path: root/lisp/emacs-lisp/cust-print.el

;; cus-print.el -- handles print-level and print-circle.

;; LCD Archive Entry:
;; custom-print|Daniel LaLiberte|liberte@cs.uiuc.edu
;; |Handle print-level, print-circle and more.
;; |$Date: Tue Mar 17, 1992$|$Revision: 1.0$|

;; Copyright (C) 1992 Free Software Foundation, Inc.

;; This file is part of GNU Emacs.

;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 1, or (at your option)
;; any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING.  If not, write to
;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

;; This package provides a general print handler for prin1 and princ
;; that supports print-level and print-circle, and by the way,
;; print-length since the standard routines are being replaced.  Also,
;; to print custom types constructed from lists and vectors, use
;; custom-print-list and custom-print-vector.  See the documentation
;; strings of these variables for more details.  

;; If the results of your expressions contain circular references to
;; other parts of the same structure, the standard Emacs print
;; subroutines may fail to print with an untrappable error,
;; "Apparently circular structure being printed".  If you only use cdr
;; circular lists (where cdrs of lists point back; what is the right
;; term here?), you can limit the length of printing with
;; print-length.  But car circular lists and circular vectors generate
;; the above mentioned untrappable error in Emacs version 18.  Version
;; 19 will support print-level, but it is often useful to get a better
;; print representation of circular structures; the print-circle
;; option may be used to print more concise representations.

;; There are two main ways to use this package.  First, you may
;; replace prin1, princ, and some subroutines that use them by calling
;; install-custom-print-funcs so that any use of these functions in
;; lisp code will be affected.  Second, you could call the custom
;; routines directly, thus only affecting the printing that requires
;; them.

;; Note that subroutines which call print subroutines directly will not
;; use the custom print functions.  In particular, the evaluation
;; functions like eval-region call the print subroutines directly.
;; Therefore, evaluating (aref circ-list 0), which calls error
;; directly (because circ-list is not an array), will jump to the top
;; level instead of printing the circular list.

;; Obviously the right way to implement this custom-print facility
;; is in C.  Please volunteer since I don't have the time or need.

;; Implementation design: we want to use the same list and vector
;; processing algorithm for all versions of prin1 and princ, since how
;; the processing is done depends on print-length, print-level, and
;; print-circle.  For circle printing, a preprocessing step is
;; required before the final printing.  Thanks to Jamie Zawinski
;; for motivation and algorithms.

;;=========================================================
;; export list:

;; print-level
;; print-circle

;; custom-print-list
;; custom-print-vector
;; add-custom-print-list
;; add-custom-print-vector

;; install-custom-print-funcs
;; uninstall-custom-print-funcs

;; custom-prin1
;; custom-princ
;; custom-prin1-to-string
;; custom-print
;; custom-format
;; custom-message
;; custom-error


(provide 'custom-print)
;; Abbreviated package name: "CP"

;;(defvar print-length nil
;;  "*Controls how many elements of a list, at each level, are printed.
;;This is defined by emacs.")

(defvar print-level nil
  "*Controls how many levels deep a nested data object will print.  

If nil, printing proceeds recursively and may lead to
max-lisp-eval-depth being exceeded or an untrappable error may occur:
\"Apparently circular structure being printed.\"   Also see
print-length and print-circle.

If non-nil, components at levels equal to or greater than print-level
are printed simply as \"#\".  The object to be printed is at level 0,
and if the object is a list or vector, its top-level components are at
level 1.")


(defvar print-circle nil
  "*Controls the printing of recursive structures.  

If nil, printing proceeds recursively and may lead to
max-lisp-eval-depth being exceeded or an untrappable error may occur:
\"Apparently circular structure being printed.\"  Also see
print-length and print-level.

If non-nil, shared substructures anywhere in the structure are printed
with \"#n=\" before the first occurance (in the order of the print
representation) and \"#n#\" in place of each subsequent occurance,
where n is a positive decimal integer.

Currently, there is no way to read this representation in Emacs.")


(defconst custom-print-list
  nil
  ;; e.g.  '((floatp . float-to-string))
  "If non-nil, an alist for printing of custom list objects.  
Pairs are of the form (pred . converter).  If the predicate is true
for an object, the converter is called with the object and should
return a string which will be printed with princ.  
Also see custom-print-vector.")

(defconst custom-print-vector
  nil
  "If non-nil, an alist for printing of custom vector objects.  
Pairs are of the form (pred . converter).  If the predicate is true
for an object, the converter is called with the object and should
return a string which will be printed with princ.  
Also see custom-print-list.")


