jupyter-mime.el

;;; jupyter-mime.el --- Insert mime types -*- lexical-binding: t -*-

;; Copyright (C) 2018-2024 Nathaniel Nicandro

;; Author: Nathaniel Nicandro <nathanielnicandro@gmail.com>
;; Created: 09 Nov 2018

;; This program 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, or (at
;; your option) any later version.

;; This program 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, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.

;;; Commentary:

;; Routines for working with MIME types.
;; Also adds the following methods which may be extended:
;;
;; - jupyter-markdown-follow-link
;; - jupyter-insert
;;
;; For working with display IDs, currently rudimentary
;;
;; - jupyter-current-display
;; - jupyter-beginning-of-display
;; - jupyter-end-of-display
;; - jupyter-next-display-with-id
;; - jupyter-delete-current-display
;; - jupyter-update-display

;;; Code:

(require 'jupyter-base)
(require 'shr)
(require 'ansi-color)

(declare-function jupyter-message-content "jupyter-messages" (msg))
(declare-function org-format-latex "org" (prefix &optional beg end dir overlays msg forbuffer processing-type))
(declare-function markdown-link-at-pos "ext:markdown-mode" (pos))
(declare-function markdown-follow-link-at-point "ext:markdown-mode")

;;; User variables

(defcustom jupyter-image-max-width 0
  "Maximum width of images in REPL.
Wider images are resized.  Special value 0 means no limit."
  :type 'integer
  :group 'jupyter-repl)

;;; Implementation

(defvar-local jupyter-display-ids nil
  "A hash table of display IDs.
Display IDs are implemented by setting the text property,
`jupyter-display', to the display ID requested by a display_data
message.  When a display is updated from an update_display_data
message, the display ID from the initial display_data message is
retrieved from this table and used to find the display in the
REPL buffer.  See `jupyter-update-display'.")

;;; Macros

;; Taken from `eshell-handle-control-codes'
(defun jupyter-handle-control-codes (beg end)
  "Handle any control sequences between BEG and END."
  (save-excursion
    (goto-char beg)
    (while (< (point) end)
      (let ((char (char-after)))
        (cond
         ((eq char ?\r)
          (if (< (1+ (point)) end)
              (if (memq (char-after (1+ (point)))
                        '(?\n ?\r))
                  (delete-char 1)
                (let ((end (1+ (point))))
                  (beginning-of-line)
                  (delete-region (point) end)))
            (add-text-properties (point) (1+ (point))
                                 '(invisible t))
            (forward-char)))
         ((eq char ?\a)
          (delete-char 1)
          (beep))
         ((eq char ?\C-h)
          (delete-region (1- (point)) (1+ (point))))
         (t
          (forward-char)))))))

(defmacro jupyter-with-control-code-handling (&rest body)
  "Handle control codes in any produced output generated by evaluating BODY.
After BODY is evaluated, call `jupyter-handle-control-codes'
on the region inserted by BODY."
  (let ((beg (make-symbol "beg"))
        (end (make-symbol "end")))
    `(jupyter-with-insertion-bounds
         ,beg ,end (progn ,@body)
       ;; Handle continuation from previous messages
       (when (eq (char-before ,beg) ?\r)
         (move-marker ,beg (1- ,beg)))
       (jupyter-handle-control-codes ,beg ,end))))

;;; Fontificiation routines

(defun jupyter-fontify-buffer-name (mode)
  "Return the buffer name for fontifying MODE."
  (format " *jupyter-fontify[%s]*" mode))

(defun jupyter-fontify-buffer (mode)
  "Return the buffer used to fontify text for MODE.
Retrieve the buffer for MODE from `jupyter-fontify-buffers'.
If no buffer for MODE exists, create a new one."
  (let ((buf (get-buffer-create (jupyter-fontify-buffer-name mode))))
    (with-current-buffer buf
      (unless (eq major-mode mode)
        (delay-mode-hooks (funcall mode))))
    buf))

(defun jupyter-fixup-font-lock-properties (beg end &optional object)
  "Fixup the text properties in the `current-buffer' between BEG END.
If OBJECT is non-nil, fixup the text properties of OBJECT.  Fixing
the text properties involves substituting any `face' property
with `font-lock-face'."
  (let ((next beg) val)
    (while (/= beg end)
      (setq val (get-text-property beg 'face object)
            next (next-single-property-change beg 'face object end))
      (remove-text-properties beg next '(face) object)
      (put-text-property beg next 'font-lock-face (or val 'default) object)
      (setq beg next))))

(defun jupyter-add-font-lock-properties (start end &optional object use-face)
  "Add font lock text properties between START and END in the `current-buffer'.
START, END, and OBJECT have the same meaning as in
`add-text-properties'.  The properties added are the ones that
mark the text between START and END as fontified according to
font lock.  Any text between START and END that does not have a
font-lock-face property will have the default face filled in for
the property and the face text property is swapped for
font-lock-face.

