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Re: emacs-26 8f18d12: Improve documentation of decoding into a unibyte b

From: Robert Pluim
Subject: Re: emacs-26 8f18d12: Improve documentation of decoding into a unibyte buffer
Date: Mon, 27 May 2019 15:02:42 +0200

>>>>> On Mon, 27 May 2019 08:24:46 -0400, Stefan Monnier <address@hidden> said:

    >> A related issue: C-h f string-as-unibyte
    >> string-as-unibyte is a built-in function in `src/fns.c'.
    >> (string-as-unibyte STRING)
    >> This function is obsolete since 26.1;
    >> use `encode-coding-string'.
    >> Probably introduced at or before Emacs version 20.3.
    >> This function does not change global state, including the match data.
    >> Having trawled through the elisp manual, for the life of me itʼs not
    >> clear which coding system I should use. 'raw-text'? 'us-ascii'?
    >> Something Else?

    Stefan> The coding that most closely corresponds to what string-as-unibyte 
    Stefan> is `emacs-internal`.  In 90% of the cases, it's not what you want, 
    Stefan> because the code shouldn't have used string-as-unibyte in the
    Stefan> first place, so you'll need to find out what the code *really* 

Almost all uses of string-as-unibyte are gone now, but the one I was
looking at is this one in international/mule-cmds.el:

    (defun encoded-string-description (str coding-system)
      "Return a pretty description of STR that is encoded by CODING-SYSTEM."
      (setq str (string-as-unibyte str))
       (if (and coding-system (eq (coding-system-type coding-system) 'iso-2022))
           ;; Try to get a pretty description for ISO 2022 escape sequences.
           (function (lambda (x) (or (cdr (assq x iso-2022-control-alist))
                                     (format "#x%02X" x))))
         (function (lambda (x) (format "#x%02X" x))))
       str " "))

If I take a string of say "β", and replace string-as-unibyte with
(encode-coding-string 'emacs-internal), `encoded-string-description'
prints "#xCE #xB2", which is the correct UTF-8 encoded
value. 'raw-text works too. Iʼm certain that there are subtle
differences between the two that I donʼt understand.


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