(defun add-custom-print-list (pred converter)
  "Add the pair, a PREDICATE and a CONVERTER, to custom-print-list.
Any pair that has the same PREDICATE is first removed."
  (setq custom-print-list (cons (cons pred converter) 
				(delq (assq pred custom-print-list)
				      custom-print-list))))
;; e.g. (add-custom-print-list 'floatp 'float-to-string)


(defun add-custom-print-vector (pred converter)
  "Add the pair, a PREDICATE and a CONVERTER, to custom-print-vector.
Any pair that has the same PREDICATE is first removed."
  (setq custom-print-vector (cons (cons pred converter) 
				  (delq (assq pred custom-print-vector)
					custom-print-vector))))


;;====================================================
;; Saving and restoring internal printing routines.

(defun CP::set-function-cell (symbol-pair)
  (fset (car symbol-pair) 
	(symbol-function (car (cdr symbol-pair)))))


(if (not (fboundp 'CP::internal-prin1))
    (mapcar 'CP::set-function-cell
	    '((CP::internal-prin1 prin1)
	      (CP::internal-princ princ)
	      (CP::internal-print print)
	      (CP::internal-prin1-to-string prin1-to-string)
	      (CP::internal-format format)
	      (CP::internal-message message)
	      (CP::internal-error error))))


(defun install-custom-print-funcs ()
  "Replace print functions with general, customizable, lisp versions.
The internal subroutines are saved away and may be recovered with
uninstall-custom-print-funcs."
  (interactive)
  (mapcar 'CP::set-function-cell
	  '((prin1 custom-prin1)
	    (princ custom-princ)
	    (print custom-print)
	    (prin1-to-string custom-prin1-to-string)
	    (format custom-format)
	    (message custom-message)
	    (error custom-error)
	    )))
  
(defun uninstall-custom-print-funcs ()
  "Reset print functions to their internal subroutines."
  (interactive)
  (mapcar 'CP::set-function-cell
	  '((prin1 CP::internal-prin1)
	    (princ CP::internal-princ)
	    (print CP::internal-print)
	    (prin1-to-string CP::internal-prin1-to-string)
	    (format CP::internal-format)
	    (message CP::internal-message)
	    (error CP::internal-error)
	    )))


;;===============================================================
;; Lisp replacements for prin1 and princ and for subrs that use prin1 
;; (or princ) -- so far only the printing and formatting subrs.

(defun custom-prin1 (object &optional stream)
  "Replacement for standard prin1 that uses the appropriate
printer depending on the values of print-level and print-circle (which see).

Output the printed representation of OBJECT, any Lisp object.
Quoting characters are printed when needed to make output that `read'
can handle, whenever this is possible.
Output stream is STREAM, or value of `standard-output' (which see)."
  (CP::top-level object stream 'CP::internal-prin1))


(defun custom-princ (object &optional stream)
  "Same as custom-prin1 except no quoting."
  (CP::top-level object stream 'CP::internal-princ))

(defun custom-prin1-to-string-func (c)
  "Stream function for custom-prin1-to-string."
  (setq prin1-chars (cons c prin1-chars)))

(defun custom-prin1-to-string (object)
  "Replacement for standard prin1-to-string."
  (let ((prin1-chars nil))
    (custom-prin1 object 'custom-prin1-to-string-func)
    (concat (nreverse prin1-chars))))


(defun custom-print (object &optional stream)
  "Replacement for standard print."
  (CP::internal-princ "\n")
  (custom-prin1 object stream)
  (CP::internal-princ "\n"))


(defun custom-format (fmt &rest args)
  "Replacement for standard format.

Calls format after first making strings for list or vector args.
The format specification for such args should be %s in any case, so a
string argument will also work.  The string is generated with
custom-prin1-to-string, which quotes quotable characters."
  (apply 'CP::internal-format fmt
	 (mapcar (function (lambda (arg)
			     (if (or (listp arg) (vectorp arg))
				 (custom-prin1-to-string arg)
			       arg)))
		 args)))
	    
  

(defun custom-message (fmt &rest args)
  "Replacement for standard message that works like custom-format."
  ;; It doesnt work to princ the result of custom-format
  ;; because the echo area requires special handling
  ;; to avoid duplicating the output.  CP::internal-message does it right.
  ;; (CP::internal-princ (apply 'custom-format fmt args))
  (apply 'CP::internal-message  fmt
	 (mapcar (function (lambda (arg)
			     (if (or (listp arg) (vectorp arg))
				 (custom-prin1-to-string arg)
			       arg)))
		 args)))
	    

(defun custom-error (fmt &rest args)
  "Replacement for standard error that uses custom-format"
  (signal 'error (list (apply 'custom-format fmt args))))