If USE-FACE is non-nil, do not replace the face text property
with font-lock-face."
  (unless use-face
    (jupyter-fixup-font-lock-properties start end object))
  (add-text-properties start end '(fontified t font-lock-fontified t) object))

(defun jupyter-fontify-according-to-mode (mode str &optional use-face)
  "Fontify a string according to MODE.
Return the fontified string.  In addition to fontifying STR, if
MODE has a non-default `fill-forward-paragraph-function', STR
will be filled using `fill-region'.

If USE-FACE is non-nil, do not replace the face text property
with font-lock-face in the returned string."
  (with-current-buffer (jupyter-fontify-buffer mode)
    (erase-buffer)
    (insert str)
    (font-lock-ensure)
    (jupyter-add-font-lock-properties (point-min) (point-max) nil use-face)
    (when (not (memq fill-forward-paragraph-function
                     '(forward-paragraph)))
      (fill-region (point-min) (point-max) t 'nosqueeze))
    (buffer-string)))

(defun jupyter-fontify-region-according-to-mode (mode beg end)
  "Fontify a region according to MODE.
Fontify the region between BEG and END in the current buffer
according to MODE.  This works by creating a new indirect buffer,
enabling MODE in the new buffer, ensuring the region is font
locked, adding required text properties, and finally re-enabling
the `major-mode' that was current before the call to this
function."
  (let ((restore-mode major-mode))
    (with-current-buffer
        (make-indirect-buffer
         (current-buffer) (generate-new-buffer-name
                           (jupyter-fontify-buffer-name mode)))
      (unwind-protect
          (save-restriction
            (narrow-to-region beg end)
            (delay-mode-hooks (funcall mode))
            (font-lock-ensure)
            (jupyter-fixup-font-lock-properties beg end))
        (kill-buffer)))
    (funcall restore-mode)))

;;; Special handling of ANSI sequences

(defalias 'jupyter--ansi-color-apply-on-region
  (if (version< emacs-version "28.1")
      (lambda (begin end)
        (cl-letf (((symbol-function #'delete-and-extract-region)
                   (lambda (beg end)
                     (add-text-properties beg end '(invisible t jupyter-invisible t))
                     (buffer-substring beg end))))
          (ansi-color-apply-on-region begin end)))
    (lambda (begin end)
      (ansi-color-apply-on-region begin end t)
      (dolist (ov (overlays-in begin end))
        (when (and (overlay-get ov 'invisible)
                   (<= begin (overlay-start ov)
                       (overlay-end ov) end))
          ;; Assume invisible overlay added by
          ;; `ansi-color-apply-on-region'.
          (add-text-properties (overlay-start ov) (overlay-end ov)
                               '(invisible t jupyter-invisible t))
          (delete-overlay ov))))))

(defun jupyter-ansi-color-apply-on-region (begin end &optional face-prop)
  "`ansi-color-apply-on-region' with Jupyter specific modifications.
In particular, does not delete escape sequences between BEGIN and
END from the buffer.  Instead, an invisible text property with a
value of t is added to render the escape sequences invisible.
Also, the `ansi-color-apply-face-function' is hard-coded to a
custom function that prepends to the face property of the text
and also sets the FACE-PROP to the prepended face, if FACE-PROP
is nil it defaults to `font-lock-face'.

For convenience, a jupyter-invisible property is also added with
a value of t.  This is mainly for modes like `org-mode' which
strip invisible properties during fontification.  In such cases,
the jupyter-invisible property can act as an alias to the
invisible property by adding it to `char-property-alias-alist'."
  (let ((ansi-color-apply-face-function
         (lambda (beg end face)
           (when face
             (setq face (list face))
             (font-lock-prepend-text-property beg end 'face face)
             (put-text-property beg end (or face-prop 'font-lock-face) face)))))
    (jupyter--ansi-color-apply-on-region begin end)))

;;; `jupyter-insert' method

(cl-defgeneric jupyter-insert (_mime _data &optional _metadata)
  "Insert MIME data in the current buffer.
Additions to this method should insert DATA assuming it has a
mime type of MIME.  If METADATA is non-nil, it will be a property
list containing extra properties for inserting DATA such as
:width and :height for image mime types.

