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bug#15042: 24.3.50; while-no-input and input-pending-p

From: Michael Heerdegen
Subject: bug#15042: 24.3.50; while-no-input and input-pending-p
Date: Wed, 07 Aug 2013 17:39:42 +0200


I want to discuss if the current implementation of `while-no-input':

--8<---------------cut here---------------start------------->8---
(defmacro while-no-input (&rest body)
  "Execute BODY only as long as there's no pending input.
If input arrives, that ends the execution of BODY,
and `while-no-input' returns t.  Quitting makes it return nil.
If BODY finishes, `while-no-input' returns whatever value BODY produced."
  (declare (debug t) (indent 0))
  (let ((catch-sym (make-symbol "input")))
       (catch ',catch-sym
         (let ((throw-on-input ',catch-sym))
           (or (input-pending-p)
               (progn ,@body)))))))
--8<---------------cut here---------------end--------------->8---

that uses a preliminary `input-pending-p' test is useful.  My reasons:

1.  `input-pending-p' can (and does) return t in cases were no input is
pending (see the doc).  In such cases, `while-no-input' just returns t,
although no input was given.  This contradicts the doc, is not useful
and the behavior is unforeseeable.

2.  Even if `input-pending-p' would not give false alarm sometimes - why
needs `while-no-input' to use it?

If the programmer really wants to check for input _before_ starting the
calculation, he can do so explicitly.

With the current implementation, I have to `discard-input' if I don't
want this.

2 is probably arguable, but 1 is really bad.  I experienced that
 (while-no-input code ...)

is sometimes semantically equivalent to


without any input.



In GNU Emacs (x86_64-pc-linux-gnu, GTK+ Version 3.8.2)
 of 2013-08-04 on dex, modified by Debian
 (emacs-snapshot package, version 2:20130804-1)
Windowing system distributor `The X.Org Foundation', version 11.0.11204000
System Description:     Debian GNU/Linux testing (jessie)

Configured using:
 `configure --build x86_64-linux-gnu --host x86_64-linux-gnu
 --prefix=/usr --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var --infodir=/usr/share/info --mandir=/usr/share/man
 --without-compress-info --with-crt-dir=/usr/lib/x86_64-linux-gnu/
 --with-x=yes --with-x-toolkit=gtk3 --with-imagemagick=yes

Important settings:
  value of $LC_ALL: de_DE.utf8
  value of $LANG: de_DE.utf8
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

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