+;;; Backquote implementation.
+;;;
+;;; Author: Guy L. Steele Jr. Date: 27 December 1985
+;;; Tested under Symbolics Common Lisp and Lucid Common Lisp.
+;;; This software is in the public domain.
+
+;;; The following are unique tokens used during processing.
+;;; They need not be symbols; they need not even be atoms.
+(defvar *comma* 'unquote)
+(defvar *comma-atsign* 'unquote-splicing)
+
+(defvar *bq-list* (make-symbol "BQ-LIST"))
+(defvar *bq-append* (make-symbol "BQ-APPEND"))
+(defvar *bq-list** (make-symbol "BQ-LIST*"))
+(defvar *bq-nconc* (make-symbol "BQ-NCONC"))
+(defvar *bq-clobberable* (make-symbol "BQ-CLOBBERABLE"))
+(defvar *bq-quote* (make-symbol "BQ-QUOTE"))
+(defvar *bq-quote-nil* (list *bq-quote* nil))
+
+;;; BACKQUOTE is an ordinary macro (not a read-macro) that processes
+;;; the expression foo, looking for occurrences of #:COMMA,
+;;; #:COMMA-ATSIGN, and #:COMMA-DOT. It constructs code in strict
+;;; accordance with the rules on pages 349-350 of the first edition
+;;; (pages 528-529 of this second edition). It then optionally
+;;; applies a code simplifier.
+
+;;; If the value of *BQ-SIMPLIFY* is non-NIL, then BACKQUOTE
+;;; processing applies the code simplifier. If the value is NIL,
+;;; then the code resulting from BACKQUOTE is exactly that
+;;; specified by the official rules.
+(defparameter *bq-simplify* t)
+
+(defmacro backquote (x)
+ (bq-completely-process x))
+
+;;; Backquote processing proceeds in three stages:
+;;;
+;;; (1) BQ-PROCESS applies the rules to remove occurrences of
+;;; #:COMMA, #:COMMA-ATSIGN, and #:COMMA-DOT corresponding to
+;;; this level of BACKQUOTE. (It also causes embedded calls to
+;;; BACKQUOTE to be expanded so that nesting is properly handled.)
+;;; Code is produced that is expressed in terms of functions
+;;; #:BQ-LIST, #:BQ-APPEND, and #:BQ-CLOBBERABLE. This is done
+;;; so that the simplifier will simplify only list construction
+;;; functions actually generated by BACKQUOTE and will not involve
+;;; any user code in the simplification. #:BQ-LIST means LIST,
+;;; #:BQ-APPEND means APPEND, and #:BQ-CLOBBERABLE means IDENTITY
+;;; but indicates places where "%." was used and where NCONC may
+;;; therefore be introduced by the simplifier for efficiency.
+;;;
+;;; (2) BQ-SIMPLIFY, if used, rewrites the code produced by
+;;; BQ-PROCESS to produce equivalent but faster code. The
+;;; additional functions #:BQ-LIST* and #:BQ-NCONC may be
+;;; introduced into the code.
+;;;
+;;; (3) BQ-REMOVE-TOKENS goes through the code and replaces
+;;; #:BQ-LIST with LIST, #:BQ-APPEND with APPEND, and so on.
+;;; #:BQ-CLOBBERABLE is simply eliminated (a call to it being
+;;; replaced by its argument). #:BQ-LIST* is replaced by either
+;;; LIST* or CONS (the latter is used in the two-argument case,
+;;; purely to make the resulting code a tad more readable).