If MIME is considered handled, but does not insert anything in
the current buffer, return a non-nil value to indicate that MIME
has been handled."
  (ignore))

(cl-defmethod jupyter-insert ((plist cons) &optional metadata)
  "Insert the content contained in PLIST.
PLIST should be a property list that contains the key :data and
optionally the key :metadata.  The value of :data shall be another
property list that contains MIME types as keys and their
representations as values.  Alternatively, PLIST can be a full
message property list or be a property list that itself contains
mimetypes.

For each MIME type in `jupyter-mime-types' call

    (jupyter-insert MIME (plist-get data MIME) (plist-get metadata MIME))

until one of the invocations inserts text into the current
buffer (tested by comparisons with `buffer-modified-tick') or
returns a non-nil value.  When either of these cases occur, return
MIME.

Note on non-graphic displays, `jupyter-nongraphic-mime-types' is
used instead of `jupyter-mime-types'.

When no valid mimetype is present, a warning is shown and nil is
returned."
  (cl-assert plist json-plist)
  (let ((content (jupyter-normalize-data plist metadata)))
    (cond
     ((let ((tick (buffer-modified-tick)))
        (jupyter-map-mime-bundle (if (display-graphic-p) jupyter-mime-types
                                   jupyter-nongraphic-mime-types)
            content
          (lambda (mime content)
            (and (or (jupyter-insert
                      mime (plist-get content :data)
                      (plist-get content :metadata))
                     (/= tick (buffer-modified-tick)))
                 mime)))))
     (t
      (prog1 nil
        (let ((warning
               (format "No valid mimetype found: %s"
                       (cl-loop for (k _v) on (plist-get content :data)
                                by #'cddr collect k))))
          (display-warning 'jupyter warning)))))))

;;; HTML

(defun jupyter--shr-put-image (spec alt &optional flags)
  "Identical to `shr-put-image', but ensure :ascent is 50.
SPEC, ALT and FLAGS have the same meaning as in `shr-put-image'.
The :ascent of an image is set to 50 so that the image center
aligns on the current line."
  (let ((image (shr-put-image spec alt flags)))
    (prog1 image
      (when image
        ;; Ensure we use an ascent of 50 so that the image center aligns with
        ;; the output prompt of a REPL buffer.
        (setf (image-property image :ascent) 50)
        (force-window-update)))))

(defun jupyter-browse-url-in-temp-file (data)
  "Insert DATA into a temp file and call `browse-url-of-file' on it."
  (let* ((secs (time-to-seconds))
         ;; Allow showing the same DATA, but only after a 10s period.  This is
         ;; so that the same data doesn't get displayed multiple times very
         ;; quickly.  See #121.
         (secs (- secs (cl-rem secs 10)))
         (hash (sha1 (concat data (format-time-string "%H%M%S" secs))))
         (file (expand-file-name
                (concat "emacs-jupyter-" hash ".html")
                temporary-file-directory)))
    (unless (file-exists-p file)
      (with-temp-file file (insert data))
      (browse-url-of-file file)
      ;; Give the external browser time to open the tmp file before deleting it
      ;; based on mm-display-external
      (run-at-time
       60 nil
       (lambda ()
         (ignore-errors (delete-file file)))))))

(defun jupyter--delete-script-tags (beg end)
  (save-excursion
    (save-restriction
      (narrow-to-region beg end)
      (goto-char beg)
      (while (re-search-forward "<script[^>]*>" nil t)
        (delete-region
         (match-beginning 0)
         (if (re-search-forward "</script>" nil t)
             (point)
           (point-max)))))))

