From: Gertjan van Heijst
Subject: returning zero values
Date: Thu, 17 Sep 1992 14:36:48 +0000
Message-ID: <1992Sep17.143648.23973@swi.psy.uva.nl>

Hi,

I was wondering how I could persuade a function to return zero
values. From CLtL2 I understood that the function call (values)
was the appropriate way but it doesn't solve my problem.

what I want to do is the following

> (setq *var1* nil
        *var2* 'var2)
VAR2

> (list (if *var1* 
            *var1*
          (values))
        (if *var2*
            *var2*
	  (values)))
(VAR2)


however my lisp (LCL4.0) returns (NIL VAR2) in stead
of (VAR2) when I evaluate the forms above. What am I
doing wrong????





-- 
--------------------------------------------------------------
Gertjan van Heijst			·······@swi.psy.uva.nl
Social Science Informatics
Department of Psychology

• returning zero values Scott McKay
• Re: returning zero values Len Charest

From: Scott McKay
Subject: returning zero values
Date: Mon, 21 Sep 1992 16:12:22 +0000
Message-ID: <19920921161302.0.SWM@SUMMER.SCRC.Symbolics.COM>

    Date: Thu, 17 Sep 1992 10:36 EDT
    From: Gertjan van Heijst <·······@swi.psy.uva.nl>

    Hi,

    I was wondering how I could persuade a function to return zero
    values. From CLtL2 I understood that the function call (values)
    was the appropriate way but it doesn't solve my problem.

    what I want to do is the following

    > (setq *var1* nil
	    *var2* 'var2)
    VAR2

    > (list (if *var1* 
		*var1*
	      (values))
	    (if *var2*
		*var2*
	      (values)))
    (VAR2)


`(,@(and *var1* (list *var1*))
  ,@(and *var2* (list *var2*)))

or

(multiple-value-call #'list
  (if *var1* *var1* (values))
  (if *var2* *var2* (values)))

    however my lisp (LCL4.0) returns (NIL VAR2) instead
    of (VAR2) when I evaluate the forms above. What am I
    doing wrong????





    -- 
    --------------------------------------------------------------
    Gertjan van Heijst			·······@swi.psy.uva.nl
    Social Science Informatics
    Department of Psychology

From: Len Charest
Subject: Re: returning zero values
Date: Mon, 21 Sep 1992 18:43:32 +0000
Message-ID: <1992Sep21.184332.8161@jpl-devvax.jpl.nasa.gov>

In article <······················@swi.psy.uva.nl>, ·······@swi.psy.uva.nl (Gertjan van Heijst) writes:
|> Hi,
|> 
|> I was wondering how I could persuade a function to return zero
|> values. From CLtL2 I understood that the function call (values)
|> was the appropriate way but it doesn't solve my problem.
|> [ example showing how use of (VALUES) actually returns NIL ]

Scott McKay has already posted some code to work around your problem, but this
excerpt from CLtL2 may shed some light on why VALUES behaves so: "In an ordinary
function call, each argument form produces exactly *one* argument; if such a form
returns zero values, NIL is used for the argument..." (page 186). 

I suppose one example of an extra-ordinary function call would be a form typed to
the Lisp top-level (aka listener) since (VALUES) actually returns zero values
there.
..................................................
                                  Len Charest, Jr.
                 JPL Artificial Intelligence Group
                          ·······@aig.jpl.nasa.gov