Jens Lechtenboerger <lech...@wi.uni-muenster.de> writes:

> Without the second argument I get an error “Wrong type argument:
> stringp,” when evaluating regular expressions against the cons cell
> that is returned as title.
>
> As I see now, author and title are cons cells, which is why
> org-element-interpret-data is necessary to produce strings with Org
> syntax.
>
> Also, after fixing the title, I get “wrong-type-argument sequencep
> utf-8” for “(concat "text/html;charset=" charset)”.

Thanks for testing this :) I haven't forgotten about this.

Next version!

>From 1289e381aff7562df96945aa58838ad966aa9211 Mon Sep 17 00:00:00 2001
From: TEC <t...@tecosaur.com>
Date: Thu, 17 Sep 2020 21:27:18 +0800
Subject: [PATCH] lisp/ox-html.el: make html meta func nicer

* lisp/ox-html.el (org-html--build-meta-info): Multi-line repeated
structure extracted to new function `org-html--build-meta-entry'.
The opportunity was taken to extract most metadata info to custom
variable `org-html-meta-tags', allowing for easy end-user modification.
---
 lisp/ox-html.el | 131 +++++++++++++++++++++++++++---------------------
 1 file changed, 73 insertions(+), 58 deletions(-)

diff --git a/lisp/ox-html.el b/lisp/ox-html.el
index d2f24f5c6..93014e9c7 100644
--- a/lisp/ox-html.el
+++ b/lisp/ox-html.el
@@ -1425,6 +1425,31 @@ not be modified."
 
 ;;;; Template :: Styles
 
+(defcustom org-html-meta-tags
+  '((lambda (_title author _info)
+      (when (org-string-nw-p author)
+	(list "name" "author" author)))
+    (lambda (_title _author info)
+      (when (org-string-nw-p (plist-get info :description))
+	(list "name" "description"
+	      (plist-get info :description))))
+    (lambda (_title _author info)
+      (when (org-string-nw-p (plist-get info :keywords))
+	(list "keywords" (plist-get info :keywords))))
+    ("name" "generator" "Org Mode"))
+  "A list of arguments to be passed to `org-html--build-meta-entry'.
+Each argument can either be an list which is applied, or a function which
+generates such a list with signature (TITLE AUTHOR INFO) where TITLE and AUTHOR
+are strings, and INFO a communication plist."
+  :group 'org-export-html
+  :package-version '(Org . "9.5")
+  :type '(repeat
+	  (choice
+	   (list (string :tag "Meta label")
+		 (string :tag "label value")
+		 (string :tag "Content value"))
+	   function)))
+
 (defcustom org-html-head-include-default-style t
   "Non-nil means include the default style in exported HTML files.
 The actual style is defined in `org-html-style-default' and
@@ -1835,78 +1860,68 @@ INFO is a plist used as a communication channel."
 