(defun jupyter-insert-html (html)
  "Parse and insert the HTML string using `shr'."
  (jupyter-with-insertion-bounds
      beg end (insert html)
    ;; TODO: We can't really do much about javascript so
    ;; delete those regions instead of trying to parse
    ;; them.  Maybe just re-direct to a browser like with
    ;; widgets?
    ;; NOTE: Parsing takes a very long time when the text
    ;; is > ~500000 characters.
    (jupyter--delete-script-tags beg end)
    (let ((shr-put-image-function #'jupyter--shr-put-image)
          ;; Avoid issues with proportional fonts.  Sometimes not all of the
          ;; text is rendered using proportional fonts.  See #52.
          (shr-use-fonts nil))
      (if (save-excursion
            (goto-char beg)
            (looking-at "<\\?xml"))
          ;; Be strict about syntax when the html returned explicitly asks to
          ;; be parsed as xml. `libxml-parse-html-region' converts camel cased
          ;; tags/attributes such as viewBox to viewbox in the dom since html
          ;; is case insensitive.  See #4.
          (cl-letf (((symbol-function #'libxml-parse-html-region)
                     #'libxml-parse-xml-region))
            (shr-render-region beg end))
        (shr-render-region beg end)))
    (jupyter-add-font-lock-properties beg end)))

;;; Markdown

(defvar markdown-hide-markup)
(defvar markdown-enable-math)
(defvar markdown-hide-urls)
(defvar markdown-fontify-code-blocks-natively)
(defvar markdown-mode-mouse-map)

(defvar jupyter-markdown-mouse-map
  (let ((map (make-sparse-keymap)))
    (define-key map [return] 'jupyter-markdown-follow-link-at-point)
    (define-key map [follow-link] 'mouse-face)
    (define-key map [mouse-2] 'jupyter-markdown-follow-link-at-point)
    map)
  "Keymap when `point' is over a markdown link in the REPL buffer.")

(cl-defgeneric jupyter-markdown-follow-link (_link-text _url _ref-label _title-text _bang)
  "Follow the markdown link at `point'."
  (markdown-follow-link-at-point))

(defun jupyter-markdown-follow-link-at-point ()
  "Handle markdown links specially."
  (interactive)
  (let ((link (markdown-link-at-pos (point))))
    (when (car link)
      (apply #'jupyter-markdown-follow-link (cddr link)))))

(defun jupyter-insert-markdown (text)
  "Insert TEXT, fontifying it using `markdown-mode' first."
  (let ((beg (point)))
    (insert
     (let ((markdown-hide-markup t)
           (markdown-hide-urls t)
           (markdown-enable-math t)
           (markdown-fontify-code-blocks-natively t))
       (jupyter-fontify-according-to-mode 'markdown-mode text)))
    ;; Update keymaps
    (let ((end (point)) next)
      (setq beg (next-single-property-change beg 'keymap nil end))
      (while (/= beg end)
        (setq next (next-single-property-change beg 'keymap nil end))
        (when (eq (get-text-property beg 'keymap) markdown-mode-mouse-map)
          (put-text-property beg next 'keymap jupyter-markdown-mouse-map))
        (setq beg next)))))

;;; LaTeX

(defvar org-format-latex-options)
(defvar org-preview-latex-image-directory)
(defvar org-babel-jupyter-resource-directory)
(defvar org-preview-latex-default-process)

(defun jupyter-insert-latex (tex)
  "Generate and insert a LaTeX image based on TEX.

Note that this uses `org-format-latex' to generate the LaTeX
image."
  ;; FIXME: Getting a weird error when killing the temp buffers created by
  ;; `org-format-latex'.  When generating the image, it seems that the temp
  ;; buffers created have the same major mode and local variables as the REPL
  ;; buffer which causes the query function to ask to kill the kernel client
  ;; when the temp buffers are killed!
  (let ((kill-buffer-query-functions nil)
        ;; This is added to in `org-babel-jupyter-initiate-session-by-key'
        (kill-buffer-hook nil)
        (org-format-latex-options
         `(:foreground
           default
           :background default :scale 2.0
           :matchers ,(plist-get org-format-latex-options :matchers))))
    (jupyter-with-insertion-bounds
        beg end (insert tex)
      ;; FIXME: Best way to cleanup these files? Just delete them by reading
      ;; the image data and using that for the image instead?
      (org-format-latex
       "ltximg" beg end org-babel-jupyter-resource-directory
       'overlays nil 'forbuffer
       ;; Use the default method for creating image files
       org-preview-latex-default-process)
      ;; Avoid deleting the image overlays due to text property changes
      (dolist (o (overlays-in beg end))
        (when (eq (overlay-get o 'org-overlay-type)
                  'org-latex-overlay)
          (overlay-put o 'modification-hooks nil)))
      (overlay-recenter end)
      (goto-char end))))

