[O] [RFC] ox-ascii.el: fixing variable width character handling

[Yasushi SHOJI]

Hi,

I've been trying to fix ASCII export back-end for variable width
chars. It is basically replacing `length' with `string-width', but the
behavior of those two functions differ when you give nil as an
argument; `length' returns 0, `string-width' yields a type error:
"eval: Wrong type argument: stringp, nil"

While I came up with the following experimental patch, I have a few
questions:

 - What is the lisp idiom to handle type error?  In the following
   patch, I've created a new wrapper function
   `org-ascii--string-width', it's a thin wrapper with if-then-else.
   I thought placing if-then-else clause in the original place clutter
   the code.

 - If wrapped string-width is good idea, would it be also a good idea
   to merge it with `org-string-width' in org.el?  Because it is not
   ox-ascii specific.  The current `org-string-width' does not handle
   nil, neither.

 - The width of the underline for headlines should be checked with
   string-width.

   As defined in Unicode Standard Annex #41[1], character width is
   environment dependent.  So, the calculation of character width at
   export time might not be sufficient enough.  We can check to see
   the exported document contains cjk chars more than some thresholds
   or not, I haven't gone that far.

 - Does anyone using ox-ascii.el depends on the clipped marker `=>'
   for fixed width columns?  I thought `...' would be much readable.

 - BTW, while looking at table handling, I noticed fixed column width
   doesn't work with the code at the current git HEAD.  That's because
   width calculation is mixed with `length' and `string-width', and
   pass out-of-range arguments to `add-text-properties'.


[1]: http://www.unicode.org/reports/tr41/

diff --git a/lisp/ox-ascii.el b/lisp/ox-ascii.el
index 8e75007..35d58fc 100644
--- a/lisp/ox-ascii.el
+++ b/lisp/ox-ascii.el
@@ -630,7 +630,8 @@ possible.  It doesn't apply to `inlinetask' elements."
                              org-ascii-underline)))))
         (and under-char
              (concat "\n"
-                     (make-string (length first-part) under-char))))))))
+                     (make-string (/ (string-width first-part) (char-width 
under-char))
+                                  under-char))))))))
 
 (defun org-ascii--has-caption-p (element info)
   "Non-nil when ELEMENT has a caption affiliated keyword.
@@ -1704,7 +1705,7 @@ are ignored."
               (org-element-map table 'table-row
                 (lambda (row)
                   (setq max-width
-                        (max (length
+                        (max (string-width
                               (org-export-data
                                (org-element-contents
                                 (elt (org-element-contents row) col))
@@ -1714,6 +1715,11 @@ are ignored."
               max-width))
         cache))))
 
+(defun org-ascii--string-width (str)
+  (if str
+      (string-width str)
+    0))
+
 (defun org-ascii-table-cell (table-cell contents info)
   "Transcode a TABLE-CELL object from Org to ASCII.
 CONTENTS is the cell contents.  INFO is a plist used as
@@ -1724,16 +1730,18 @@ a communication channel."
   ;; each cell in the column.
   (let ((width (org-ascii--table-cell-width table-cell info)))
     ;; When contents are too large, truncate them.
-    (unless (or org-ascii-table-widen-columns (<= (length contents) width))
-      (setq contents (concat (substring contents 0 (- width 2)) "=>")))
+    (unless (or org-ascii-table-widen-columns
+               (<= (org-ascii--string-width contents) width))
+      (setq contents (truncate-string-to-width contents width nil ?. t)))
     ;; Align contents correctly within the cell.
     (let* ((indent-tabs-mode nil)
           (data
            (when contents
              (org-ascii--justify-string
               contents width
-              (org-export-table-cell-alignment table-cell info)))))
-      (setq contents (concat data (make-string (- width (length data)) ? ))))
+              (org-export-table-cell-alignment table-cell info))))
+          (trailing-space (make-string (- width (org-ascii--string-width 
data)) ? )))
+      (setq contents (concat data trailing-space)))
     ;; Return cell.
     (concat (format " %s " contents)
            (when (memq 'right (org-export-table-cell-borders table-cell info))