 ;;; Template
 
+(defun org-html--build-meta-entry (label identity &optional content-format &rest content-formatters)
+  "Construct <meta> tag with LABEL=\"IDENTITY\" and content from CONTENT-FORMAT and CONTENT-FORMATTER."
+  (concat "<meta "
+	  (format "%s=\"%s" label identity)
+	  (when content-format
+	    (concat "\" content=\""
+		    (replace-regexp-in-string
+		     "\"" "&quot;"
+		     (org-html-encode-plain-text
+		      (if content-formatters
+			  (apply #'format content-format content-formatters)
+			content-format)))))
+	  "\" />\n"))
+
 (defun org-html--build-meta-info (info)
   "Return meta tags for exported document.
 INFO is a plist used as a communication channel."
-  (let* ((protect-string
-          (lambda (str)
-            (replace-regexp-in-string
-             "\"" "&quot;" (org-html-encode-plain-text str))))
-         (title (org-export-data (plist-get info :title) info))
-         ;; Set title to an invisible character instead of leaving it
-         ;; empty, which is invalid.
-         (title (if (org-string-nw-p title) title "&lrm;"))
-         (author (and (plist-get info :with-author)
-                      (let ((auth (plist-get info :author)))
+  (let* ((title (org-html-encode-plain-text (or (car (plist-get info :title)) "Org Export")))
+	 ;; Set title to an invisible character instead of leaving it
+	 ;; empty, which is invalid.
+	 (title (if (org-string-nw-p title) title "&lrm;"))
+	 (author (and (plist-get info :with-author)
+		      (let ((auth (plist-get info :author)))
 			;; Return raw Org syntax.
-                        (and auth (org-element-interpret-data auth)))))
-         (description (plist-get info :description))
-         (keywords (plist-get info :keywords))
-         (charset (or (and org-html-coding-system
-                           (fboundp 'coding-system-get)
-                           (coding-system-get org-html-coding-system
-                                              'mime-charset))
-                      "iso-8859-1")))
+			(and auth (org-element-interpret-data auth)))))
+	 (charset (or (and org-html-coding-system
+			   (fboundp 'coding-system-get)
+			   (symbol-name
+			    (coding-system-get org-html-coding-system
+					       'mime-charset)))
+		      "iso-8859-1")))
     (concat
      (when (plist-get info :time-stamp-file)
        (format-time-string
 	(concat "<!-- "
 		(plist-get info :html-metadata-timestamp-format)
 		" -->\n")))
-     (format
-      (if (org-html-html5-p info)
-	  (org-html-close-tag "meta" "charset=\"%s\"" info)
-	(org-html-close-tag
-	 "meta" "http-equiv=\"Content-Type\" content=\"text/html;charset=%s\""
-	 info))
-      charset) "\n"
+
+     (if (org-html-html5-p info)
+	 (org-html--build-meta-entry "charset" charset)
+       (org-html--build-meta-entry "http-equiv" "Content-Type"
+				   (concat "text/html;charset=" charset)))
+
      (let ((viewport-options
 	    (cl-remove-if-not (lambda (cell) (org-string-nw-p (cadr cell)))
 			      (plist-get info :html-viewport))))
-       (and viewport-options
-	    (concat
-	     (org-html-close-tag
-	      "meta"
-	      (format "name=\"viewport\" content=\"%s\""
-		      (mapconcat
-		       (lambda (elm) (format "%s=%s" (car elm) (cadr elm)))
-		       viewport-options ", "))
-	      info)
-	     "\n")))
+       (if viewport-options
+	   (org-html--build-meta-entry "name" "viewport"
+				       (mapconcat
+					(lambda (elm) (format "%s=%s" (car elm) (cadr elm)))
+					viewport-options ", "))))
+
      (format "<title>%s</title>\n" title)
-     (org-html-close-tag "meta" "name=\"generator\" content=\"Org mode\"" info)
-     "\n"
-     (and (org-string-nw-p author)
-	  (concat
-	   (org-html-close-tag "meta"
-			       (format "name=\"author\" content=\"%s\""
-				       (funcall protect-string author))
-			       info)
-	   "\n"))
-     (and (org-string-nw-p description)
-	  (concat
-	   (org-html-close-tag "meta"
-			       (format "name=\"description\" content=\"%s\"\n"
-				       (funcall protect-string description))
-			       info)
-	   "\n"))
-     (and (org-string-nw-p keywords)
-	  (concat
-	   (org-html-close-tag "meta"
-			       (format "name=\"keywords\" content=\"%s\""
-				       (funcall protect-string keywords))
-			       info)
-	   "\n")))))
+
+     (apply #'concat
+	    (mapcar
+	     (lambda (form)
+	       (when (functionp form)
+		 (setq form (funcall form title author info)))
+	       (when form
+		 (apply #'org-html--build-meta-entry form)))
+	     org-html-meta-tags)))))
 
 (defun org-html--build-head (info)
   "Return information for the <head>..</head> of the HTML output.
-- 
2.29.2

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