Re: Coding system to encode arguments to groff?

[Eli Zaretskii <eliz@gnu.org>]

> From: Tim Landscheidt <tim@tim-landscheidt.de>
> Cc: help-gnu-emacs@gnu.org
> Date: Sun, 03 Oct 2021 13:14:04 +0000
> 
> | (let
> |     ((temp-ps-buffer (generate-new-buffer "*test ps*"))
> |      (test-arg "a-o"))
> |   (with-temp-buffer
> |     (insert ".fam H\n\\*[test-arg]\n")
> |     (call-process-region
> |      (point-min)
> |      (point-max)
> |      "groff"
> |      nil
> |      temp-ps-buffer
> |      nil
> |      "-Tps"
> |      "-d" (concat "test-arg=" test-arg)))
> |   (switch-to-buffer temp-ps-buffer)
> |   (ps-mode)
> |   (doc-view-mode))
> 
> produces a PostScript buffer with the text "a-o".
> 
> With test-arg = "ä-ö" (ä minus ö), it produces gibberish mi-
> nus gibberish.
> 
> With test-arg = (encode-coding-string "ä-ö" 'iso-latin-1) (ä
> minus ö), it produces the text "ä-ö".
> 
> With test-arg = (encode-coding-string "ä–ö" 'iso-latin-1) (ä
> endash ö), it produces the text "ä[white space]ö".
> 
> With test-arg = (shell-command-to-string (concat "preconv -r
> <(echo " (shell-quote-argument "ä–ö") ")")) (ä endash ö), it
> produces the intended text "ä–ö".

So the problem is that troff doesn't accept non-ASCII command-line
arguments, and so you want to convert non-ASCII characters into a
series of characters encoded in the [\uNNNN] form, is that right?

Then I guess mapconcat is your friend, something like

  (mapconcat (lambda (ch)
	       (format "[\\u%4.4X]" ch))
	     "ä–ö" "")

There's no need to use preconv at all, as Emacs can do that by itself.
And this isn't an encoding, because codepoints are not encoded in any
sense of that word.