;;; Images

(defun jupyter-insert-image (data type &optional metadata)
  "Insert image DATA as TYPE in the current buffer.
TYPE has the same meaning as in `create-image'.  METADATA is a
plist containing :width and :height keys that will be used as the
width and height of the image."
  (cl-destructuring-bind (&key width height needs_background &allow-other-keys)
      metadata
    (let ((img (create-image
                data type 'data :width width :height height
                :max-width (when (> jupyter-image-max-width 0)
                             jupyter-image-max-width)
                :mask (when needs_background
                        '(heuristic t)))))
      (insert-image img))))

;;; Plain text

(defun jupyter-insert-ansi-coded-text (text)
  "Insert TEXT, converting ANSI color codes to font lock faces."
  (jupyter-with-insertion-bounds
      beg end (insert (ansi-color-apply text))
    (jupyter-fixup-font-lock-properties beg end)))

;;; `jupyter-insert' method additions

(cl-defmethod jupyter-insert ((_mime (eql :text/html)) data
                              &optional _metadata)
  (if (not (functionp 'libxml-parse-html-region))
      (cl-call-next-method)
    (jupyter-insert-html data)
    (insert "\n")))

(cl-defmethod jupyter-insert ((_mime (eql :text/markdown)) data
                              &context ((require 'markdown-mode nil t)
                                        (eql markdown-mode))
                              &optional _metadata)
  (jupyter-insert-markdown data))

(cl-defmethod jupyter-insert ((_mime (eql :text/latex)) data
                              &context ((require 'org nil t)
                                        (eql org))
                              &optional _metadata)
  (jupyter-insert-latex data)
  (insert "\n"))

(cl-defmethod jupyter-insert ((_mime (eql :image/svg+xml)) data
                              &context ((and (image-type-available-p 'svg) t)
                                        (eql t))
                              &optional metadata)
  (jupyter-insert-image data 'svg metadata)
  (insert "\n"))

(cl-defmethod jupyter-insert ((_mime (eql :image/jpeg)) data
                              &context ((and (image-type-available-p 'jpeg) t)
                                        (eql t))
                              &optional metadata)
  (jupyter-insert-image (base64-decode-string data) 'jpeg metadata)
  (insert "\n"))

(cl-defmethod jupyter-insert ((_mime (eql :image/png)) data
                              &context ((and (image-type-available-p 'png) t)
                                        (eql t))
                              &optional metadata)
  (jupyter-insert-image (base64-decode-string data) 'png metadata)
  (insert "\n"))

(cl-defmethod jupyter-insert ((_mime (eql :text/plain)) data
                              &optional _metadata)
  ;; Reset the context so that it doesn't leak into DATA if DATA has
  ;; no ANSI sequences.
  (setq ansi-color-context nil)
  (jupyter-insert-ansi-coded-text data)
  (insert "\n"))

;;; Insert with display IDs

(cl-defmethod jupyter-insert :before ((_display-id string) &rest _ignore)
  "Initialize `juptyer-display-ids'"
  ;; FIXME: Set the local display ID hash table for the current buffer, or
  ;; should display IDs be global? Then we would have to associate marker
  ;; positions as well in this table.
  (unless jupyter-display-ids
    (setq jupyter-display-ids (make-hash-table
                               :test #'equal
                               :weakness 'value))))

(cl-defmethod jupyter-insert ((display-id string) data &optional metadata)
  "Associate DISPLAY-ID with DATA when inserting DATA.
DATA and METADATA have the same meaning as in
`jupyter-insert'.

The default implementation adds a jupyter-display text property
to any inserted text and a jupyter-display-begin property to the
first character.

Currently there is no support for associating a DISPLAY-ID if
DATA is displayed as a widget."
  (jupyter-with-insertion-bounds
      beg end (jupyter-insert data metadata)
    ;; Don't add display IDs to widgets since those are currently implemented
    ;; using an external browser and not in the current buffer.
    (when (and (not (memq :application/vnd.jupyter.widget-view+json data))
               (< beg end))
      (let ((id (gethash display-id jupyter-display-ids)))
        (unless id
          (setq id (puthash display-id display-id jupyter-display-ids)))
        (put-text-property beg end 'jupyter-display id)
        (put-text-property beg (1+ beg) 'jupyter-display-begin t)))))

(cl-defgeneric jupyter-current-display ()
  "Return the display ID for the display at `point'.