+
+(defun bq-completely-process (x)
+ (let ((raw-result (bq-process x)))
+ (bq-remove-tokens (if *bq-simplify*
+ (bq-simplify raw-result)
+ raw-result))))
+
+(defun bq-process (x)
+ (cond ((atom x)
+ (list *bq-quote* x))
+ ((eq (car x) 'backquote)
+ (bq-process (bq-completely-process (cadr x))))
+ ((eq (car x) *comma*) (cadr x))
+ ((eq (car x) *comma-atsign*)
+ ;; (error ",@~S after `" (cadr x))
+ (error "ill-formed"))
+ ;; ((eq (car x) *comma-dot*)
+ ;; ;; (error ",.~S after `" (cadr x))
+ ;; (error "ill-formed"))
+ (t (do ((p x (cdr p))
+ (q '() (cons (bracket (car p)) q)))
+ ((atom p)
+ (cons *bq-append*
+ (nreconc q (list (list *bq-quote* p)))))
+ (when (eq (car p) *comma*)
+ (unless (null (cddr p))
+ ;; (error "Malformed ,~S" p)
+ (error "Malformed"))
+ (return (cons *bq-append*
+ (nreconc q (list (cadr p))))))
+ (when (eq (car p) *comma-atsign*)
+ ;; (error "Dotted ,@~S" p)
+ (error "Dotted"))
+ ;; (when (eq (car p) *comma-dot*)
+ ;; ;; (error "Dotted ,.~S" p)
+ ;; (error "Dotted"))
+ ))))
+
+;;; This implements the bracket operator of the formal rules.
+(defun bracket (x)
+ (cond ((atom x)
+ (list *bq-list* (bq-process x)))
+ ((eq (car x) *comma*)
+ (list *bq-list* (cadr x)))
+ ((eq (car x) *comma-atsign*)
+ (cadr x))
+ ;; ((eq (car x) *comma-dot*)
+ ;; (list *bq-clobberable* (cadr x)))
+ (t (list *bq-list* (bq-process x)))))
+
+;;; This auxiliary function is like MAPCAR but has two extra
+;;; purposes: (1) it handles dotted lists; (2) it tries to make
+;;; the result share with the argument x as much as possible.
+(defun maptree (fn x)
+ (if (atom x)
+ (funcall fn x)
+ (let ((a (funcall fn (car x)))
+ (d (maptree fn (cdr x))))
+ (if (and (eql a (car x)) (eql d (cdr x)))
+ x
+ (cons a d)))))
+
+;;; This predicate is true of a form that when read looked
+;;; like %@foo or %.foo.
+(defun bq-splicing-frob (x)
+ (and (consp x)
+ (or (eq (car x) *comma-atsign*)
+ ;; (eq (car x) *comma-dot*)
+ )))
+
+;;; This predicate is true of a form that when read
+;;; looked like %@foo or %.foo or just plain %foo.
+(defun bq-frob (x)
+ (and (consp x)
+ (or (eq (car x) *comma*)
+ (eq (car x) *comma-atsign*)
+ ;; (eq (car x) *comma-dot*)
+ )))
+
+;;; The simplifier essentially looks for calls to #:BQ-APPEND and
+;;; tries to simplify them. The arguments to #:BQ-APPEND are
+;;; processed from right to left, building up a replacement form.
