[idn] Why IDNA breaks copy-and-paste

["D. J. Bernstein" <djb@cr.yp.to>]

Kent Karlsson writes:
> What is displayed is glyphs.  In many cases there is no (uniquely)
> corresponding character.

You misunderstand.

Look at the copy-and-paste supported by the UNIX xterm program. Does it
read dots off the display and attempt to convert them to characters? Of
course not. It obtains characters directly from xterm.

This does _not_ mean, however, that it obtains characters directly from
``the application,'' such as the MH/NMH ``show'' program that Keith uses
to read his mail. The system has been engineered so that

   * programs such as ``show'' simply print characters, without worrying
     about dots on the display, copy-and-paste, etc.;

   * programs such as ``xterm'' read characters and manage the display,
     without worrying about where the characters came from.

This modularity speeds software development; let me again recommend
Gancarz's ``UNIX Philosophy'' book. The system doesn't have, and doesn't
need, copy-and-paste in programs such as ``show''.

Web-mail systems are engineered the same way:

   * The web server converts your mail to HTML, without worrying about
     your display.

   * The browser displays the HTML, without worrying about how the HTML
     was generated.

This modularity again speeds software development---but it again means
that there's no copy-and-paste support in the web-mail system.

We seem to have many novice programmers here, people under the delusion
that copy-and-paste is always handled by ``the application,'' which can
provide characters in some dorky 7-bit encoding if it wants to. But the
real world has, in many cases, advanced beyond that primitive model.

---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago