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bug#27270: display-raw-bytes-as-hex generates ambiguous output for Emacs

From: Paul Eggert
Subject: bug#27270: display-raw-bytes-as-hex generates ambiguous output for Emacs strings
Date: Sat, 10 Jun 2017 17:04:40 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0

On 06/10/2017 12:24 AM, Eli Zaretskii wrote:
So your proposal would mean a change to the Lisp reader to support
such escapes, right?  If so, isn't such a change

Yes, but only in the sense that undocumented escapes evaluate to themselves, e.g., "\F" is currently the same as "F" in Emacs Lisp because there is no escape sequence \F currently defined for character constants. But there's nothing new here, e.g., when we added "\N{...}" last year we changed the interpretation of the formerly-undocumented \N escape.

Also, display-raw-bytes-as-hex would cause raw bytes to be displayed with this
new X escape, rather than with with the x escape.
It could only do that for codepoints below 256 decimal, so that
limitation should be taken into account when deciding on the proposal.

Ouch, I hadn't thought of that.

Wait -- doesn't that mean that "display-raw-bytes-as-hex" is a misleading name, because it affects the display not only of raw bytes, but of other undisplayable characters? Shouldn't we change its name to something more generic and more accurate, like "display-characters-as-hex"?

Anyway, to address the point you raised: how about a different idea? We extend the existing \x syntax in strings so that \x{dddd} has the same meaning as "\xdddd", except that the "}" terminates the escape. This syntax is used by Perl and so is in the same family as \N{...}. We also change display-raw-bytes-as-hex to use this new syntax when a character is immediately followed by a hexadecimal digit. That way, most characters are displayed as before, but my problematic example is displayed as "x\x{90}5y", which is a good visual cue of the unusual situation.

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