Amiga Format CD 7/Amiga Format AFCD07 (Dec 1996, Issue 91).iso/serious/shareware/programming/emacs-complete/fsf/emacs/lisp/cust-print.el

Text File | 1994-04-09 | 26KB | 737 lines

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

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

;; Author: Daniel LaLiberte <[e-mail]>
;; Adapted-By: ESR
;; Keywords: extensions

;; LCD Archive Entry:
;; cust-print|Daniel LaLiberte|[e-mail]
;; |Handle print-level, print-circle and more.
;; |$Date: 1994/04/09 22:24:43 $|$Revision: 2.1.1.1 $|

;; 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 2, 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.

;;; ===============================
;;; $Header: /home/gd/gnu/emacs/19.0/lisp/RCS/cust-print.el,v 2.1.1.1 1994/04/09 22:24:43 liberte Exp liberte $
;;; $Log: cust-print.el,v $
;;; Revision 2.1.1.1  1994/04/09  22:24:43  liberte
;;; Branch for FSF mods.
;;;
;;; Revision 2.1  1994/04/09  22:19:10  liberte
;;; Jumping up to new revision.
;;; Simplify definition of defalias for Emacs 18.
;;;
;;; Revision 2.1  1994/04/09  22:19:10  liberte
;;; Jumping up to new revision.
;;; Simplify definition of defalias for Emacs 18.
;;;
;;; Revision 1.14  1994/04/05  21:05:09  liberte
;;; Change install- and uninstall- to -install and -uninstall.
;;;
;;; Revision 1.13  1994/03/24  20:26:05  liberte
;;; Change "internal" to "original" throughout.
;;;         (add-custom-printer, delete-custom-printer) replace customizers.
;;;         (with-custom-print) new
;;;         (custom-prin1-to-string) Made it more robust.
;;;
;;; Revision 1.4  1994/03/23  20:34:29  liberte
;;; * Change "emacs" to "original" - I just can't decide.
;;;
;;; Revision 1.3  1994/02/21  21:25:36  liberte
;;; * Make custom-prin1-to-string more robust when errors occur.
;;; * Change "internal" to "emacs".
;;;
;;; Revision 1.2  1993/11/22  22:36:36  liberte
;;; * Simplified and generalized printer customization.
;;;     custom-printers is an alist of (PREDICATE . PRINTER) pairs
;;;     for any data types.  The PRINTER function should print to
;;;     `standard-output'  add-custom-printer and delete-custom-printer
;;;     change custom-printers.
;;;
;;; * Installation function now called install-custom-print.  The
;;;     old name is still around for now.
;;;
;;; * New macro with-custom-print (added earlier) - executes like
;;;     progn but with custom-print activated temporarily.
;;;
;;; * Cleaned up comments for replacements of standardard printers.
;;;
;;; * Changed custom-prin1-to-string to use a temporary buffer.
;;;
;;; * Option custom-print-vectors (added earlier) - controls whether
;;;     vectors should be printed according to print-length and
;;;     print-length.  Emacs doesnt do this, but cust-print would
;;;     otherwise do it only if custom printing is required.
;;;
;;; * Uninterned symbols are treated as non-read-equivalent.
;;;


;;; Commentary:

;; 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 error in Emacs version 18.  Version
;; 19 supports print-level, but it is often useful to get a better
;; print representation of circular and shared structures; the print-circle
;; option may be used to print more concise representations.

;; There are three main ways to use this package.  First, you may
;; replace prin1, princ, and some subroutines that use them by calling
;; install-custom-print so that any use of these functions in
;; Lisp code will be affected; you can later reset with
;; uninstall-custom-print.  Second, you may temporarily install
;; these functions with the macro with-custom-print.  Third, 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, if you evaluate (aref circ-list 0), where circ-list is a
;; circular list rather than an array, aref calls error directly which
;; will jump to the top level instead of printing the circular list.

;; Uninterned symbols are recognized when print-circle is non-nil,
;; but they are not printed specially here.  Use the cl-packages package
;; to print according to print-gensym.

;; Obviously the right way to implement this custom-print facility is
;; in C or with hooks into the standard printer.  Please volunteer
;; since I don't have the time or need.  More CL-like printing
;; capabilities could be added in the future.

