Add DOI links export

* lisp/ol-doi.el: New file.
* lisp/ol.el (org-link-doi-server-url): Move to new file.
(org-link--open-doi): Remove function.
* lisp/org.el (org-modules): Autoload new library for backward
compatibility.

Reported-by: yarnton--- via "General discussions about Org-mode." <emacs-orgmode@gnu.org>
<http://lists.gnu.org/r/emacs-orgmode/2020-09/msg00908.html>

lisp/ol-doi.el

@@ -0,0 +1,70 @@
+;;; ol-doi.el --- DOI links support in Org           -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2021  Free Software Foundation, Inc.
+
+;; Author: Nicolas Goaziou <mail@nicolasgoaziou.fr>
+
+;; 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 of the License, 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 this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This library introduces the "doi" link type in Org, and provides
+;; code for opening and exporting such links.
+
+;;; Code:
+
+(require 'ol)
+
+(defcustom org-link-doi-server-url "https://doi.org/"
+  "The URL of the DOI server."
+  :group 'org-link-follow
+  :version "24.3"
+  :type 'string
+  :safe #'stringp)
+
+(defun org-link-doi-open (path arg)
+  "Open a \"doi\" type link.
+PATH is a the path to search for, as a string."
+  (browse-url (url-encode-url (concat org-link-doi-server-url path)) arg))
+
+(defun org-link-doi-export (path desc backend info)
+  "Export a \"doi\" type link.
+PATH is the DOI name.  DESC is the description of the link, or
+nil.  BACKEND is a symbol representing the backend used for
+export.  INFO is a a plist containing the export parameters."
+  (let ((uri (concat org-link-doi-server-url path)))
+    (pcase backend
+      (`html
+       (format "<a href=\"%s\">%s</a>" uri (or desc uri)))
+      (`latex
+       (if desc (format "\\href{%s}{%s}" uri desc)
+	 (format "\\url{%s}" uri)))
+      (`ascii
+       (if (not desc) (format "<%s>" uri)
+         (concat (format "[%s]" desc)
+		 (and (not (plist-get info :ascii-links-to-notes))
+		      (format " (<%s>)" uri)))))
+      (`texinfo
+       (if (not desc) (format "@uref{%s}" uri)
+         (format "@uref{%s, %s}" uri desc)))
+      (_ uri))))
+
+(org-link-set-parameters "doi"
+                         :follow #'org-link-doi-open
+                         :export #'org-link-doi-export)
+
+
+(provide 'org-link-doi)
+(provide 'ol-doi)
+;;; ol-doi.el ends here

lisp/ol.el

@@ -282,13 +282,6 @@ links created by planner."
   :type '(choice (const nil) (function))
   :safe #'null)
 
-(defcustom org-link-doi-server-url "https://doi.org/"
-  "The URL of the DOI server."
-  :group 'org-link-follow
-  :version "24.3"
-  :type 'string
-  :safe #'stringp)
-
 (defcustom org-link-frame-setup
   '((vm . vm-visit-folder-other-frame)
     (vm-imap . vm-visit-imap-folder-other-frame)
@@ -1306,14 +1299,6 @@ If there is no description, use the link target."
 
 ;;; Built-in link types
 
-;;;; "doi" link type
-(defun org-link--open-doi (path arg)
-  "Open a \"doi\" type link.
-PATH is a the path to search for, as a string."
-  (browse-url (url-encode-url (concat org-link-doi-server-url path)) arg))
-
-(org-link-set-parameters "doi" :follow #'org-link--open-doi)
-
 ;;;; "elisp" link type
 (defun org-link--open-elisp (path _)
   "Open a \"elisp\" type link.

lisp/org.el

@@ -680,7 +680,7 @@ defined in org-duration.el.")
     (org-load-modules-maybe 'force)
     (org-element-cache-reset 'all)))
 
-(defcustom org-modules '(ol-w3m ol-bbdb ol-bibtex ol-docview ol-gnus ol-info ol-irc ol-mhe ol-rmail ol-eww)
+(defcustom org-modules '(ol-doi ol-w3m ol-bbdb ol-bibtex ol-docview ol-gnus ol-info ol-irc ol-mhe ol-rmail ol-eww)
   "Modules that should always be loaded together with org.el.
 
 If a description starts with <C>, the file is not part of Emacs
@@ -697,8 +697,7 @@ to add the symbol `xyz', and the package must have a call to:
 For export specific modules, see also `org-export-backends'."
   :group 'org
   :set 'org-set-modules
-  :version "26.1"
-  :package-version '(Org . "9.2")
+  :package-version '(Org . "9.5")
   :type
   '(set :greedy t
 	(const :tag "   bbdb:              Links to BBDB entries" ol-bbdb)
@@ -706,6 +705,7 @@ For export specific modules, see also `org-export-backends'."
 	(const :tag "   crypt:             Encryption of subtrees" org-crypt)
 	(const :tag "   ctags:             Access to Emacs tags with links" org-ctags)
 	(const :tag "   docview:           Links to Docview buffers" ol-docview)
+        (const :tag "   doi:               Links to DOI references" ol-docview)
 	(const :tag "   eww:               Store link to URL of Eww" ol-eww)
 	(const :tag "   gnus:              Links to GNUS folders/messages" ol-gnus)
 	(const :tag "   habit:             Track your consistency with habits" org-habit)