The default implementation returns the jupyter-display text
property at `point'."
  (get-text-property (point) 'jupyter-display))

(cl-defgeneric jupyter-beginning-of-display ()
  "Go to the beginning of the current Jupyter display.

The default implementation moves `point' to the position of the
character with a jupyter-display-begin property.  If `point' is
already at a character with such a property, then `point' is
returned."
  (if (get-text-property (point) 'jupyter-display-begin) (point)
    (goto-char
     (previous-single-property-change
      (point) 'jupyter-display-begin nil (point-min)))))

(cl-defgeneric jupyter-end-of-display ()
  "Go to the end of the current Jupyter display."
  (goto-char
   (min (next-single-property-change
         (point) 'jupyter-display nil (point-max))
        (next-single-property-change
         (min (1+ (point)) (point-max))
         'jupyter-display-begin nil (point-max)))))

(cl-defgeneric jupyter-next-display-with-id (id)
  "Go to the start of the next display matching ID.
Return non-nil if successful.  If no display with ID is found,
return nil without moving `point'.

The default implementation searches the current buffer for text
with a jupyter-display text property matching ID."
  (or (and (bobp) (eq id (get-text-property (point) 'jupyter-display)))
      (let ((pos (next-single-property-change (point) 'jupyter-display-begin)))
        (while (and pos (not (eq (get-text-property pos 'jupyter-display) id)))
          (setq pos (next-single-property-change pos 'jupyter-display-begin)))
        (and pos (goto-char pos)))))

(cl-defgeneric jupyter-delete-current-display ()
  "Delete the current Jupyter display.

The default implementation checks if `point' has a non-nil
jupyter-display text property, if so, it deletes the surrounding
region around `point' containing that same jupyter-display
property."
  (when (jupyter-current-display)
    (delete-region
     (save-excursion (jupyter-beginning-of-display) (point))
     (save-excursion (jupyter-end-of-display) (point)))))

(cl-defmethod jupyter-update-display ((display-id string) data &optional metadata)
  "Update the display with DISPLAY-ID using DATA.
DATA and METADATA have the same meaning as in a display_data
message."
  (let ((id (and jupyter-display-ids
                 (gethash display-id jupyter-display-ids))))
    (unless id
      (error "Display ID not found (%s)" display-id))
    (save-excursion
      (goto-char (point-min))
      (let (bounds)
        (while (jupyter-next-display-with-id id)
          (jupyter-delete-current-display)
          (jupyter-with-insertion-bounds
              beg end (if bounds (insert-buffer-substring
                                  (current-buffer) (car bounds) (cdr bounds))
                        (jupyter-insert id data metadata))
            (unless bounds
              (setq bounds (cons (copy-marker beg) (copy-marker end))))
            (pulse-momentary-highlight-region beg end 'secondary-selection)))
        (when bounds
          (set-marker (car bounds) nil)
          (set-marker (cdr bounds) nil)))
      (when (= (point) (point-min))
        (error "No display matching id (%s)" id)))))

;;; Pandoc

(defun jupyter-pandoc-convert (from to from-string &optional callback)
  "Use pandoc to convert a string in FROM format to TO format.
Starts a process and converts FROM-STRING, assumed to be in FROM
format, to a string in TO format and returns the converted
string.

If CALLBACK is specified, return the process object.  When the
process exits, call CALLBACK with zero arguments and with the
buffer containing the converted string current."
  (cl-assert (executable-find "pandoc"))
  (let* ((process-connection-type nil)
         (proc (start-process
                "jupyter-pandoc"
                (generate-new-buffer " *jupyter-pandoc*")
                "pandoc" "-f" from "-t" to "--")))
    (set-process-sentinel
     proc (lambda (proc _)
            (when (memq (process-status proc) '(exit signal))
              (with-current-buffer (process-buffer proc)
                (funcall callback)
                (kill-buffer (process-buffer proc))))))
    (process-send-string proc from-string)
    (process-send-eof proc)
    (if callback proc
      (let ((to-string ""))
        (setq callback (lambda () (setq to-string (buffer-string))))
        (while (zerop (length to-string))
          (accept-process-output nil 1))
        to-string))))

(provide 'jupyter-mime)

;;; jupyter-mime.el ends here