;; 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.


;;; Code:
;;=========================================================

;; If using cl-packages:

'(defpackage "cust-print"
   (:nicknames "CP" "custom-print")
   (:use "el")
   (:export
    print-level
    print-circle

    custom-print-install
    custom-print-uninstall
    custom-print-installed-p
    with-custom-print

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

    custom-printers
    add-custom-printer
    ))

'(in-package cust-print)

(require 'backquote)

;; Emacs 18 doesnt have defalias.
;; Provide def for byte compiler.
(eval-and-compile
  (or (fboundp 'defalias) (fset 'defalias 'fset)))


;; Variables:
;;=========================================================

;;(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 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 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 occurrence (in the order of the print
representation) and `#N#' in place of each subsequent occurrence,
where N is a positive decimal integer.

There is no way to read this representation in standard Emacs,
but if you need to do so, try the cl-read.el package.")


(defvar custom-print-vectors nil
  "*Non-nil if printing of vectors should obey print-level and print-length.

For Emacs 18, setting print-level, or adding custom print list or
vector handling will make this happen anyway.  Emacs 19 obeys
print-level, but not for vectors.")


;; Custom printers
;;==========================================================

(defconst custom-printers nil
  ;; e.g. '((symbolp . pkg::print-symbol))
  "An alist for custom printing of any type.
Pairs are of the form (PREDICATE . PRINTER).  If PREDICATE is true
for an object, then PRINTER is called with the object.
PRINTER should print to `standard-output' using cust-print-original-princ
if the standard printer is sufficient, or cust-print-prin for complex things.
The PRINTER should return the object being printed.

Don't modify this variable directly.  Use `add-custom-printer' and
`delete-custom-printer'")
;; Should cust-print-original-princ and cust-print-prin be exported symbols?
;; Or should the standard printers functions be replaced by
;; CP ones in elisp so that CP internal functions need not be called?

(defun add-custom-printer (pred printer)
  "Add a pair of PREDICATE and PRINTER to `custom-printers'.
Any pair that has the same PREDICATE is first removed."
  (setq custom-printers (cons (cons pred printer)
                  (delq (assq pred custom-printers)
                    custom-printers)))
  ;; Rather than updating here, we could wait until cust-print-top-level is called.
  (cust-print-update-custom-printers))

(defun delete-custom-printer (pred)
  "Delete the custom printer associated with PREDICATE."
  (setq custom-printers (delq (assq pred custom-printers)
                  custom-printers))
  (cust-print-update-custom-printers))


(defun cust-print-use-custom-printer (object)
  ;; Default function returns nil.
  nil)

(defun cust-print-update-custom-printers ()
  ;; Modify the definition of cust-print-use-custom-printer
  (defalias 'cust-print-use-custom-printer
    ;; We dont really want to require the byte-compiler.
    ;; (byte-compile
     (` (lambda (object)
      (cond
       (,@ (mapcar (function
            (lambda (pair)
              (` (((, (car pair)) object)
                  ((, (cdr pair)) object)))))
               custom-printers))
       ;; Otherwise return nil.
       (t nil)
       )))
     ;; )
  ))


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

(defun cust-print-set-function-cell (symbol-pair)
  (defalias (car symbol-pair)
    (symbol-function (car (cdr symbol-pair)))))

(defun cust-print-original-princ (object &optional stream)) ; dummy def

;; Save emacs routines.
(if (not (fboundp 'cust-print-original-prin1))
    (mapcar 'cust-print-set-function-cell
        '((cust-print-original-prin1 prin1)
          (cust-print-original-princ princ)
          (cust-print-original-print print)
          (cust-print-original-prin1-to-string prin1-to-string)
          (cust-print-original-format format)
          (cust-print-original-message message)
          (cust-print-original-error error))))


(defun custom-print-install ()
  "Replace print functions with general, customizable, Lisp versions.
The emacs subroutines are saved away, and you can reinstall them
by running `custom-print-uninstall'."
  (interactive)
  (mapcar 'cust-print-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)
        ))
  t)

