emacs/lisp/emacs-lisp/cl-print.el

;;; cl-print.el --- CL-style generic printing  -*- lexical-binding: t; -*-

;; Copyright (C) 2017  Free Software Foundation, Inc.

;; Author: Stefan Monnier <monnier@iro.umontreal.ca>
;; Keywords:
;; Version: 1.0
;; Package-Requires: ((emacs "25"))

;; 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 3 of the License, 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.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:

;; Customizable print facility.
;;
;; The heart of it is the generic function `cl-print-object' to which you
;; can add any method you like.
;;
;; The main entry point is `cl-prin1'.

;;; Code:

(require 'button)

(defvar cl-print-readably nil
  "If non-nil, try and make sure the result can be `read'.")

(defvar cl-print--number-table nil)
(defvar cl-print--currently-printing nil)

;;;###autoload
(cl-defgeneric cl-print-object (object stream)
  "Dispatcher to print OBJECT on STREAM according to its type.
You can add methods to it to customize the output.
But if you just want to print something, don't call this directly:
call other entry points instead, such as `cl-prin1'."
  ;; This delegates to the C printer.  The C printer will not call us back, so
  ;; we should only use it for objects which don't have nesting.
  (prin1 object stream))

(cl-defmethod cl-print-object ((object cons) stream)
  (let ((car (pop object)))
    (if (and (memq car '(\, quote \` \,@ \,.))
             (consp object)
             (null (cdr object)))
        (progn
          (princ (if (eq car 'quote) '\' car) stream)
          (cl-print-object (car object) stream))
      (princ "(" stream)
      (cl-print-object car stream)
      (while (and (consp object)
                  (not (if cl-print--number-table
                           (numberp (gethash object cl-print--number-table))
                         (memq object cl-print--currently-printing))))
        (princ " " stream)
        (cl-print-object (pop object) stream))
      (when object
        (princ " . " stream) (cl-print-object object stream))
      (princ ")" stream))))

(cl-defmethod cl-print-object ((object vector) stream)
  (princ "[" stream)
  (dotimes (i (length object))
    (unless (zerop i) (princ " " stream))
    (cl-print-object (aref object i) stream))
  (princ "]" stream))

(cl-defmethod cl-print-object ((object hash-table) stream)
  (princ "#<hash-table " stream)
  (princ (hash-table-test object) stream)
  (princ " " stream)
  (princ (hash-table-count object) stream)
  (princ "/" stream)
  (princ (hash-table-size object) stream)
  (princ (format " 0x%x" (sxhash object)) stream)
  (princ ">" stream))

(define-button-type 'help-byte-code
  'follow-link t
  'action (lambda (button)
            (disassemble (button-get button 'byte-code-function)))
  'help-echo (purecopy "mouse-2, RET: disassemble this function"))

(defvar cl-print-compiled nil
  "Control how to print byte-compiled functions.
Acceptable values include:
- `static' to print the vector of constants.
- `disassemble' to print the disassembly of the code.
- nil to skip printing any details about the code.")

(defvar cl-print-compiled-button t
  "Control how to print byte-compiled functions into buffers.
When the stream is a buffer, make the bytecode part of the output
into a button whose action shows the function's disassembly.")

(autoload 'disassemble-1 "disass")

(cl-defmethod cl-print-object ((object compiled-function) stream)
  (unless stream (setq stream standard-output))
  ;; We use "#f(...)" rather than "#<...>" so that pp.el gives better results.
  (princ "#f(compiled-function " stream)
  (let ((args (help-function-arglist object 'preserve-names)))
    (if args
        (prin1 args stream)
      (princ "()" stream)))
  (pcase (help-split-fundoc (documentation object 'raw) object)
    ;; Drop args which `help-function-arglist' already printed.
    (`(,_usage . ,(and doc (guard (stringp doc))))
     (princ " " stream)
     (prin1 doc stream)))
  (let ((inter (interactive-form object)))
    (when inter
      (princ " " stream)
      (cl-print-object
       (if (eq 'byte-code (car-safe (cadr inter)))
           `(interactive ,(make-byte-code nil (nth 1 (cadr inter))
                                          (nth 2 (cadr inter))
                                          (nth 3 (cadr inter))))
         inter)
       stream)))
  (if (eq cl-print-compiled 'disassemble)
      (princ
       (with-temp-buffer
         (insert "\n")
         (disassemble-1 object 0)
         (buffer-string))
       stream)
    (princ " " stream)
    (let ((button-start (and cl-print-compiled-button
                             (bufferp stream)
                             (with-current-buffer stream (point)))))
      (princ (format "#<bytecode 0x%x>" (sxhash object)) stream)
      (when (eq cl-print-compiled 'static)
        (princ " " stream)
        (cl-print-object (aref object 2) stream))
      (when button-start
        (with-current-buffer stream
          (make-text-button button-start (point)
                            :type 'help-byte-code
                            'byte-code-function object)))))
  (princ ")" stream))