;;=========================================
;; Support for custom prin1 and princ

(defun CP::top-level (object stream internal-printer)
  "Set up for printing."
  (let ((standard-output (or stream standard-output))
	(circle-table (and print-circle (CP::preprocess-circle-tree object)))
	(level (or print-level -1))
	)

    (fset 'CP::internal-printer internal-printer)
    (fset 'CP::low-level-prin 
	  (cond
	   ((or custom-print-list
		custom-print-vector
		print-level ; comment out for version 19
		)
	    'CP::custom-object)
	   (circle-table
	    'CP::object)
	   (t 'CP::internal-printer)))
    (fset 'CP::prin (if circle-table 'CP::circular 'CP::low-level-prin))

    (CP::prin object)
    object))


(defun CP::object (object)
  "Test object type and print accordingly."
  ;; Could be called as either CP::low-level-prin or CP::prin.
  (cond 
   ((null object) (CP::internal-printer object))
   ((consp object) (CP::list object))
   ((vectorp object) (CP::vector object))
   ;; All other types, just print.
   (t (CP::internal-printer object))))


(defun CP::custom-object (object)
  "Test object type and print accordingly."
  ;; Could be called as either CP::low-level-prin or CP::prin.
  (cond 
   ((null object) (CP::internal-printer object))

   ((consp object) 
    (or (and custom-print-list
	     (CP::custom-object1 object custom-print-list))
	(CP::list object)))

   ((vectorp object) 
    (or (and custom-print-vector
	     (CP::custom-object1 object custom-print-vector))
	(CP::vector object)))

   ;; All other types, just print.
   (t (CP::internal-printer object))))


(defun CP::custom-object1 (object alist)
  "Helper for CP::custom-object.
Print the custom OBJECT using the custom type ALIST.
For the first predicate that matches the object, the corresponding
converter is evaluated with the object and the string that results is
printed with princ.  Return nil if no predicte matches the object."
  (while (and alist (not (funcall (car (car alist)) object)))
    (setq alist (cdr alist)))
  ;; If alist is not null, then something matched.
  (if alist
      (CP::internal-princ
       (funcall (cdr (car alist)) object) ; returns string
       )))


(defun CP::circular (object)
  "Printer for prin1 and princ that handles circular structures.
If OBJECT appears multiply, and has not yet been printed,
prefix with label; if it has been printed, use #n# instead.
Otherwise, print normally."
  (let ((tag (assq object circle-table)))
    (if tag
	(let ((id (cdr tag)))
	  (if (> id 0)
	      (progn
		;; Already printed, so just print id.
		(CP::internal-princ "#")
		(CP::internal-princ id)
		(CP::internal-princ "#"))
	    ;; Not printed yet, so label with id and print object.
	    (setcdr tag (- id)) ; mark it as printed
	    (CP::internal-princ "#")
	    (CP::internal-princ (- id))
	    (CP::internal-princ "=")
	    (CP::low-level-prin object)
	    ))
      ;; Not repeated in structure.
      (CP::low-level-prin object))))


;;================================================
;; List and vector processing for print functions.

(defun CP::list (list)
  "Print a list using print-length, print-level, and print-circle."
  (if (= level 0)
      (CP::internal-princ "#")
    (let ((level (1- level)))
      (CP::internal-princ "(")
      (let ((length (or print-length 0)))

	;; Print the first element always (even if length = 0).
	(CP::prin (car list))
	(setq list (cdr list))
	(if list (CP::internal-princ " "))
	(setq length (1- length))

	;; Print the rest of the elements.
	(while (and list (/= 0 length))
	  (if (and (listp list)
		   (not (assq list circle-table)))
	      (progn
		(CP::prin (car list))
		(setq list (cdr list)))

	    ;; cdr is not a list, or it is in circle-table.
	    (CP::internal-princ ". ")
	    (CP::prin list)
	    (setq list nil))

	  (setq length (1- length))
	  (if list (CP::internal-princ " ")))

	(if (and list (= length 0)) (CP::internal-princ "..."))
	(CP::internal-princ ")"))))
  list)


(defun CP::vector (vector)
  "Print a vector using print-length, print-level, and print-circle."
  (if (= level 0)
      (CP::internal-princ "#")
    (let ((level (1- level))
	  (i 0)
	  (len (length vector)))
      (CP::internal-princ "[")

      (if print-length
	  (setq len (min print-length len)))
      ;; Print the elements
      (while (< i len)
	(CP::prin (aref vector i))
	(setq i (1+ i))
	(if (< i (length vector)) (CP::internal-princ " ")))

      (if (< i (length vector)) (CP::internal-princ "..."))
      (CP::internal-princ "]")
      ))
  vector)


