Subject: 24.3.50; while-no-input and input-pending-p

From: Michael Heerdegen <[email-masked]>
Date: Wed, 07 Aug 2013 17:39:42 +0200

Hello,

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")))
    `(with-local-quit
       (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

 t

without any input.

Regards,

Michael.

In GNU Emacs 24.3.50.1 (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)

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