;; (APPEND (LIST a b c) foo) => (LIST* a b c foo)
160 ;;; provided a, b, c are not splicing frobs
161 ;;; (APPEND (LIST* a b c) foo) => (LIST* a b (APPEND c foo))
162 ;;; provided a, b, c are not splicing frobs
163 ;;; (APPEND (QUOTE (x)) foo) => (LIST* (QUOTE x) foo)
164 ;;; (APPEND (CLOBBERABLE x) foo) => (NCONC x foo)
165 (defun bq-simplify (x)
168 (let ((x (if (eq (car x) *bq-quote*)
170 (maptree #'bq-simplify x))))
171 (if (not (eq (car x) *bq-append*))
173 (bq-simplify-args x)))))
175 (defun bq-simplify-args (x)
176 (do ((args (reverse (cdr x)) (cdr args))
179 (cond ((atom (car args))
180 (bq-attach-append *bq-append* (car args) result))
181 ((and (eq (caar args) *bq-list*)
182 (notany #'bq-splicing-frob (cdar args)))
183 (bq-attach-conses (cdar args) result))
184 ((and (eq (caar args) *bq-list**)
185 (notany #'bq-splicing-frob (cdar args)))
187 (reverse (cdr (reverse (cdar args))))
188 (bq-attach-append *bq-append*
189 (car (last (car args)))
191 ((and (eq (caar args) *bq-quote*)
193 (not (bq-frob (cadar args)))
195 (bq-attach-conses (list (list *bq-quote*
198 ((eq (caar args) *bq-clobberable*)
199 (bq-attach-append *bq-nconc* (cadar args) result))
200 (t (bq-attach-append *bq-append*
203 ((null args) result)))
205 (defun null-or-quoted (x)
206 (or (null x) (and (consp x) (eq (car x) *bq-quote*))))
208 ;;; When BQ-ATTACH-APPEND is called, the OP should be #:BQ-APPEND
209 ;;; or #:BQ-NCONC. This produces a form (op item result) but
210 ;;; some simplifications are done on the fly:
212 ;;; (op '(a b c) '(d e f g)) => '(a b c d e f g)
213 ;;; (op item 'nil) => item, provided item is not a splicable frob
214 ;;; (op item 'nil) => (op item), if item is a splicable frob
215 ;;; (op item (op a b c)) => (op item a b c)
216 (defun bq-attach-append (op item result)
217 (cond ((and (null-or-quoted item) (null-or-quoted result))
218 (list *bq-quote* (append (cadr item) (cadr result))))
219 ((or (null result) (equal result *bq-quote-nil*))
220 (if (bq-splicing-frob item) (list op item) item))
221 ((and (consp result) (eq (car result) op))
222 (list* (car result) item (cdr result)))
223 (t (list op item result))))
225 ;;; The effect of BQ-ATTACH-CONSES is to produce a form as if by
226 ;;; `(LIST* ,@items ,result) but some simplifications are done
229 ;;; (LIST* 'a 'b 'c 'd) => '(a b c . d)
230 ;;; (LIST* a b c 'nil) => (LIST a b c)
231 ;;; (LIST* a b c (LIST* d e f g)) => (LIST* a b c d e f g)
232 ;;; (LIST* a b c (LIST d e f g)) => (LIST a b c d e f g)
233 (defun bq-attach-conses (items result)
234 (cond ((and (every #'null-or-quoted items)
235 (null-or-quoted result))
237 (append (mapcar #'cadr items) (cadr result))))
238 ((or (null result) (equal result *bq-quote-nil*))
239 (cons *bq-list* items))
241 (or (eq (car result) *bq-list*)
242 (eq (car result) *bq-list**)))
243 (cons (car result) (append items (cdr result))))
244 (t (cons *bq-list** (append items (list result))))))
246 ;;; Removes funny tokens and changes (#:BQ-LIST* a b) into
247 ;;; (CONS a b) instead of (LIST* a b), purely for readability.
248 (defun bq-remove-tokens (x)
249 (cond ((eq x *bq-list*) 'list)
250 ((eq x *bq-append*) 'append)
251 ((eq x *bq-nconc*) 'nconc)
252 ((eq x *bq-list**) 'list*)
253 ((eq x *bq-quote*) 'quote)
255 ((eq (car x) *bq-clobberable*)
256 (bq-remove-tokens (cadr x)))
257 ((and (eq (car x) *bq-list**)
260 (cons 'cons (maptree #'bq-remove-tokens (cdr x))))
261 (t (maptree #'bq-remove-tokens x))))