(defun custom-print-uninstall ()
  "Reset print functions to their emacs subroutines."
  (interactive)
  (mapcar 'cust-print-set-function-cell
      '((prin1 cust-print-original-prin1)
        (princ cust-print-original-princ)
        (print cust-print-original-print)
        (prin1-to-string cust-print-original-prin1-to-string)
        (format cust-print-original-format)
        (message cust-print-original-message)
        (error cust-print-original-error)
        ))
  t)

(defalias 'custom-print-funcs-installed-p 'custom-print-installed-p)
(defun custom-print-installed-p ()
  "Return t if custom-print is currently installed, nil otherwise."
  (eq (symbol-function 'custom-prin1) (symbol-function 'prin1)))

(put 'with-custom-print-funcs 'edebug-form-spec '(body))
(put 'with-custom-print 'edebug-form-spec '(body))

(defalias 'with-custom-print-funcs 'with-custom-print)
(defmacro with-custom-print (&rest body)
  "Temporarily install the custom print package while executing BODY."
  (` (unwind-protect
     (progn
       (custom-print-install)
       (,@ body))
       (custom-print-uninstall))))


;; Lisp replacements for prin1 and princ, and for some subrs that use them
;;===============================================================
;; - so far only the printing and formatting subrs.

(defun custom-prin1 (object &optional stream)
  "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).

This is the custom-print replacement for the standard `prin1'.  It
uses the appropriate printer depending on the values of `print-level'
and `print-circle' (which see)."
  (cust-print-top-level object stream 'cust-print-original-prin1))


(defun custom-princ (object &optional stream)
  "Output the printed representation of OBJECT, any Lisp object.
No quoting characters are used; no delimiters are printed around
the contents of strings.
Output stream is STREAM, or value of `standard-output' (which see).

This is the custom-print replacement for the standard `princ'."
  (cust-print-top-level object stream 'cust-print-original-princ))


(defun custom-prin1-to-string (object)
  "Return a string containing the printed representation of OBJECT,
any Lisp object.  Quoting characters are used when needed to make output
that `read' can handle, whenever this is possible.

This is the custom-print replacement for the standard `prin1-to-string'."
  (let ((buf (get-buffer-create " *custom-print-temp*")))
    ;; We must erase the buffer before printing in case an error
    ;; occured during the last prin1-to-string and we are in debugger.
    (save-excursion
      (set-buffer buf)
      (erase-buffer))
    ;; We must be in the current-buffer when the print occurs.
    (custom-prin1 object buf)
    (save-excursion
      (set-buffer buf)
      (buffer-string)
      ;; We could erase the buffer again, but why bother?
      )))


(defun custom-print (object &optional stream)
  "Output the printed representation of OBJECT, with newlines around it.
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).

This is the custom-print replacement for the standard `print'."
  (cust-print-original-princ "\n" stream)
  (custom-prin1 object stream)
  (cust-print-original-princ "\n" stream))


(defun custom-format (fmt &rest args)
  "Format a string out of a control-string and arguments.
The first argument is a control string.  It, and subsequent arguments
substituted into it, become the value, which is a string.
It may contain %s or %d or %c to substitute successive following arguments.
%s means print an argument as a string, %d means print as number in decimal,
%c means print a number as a single character.
The argument used by %s must be a string or a symbol;
the argument used by %d, %b, %o, %x or %c must be a number.

This is the custom-print replacement for the standard `format'.  It
calls the emacs `format' after first making strings for list,
vector, or symbol 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 'cust-print-original-format fmt
     (mapcar (function (lambda (arg)
                 (if (or (listp arg) (vectorp arg) (symbolp arg))
                 (custom-prin1-to-string arg)
                   arg)))
         args)))


(defun custom-message (fmt &rest args)
  "Print a one-line message at the bottom of the screen.
The first argument is a control string.
It may contain %s or %d or %c to print successive following arguments.
%s means print an argument as a string, %d means print as number in decimal,
%c means print a number as a single character.
The argument used by %s must be a string or a symbol;
the argument used by %d or %c must be a number.

This is the custom-print replacement for the standard `message'.
See `custom-format' for the details."
  ;; It doesn't work to princ the result of custom-format as in:
  ;; (cust-print-original-princ (apply 'custom-format fmt args))
  ;; because the echo area requires special handling
  ;; to avoid duplicating the output.
  ;; cust-print-original-message does it right.
  (apply 'cust-print-original-message  fmt
     (mapcar (function (lambda (arg)
                 (if (or (listp arg) (vectorp arg) (symbolp arg))
                 (custom-prin1-to-string arg)
                   arg)))
         args)))


(defun custom-error (fmt &rest args)
  "Signal an error, making error message by passing all args to `format'.