;; This belongs in nadvice.el, of course, but some load-ordering issues make it
;; complicated: cl-generic uses macros from cl-macs and cl-macs uses advice-add
;; from nadvice, so nadvice needs to be loaded before cl-generic and hence
;; can't use cl-defmethod.
(cl-defmethod cl-print-object :extra "nadvice"
              ((object compiled-function) stream)
  (if (not (advice--p object))
      (cl-call-next-method)
    (princ "#f(advice-wrapper " stream)
    (when (fboundp 'advice--where)
      (princ (advice--where object) stream)
      (princ " " stream))
    (cl-print-object (advice--cdr object) stream)
    (princ " " stream)
    (cl-print-object (advice--car object) stream)
    (let ((props (advice--props object)))
      (when props
        (princ " " stream)
        (cl-print-object props stream)))
    (princ ")" stream)))

(cl-defmethod cl-print-object ((object cl-structure-object) stream)
  (princ "#s(" stream)
  (let* ((class (cl-find-class (type-of object)))
         (slots (cl--struct-class-slots class)))
    (princ (cl--struct-class-name class) stream)
    (dotimes (i (length slots))
      (let ((slot (aref slots i)))
        (princ " :" stream)
        (princ (cl--slot-descriptor-name slot) stream)
        (princ " " stream)
        (cl-print-object (aref object (1+ i)) stream))))
  (princ ")" stream))

;;; Circularity and sharing.

;; I don't try to support the `print-continuous-numbering', because
;; I think it's ill defined anyway: if an object appears only once in each call
;; its sharing can't be properly preserved!

(cl-defmethod cl-print-object :around (object stream)
  ;; FIXME: Only put such an :around method on types where it's relevant.
  (cond
   (print-circle
    (let ((n (gethash object cl-print--number-table)))
      (if (not (numberp n))
          (cl-call-next-method)
        (if (> n 0)
            ;; Already printed.  Just print a reference.
            (progn (princ "#" stream) (princ n stream) (princ "#" stream))
          (puthash object (- n) cl-print--number-table)
          (princ "#" stream) (princ (- n) stream) (princ "=" stream)
          (cl-call-next-method)))))
   ((let ((already-printing (memq object cl-print--currently-printing)))
      (when already-printing
        ;; Currently printing, just print reference to avoid endless
        ;; recursion.
        (princ "#" stream)
        (princ (length (cdr already-printing)) stream))))
    (t (let ((cl-print--currently-printing
              (cons object cl-print--currently-printing)))
         (cl-call-next-method)))))

(defvar cl-print--number-index nil)

(defun cl-print--find-sharing (object table)
  ;; Avoid recursion: not only because it's too easy to bump into
  ;; `max-lisp-eval-depth', but also because function calls are fairly slow.
  ;; At first, I thought using a list for our stack would cause too much
  ;; garbage to generated, but I didn't notice any such problem in practice.
  ;; I experimented with using an array instead, but the result was slightly
  ;; slower and the reduction in GC activity was less than 1% on my test.
  (let ((stack (list object)))
    (while stack
      (let ((object (pop stack)))
        (unless
            ;; Skip objects which don't have identity!
            (or (floatp object) (numberp object)
                (null object) (if (symbolp object) (intern-soft object)))
          (let ((n (gethash object table)))
            (cond
             ((numberp n))                   ;All done.
             (n                              ;Already seen, but only once.
              (let ((n (1+ cl-print--number-index)))
                (setq cl-print--number-index n)
                (puthash object (- n) table)))
             (t
              (puthash object t table)
              (pcase object
                (`(,car . ,cdr)
                 (push cdr stack)
                 (push car stack))
                ((pred stringp)
                 ;; We presumably won't print its text-properties.
                 nil)
                ((or (pred arrayp) (pred byte-code-function-p))
                 ;; FIXME: Inefficient for char-tables!
                 (dotimes (i (length object))
                   (push (aref object i) stack))))))))))))

(defun cl-print--preprocess (object)
  (let ((print-number-table (make-hash-table :test 'eq :rehash-size 2.0)))
    (if (fboundp 'print--preprocess)
        ;; Use the predefined C version if available.
        (print--preprocess object)           ;Fill print-number-table!
      (let ((cl-print--number-index 0))
        (cl-print--find-sharing object print-number-table)))
    print-number-table))

;;;###autoload
(defun cl-prin1 (object &optional stream)
  "Print OBJECT on STREAM according to its type.
Output is further controlled by the variables
`cl-print-readably', `cl-print-compiled', along with output
variables for the standard printing functions.  See Info
node `(elisp)Output Variables'."
  (cond
   (cl-print-readably (prin1 object stream))
   ((not print-circle) (cl-print-object object stream))
   (t
    (let ((cl-print--number-table (cl-print--preprocess object)))
      (cl-print-object object stream)))))

;;;###autoload
(defun cl-prin1-to-string (object)
  "Return a string containing the `cl-prin1'-printed representation of OBJECT."
  (with-temp-buffer
    (cl-prin1 object (current-buffer))
    (buffer-string)))

(provide 'cl-print)
;;; cl-print.el ends here