+;;; At each step a number of special cases are handled that,
+;;; loosely speaking, look like this:
+;;;
+;;; (APPEND (LIST a b c) foo) => (LIST* a b c foo)
+;;; provided a, b, c are not splicing frobs
+;;; (APPEND (LIST* a b c) foo) => (LIST* a b (APPEND c foo))
+;;; provided a, b, c are not splicing frobs
+;;; (APPEND (QUOTE (x)) foo) => (LIST* (QUOTE x) foo)
+;;; (APPEND (CLOBBERABLE x) foo) => (NCONC x foo)
+(defun bq-simplify (x)
+ (if (atom x)
+ x
+ (let ((x (if (eq (car x) *bq-quote*)
+ x
+ (maptree #'bq-simplify x))))
+ (if (not (eq (car x) *bq-append*))
+ x
+ (bq-simplify-args x)))))
+
+(defun bq-simplify-args (x)
+ (do ((args (reverse (cdr x)) (cdr args))
+ (result
+ nil
+ (cond ((atom (car args))
+ (bq-attach-append *bq-append* (car args) result))
+ ((and (eq (caar args) *bq-list*)
+ (notany #'bq-splicing-frob (cdar args)))
+ (bq-attach-conses (cdar args) result))
+ ((and (eq (caar args) *bq-list**)
+ (notany #'bq-splicing-frob (cdar args)))
+ (bq-attach-conses
+ (reverse (cdr (reverse (cdar args))))
+ (bq-attach-append *bq-append*
+ (car (last (car args)))
+ result)))
+ ((and (eq (caar args) *bq-quote*)
+ (consp (cadar args))
+ (not (bq-frob (cadar args)))
+ (null (cddar args)))
+ (bq-attach-conses (list (list *bq-quote*
+ (caadar args)))
+ result))
+ ((eq (caar args) *bq-clobberable*)
+ (bq-attach-append *bq-nconc* (cadar args) result))
+ (t (bq-attach-append *bq-append*
+ (car args)
+ result)))))
+ ((null args) result)))
+
+(defun null-or-quoted (x)
+ (or (null x) (and (consp x) (eq (car x) *bq-quote*))))
+
+;;; When BQ-ATTACH-APPEND is called, the OP should be #:BQ-APPEND
+;;; or #:BQ-NCONC. This produces a form (op item result) but
+;;; some simplifications are done on the fly:
+;;;
+;;; (op '(a b c) '(d e f g)) => '(a b c d e f g)
+;;; (op item 'nil) => item, provided item is not a splicable frob
+;;; (op item 'nil) => (op item), if item is a splicable frob
+;;; (op item (op a b c)) => (op item a b c)
+(defun bq-attach-append (op item result)
+ (cond ((and (null-or-quoted item) (null-or-quoted result))
+ (list *bq-quote* (append (cadr item) (cadr result))))
+ ((or (null result) (equal result *bq-quote-nil*))
+ (if (bq-splicing-frob item) (list op item) item))
+ ((and (consp result) (eq (car result) op))
+ (list* (car result) item (cdr result)))
+ (t (list op item result))))
+
+;;; The effect of BQ-ATTACH-CONSES is to produce a form as if by
+;;; `(LIST* ,@items ,result) but some simplifications are done
+;;; on the fly.
+;;;
+;;; (LIST* 'a 'b 'c 'd) => '(a b c . d)
+;;; (LIST* a b c 'nil) => (LIST a b c)
+;;; (LIST* a b c (LIST* d e f g)) => (LIST* a b c d e f g)
+;;; (LIST* a b c (LIST d e f g)) => (LIST a b c d e f g)
+(defun bq-attach-conses (items result)
+ (cond ((and (every #'null-or-quoted items)
+ (null-or-quoted result))
+ (list *bq-quote*
+ (append (mapcar #'cadr items) (cadr result))))
+ ((or (null result) (equal result *bq-quote-nil*))
+ (cons *bq-list* items))
+ ((and (consp result)
+ (or (eq (car result) *bq-list*)
+ (eq (car result) *bq-list**)))
+ (cons (car result) (append items (cdr result))))
+ (t (cons *bq-list** (append items (list result))))))
+
+;;; Removes funny tokens and changes (#:BQ-LIST* a b) into
+;;; (CONS a b) instead of (LIST* a b), purely for readability.
+(defun bq-remove-tokens (x)
+ (cond ((eq x *bq-list*) 'list)
+ ((eq x *bq-append*) 'append)
+ ((eq x *bq-nconc*) 'nconc)
+ ((eq x *bq-list**) 'list*)
+ ((eq x *bq-quote*) 'quote)
+ ((atom x) x)
+ ((eq (car x) *bq-clobberable*)
+ (bq-remove-tokens (cadr x)))
+ ((and (eq (car x) *bq-list**)
+ (consp (cddr x))
+ (null (cdddr x)))
+ (cons 'cons (maptree #'bq-remove-tokens (cdr x))))
+ (t (maptree #'bq-remove-tokens x))))
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