;;==================================
;; Circular structure preprocessing

(defun CP::preprocess-circle-tree (object)
  ;; Fill up the table.  
  (let (;; Table of tags for each object in an object to be printed.
	;; A tag is of the form:
	;; ( <object> <nil-t-or-id-number> )
	;; The id-number is generated after the entire table has been computed.
	;; During walk through, the real circle-table lives in the cdr so we
	;; can use setcdr to add new elements instead of having to setq the
	;; variable sometimes (poor man's locf).
	(circle-table (list nil)))
    (CP::walk-circle-tree object)

    ;; Reverse table so it is in the order that the objects will be printed.
    ;; This pass could be avoided if we always added to the end of the
    ;; table with setcdr in walk-circle-tree.
    (setcdr circle-table (nreverse (cdr circle-table)))

    ;; Walk through the table, assigning id-numbers to those
    ;; objects which will be printed using #N= syntax.  Delete those
    ;; objects which will be printed only once (to speed up assq later).
    (let ((rest circle-table)
	  (id -1))
      (while (cdr rest)
	(let ((tag (car (cdr rest))))
	  (cond ((cdr tag)
		 (setcdr tag id)
		 (setq id (1- id))
		 (setq rest (cdr rest)))
		;; Else delete this object.
		(t (setcdr rest (cdr (cdr rest))))))
	))
    ;; Drop the car.
    (cdr circle-table)
    ))



(defun CP::walk-circle-tree (object)
  (let (read-equivalent-p tag)
    (while object
      (setq read-equivalent-p (or (numberp object) (symbolp object))
	    tag (and (not read-equivalent-p)
		     (assq object (cdr circle-table))))
      (cond (tag
	     ;; Seen this object already, so note that.
	     (setcdr tag t))

	    ((not read-equivalent-p)
	     ;; Add a tag for this object.
	     (setcdr circle-table
		     (cons (list object)
			   (cdr circle-table)))))
      (setq object
	    (cond 
	     (tag ;; No need to descend since we have already.
	      nil)

	     ((consp object)
	      ;; Walk the car of the list recursively.
	      (CP::walk-circle-tree (car object))
	      ;; But walk the cdr with the above while loop
	      ;; to avoid problems with max-lisp-eval-depth.
	      ;; And it should be faster than recursion.
	      (cdr object))

	     ((vectorp object)
	      ;; Walk the vector.
	      (let ((i (length object))
		    (j 0))
		(while (< j i)
		  (CP::walk-circle-tree (aref object j))
		  (setq j (1+ j))))))))))



;;=======================================

(quote 
 examples
 
 (progn
   ;; Create some circular structures.
   (setq circ-sym (let ((x (make-symbol "FOO"))) (list x x)))
   (setq circ-list (list 'a 'b (vector 1 2 3 4) 'd 'e 'f))
   (setcar (nthcdr 3 circ-list) circ-list)
   (aset (nth 2 circ-list) 2 circ-list)
   (setq dotted-circ-list (list 'a 'b 'c))
   (setcdr (cdr (cdr dotted-circ-list)) dotted-circ-list)
   (setq circ-vector (vector 1 2 3 4 (list 'a 'b 'c 'd) 6 7))
   (aset circ-vector 5 (make-symbol "-gensym-"))
   (setcar (cdr (aref circ-vector 4)) (aref circ-vector 5))
   nil)

 (install-custom-print-funcs)
 ;; (setq print-circle t)

 (let ((print-circle t))
   (or (equal (prin1-to-string circ-list) "#1=(a b [1 2 #1# 4] #1# e f)")
       (error "circular object with array printing")))

 (let ((print-circle t))
   (or (equal (prin1-to-string dotted-circ-list) "#1=(a b c . #1#)")
       (error "circular object with array printing")))

 (let* ((print-circle t)
	(x (list 'p 'q))
	(y (list (list 'a 'b) x 'foo x)))
   (setcdr (cdr (cdr (cdr y))) (cdr y))
   (or (equal (prin1-to-string y) "((a b) . #1=(#2=(p q) foo #2# . #1#))"
	      )
       (error "circular list example from CL manual")))

 ;; There's no special handling of uninterned symbols in custom-print.
 (let ((print-circle nil))
   (or (equal (prin1-to-string circ-sym) "(#:FOO #:FOO)")
       (error "uninterned symbols in list")))
 (let ((print-circle t))
   (or (equal (prin1-to-string circ-sym) "(#1=FOO #1#)")
       (error "circular uninterned symbols in list")))

 (uninstall-custom-print-funcs)
 )

;;; cus-print.el ends here