Modification to LISTEN

[Mike McMahon <MMcM@SCRC-STONY-BROOK.ARPA>]

    Date: Wed, 18 Sep 1985  12:56 EDT
    From: "Scott E. Fahlman" <Fahlman@C.CS.CMU.EDU>

	I would like to have LISTEN return two values.  The first value is the same as
	it is now and the second value is true if end of file is encountered.  The
	new definition allows READ-CHAR-NO-HANG to be implemented using only READ-CHAR
	and LISTEN.

    Looks like a good suggestion to me.  It could be implemented in a
    completely upward-compatible way.

In good lisp machine style, programs use both READ-CHAR-NO-HANG and
LISTEN.  LISTEN is treated only as a (possibly spurious) wakeup.
Imagine a network stream interface, where LISTEN returns when some
packet-ready-for-reciept hardware flag gets set.  READ-CHAR-NO-HANG
might return NIL if the packet that it reads out is damaged, out of
sequence, etc.  The rule is that LISTEN is allowed to return T when
READ-CHAR-NO-HANG will return NIL, provided that READ-CHAR-NO-HANG will
tend to clear the spurious condition.

I'm not sure this advocates against the above proposal, but I do feel
that there is good reason for having both primitives.