This is the custom-print replacement for the standard `error'.
See `custom-format' for the details."
  (signal 'error (list (apply 'custom-format fmt args))))



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

;; Defs to quiet byte-compiler.
(defvar circle-table)
(defvar cust-print-current-level)

(defun cust-print-original-printer (object))  ; One of the standard printers.
(defun cust-print-low-level-prin (object))    ; Used internally.
(defun cust-print-prin (object))              ; Call this to print recursively.

(defun cust-print-top-level (object stream emacs-printer)
  ;; Set up for printing.
  (let ((standard-output (or stream standard-output))
    ;; circle-table will be non-nil if anything is circular.
    (circle-table (and print-circle
               (cust-print-preprocess-circle-tree object)))
    (cust-print-current-level (or print-level -1)))

    (defalias 'cust-print-original-printer emacs-printer)
    (defalias 'cust-print-low-level-prin
      (cond
       ((or custom-printers
        circle-table
        print-level            ; comment out for version 19
        ;; Emacs doesn't use print-level or print-length
        ;; for vectors, but custom-print can.
        (if custom-print-vectors
        (or print-level print-length)))
    'cust-print-print-object)
       (t 'cust-print-original-printer)))
    (defalias 'cust-print-prin
      (if circle-table 'cust-print-print-circular 'cust-print-low-level-prin))

    (cust-print-prin object)
    object))


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


(defun cust-print-print-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.
        (cust-print-original-princ "#")
        (cust-print-original-princ id)
        (cust-print-original-princ "#"))
        ;; Not printed yet, so label with id and print object.
        (setcdr tag (- id)) ; mark it as printed
        (cust-print-original-princ "#")
        (cust-print-original-princ (- id))
        (cust-print-original-princ "=")
        (cust-print-low-level-prin object)
        ))
      ;; Not repeated in structure.
      (cust-print-low-level-prin object))))


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

(defun cust-print-list (list)
  ;; Print a list using print-length, print-level, and print-circle.
  (if (= cust-print-current-level 0)
      (cust-print-original-princ "#")
    (let ((cust-print-current-level (1- cust-print-current-level)))
      (cust-print-original-princ "(")
      (let ((length (or print-length 0)))

    ;; Print the first element always (even if length = 0).
    (cust-print-prin (car list))
    (setq list (cdr list))
    (if list (cust-print-original-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
        (cust-print-prin (car list))
        (setq list (cdr list)))

        ;; cdr is not a list, or it is in circle-table.
        (cust-print-original-princ ". ")
        (cust-print-prin list)
        (setq list nil))

      (setq length (1- length))
      (if list (cust-print-original-princ " ")))

    (if (and list (= length 0)) (cust-print-original-princ "..."))
    (cust-print-original-princ ")"))))
  list)


(defun cust-print-vector (vector)
  ;; Print a vector according to print-length, print-level, and print-circle.
  (if (= cust-print-current-level 0)
      (cust-print-original-princ "#")
    (let ((cust-print-current-level (1- cust-print-current-level))
      (i 0)
      (len (length vector)))
      (cust-print-original-princ "[")

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

      (if (< i (length vector)) (cust-print-original-princ "..."))
      (cust-print-original-princ "]")
      ))
  vector)



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

(defun cust-print-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)))
    (cust-print-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 cust-print-walk-circle-tree (object)
  (let (read-equivalent-p tag)
    (while object
      (setq read-equivalent-p
        (or (numberp object)
        (and (symbolp object)
             ;; Check if it is uninterned.
             (eq object (intern-soft (symbol-name 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.
          (cust-print-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)
          (cust-print-walk-circle-tree (aref object j))
          (setq j (1+ j))))))))))


;; Example.
;;=======================================

'(progn
   (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)
   ;; (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")))

   (let ((print-circle nil))
     ;; cl-packages.el is required to print uninterned symbols like #:FOO.
     ;; (require 'cl-packages)
     (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)
   )

(provide 'cust-print)

;;; cust-print.el ends here