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From: Joshua Taylor <tay...@cs.rpi.edu>
Newsgroups: comp.lang.lisp
Subject: Re: Does go have a return value ?
Date: Mon, 30 Nov 2009 11:25:08 -0500
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Spiros Bousbouras wrote:
> On Mon, 30 Nov 2009 03:31:14 +0000 (UTC)
> Kaz Kylheku <kkylh...@gmail.com> wrote:
>> On 2009-11-30, Spiros Bousbouras <spi...@gmail.com> wrote:
>>> Does a go form have a return value ?
>> Does a one-handed clap make a sound?
> 
> If you hit it against your thigh it most definitely does.
> 
>> Why would something that doesn't return have a return value?
> 
> Since the vast majority of Lisp forms return a value for reasons of
> uniformity perhaps ? To make the language more functional ? Which makes
> me wonder whether pure functional languages gave gotos and if they do
> how they handle similar constructs.

It's true that the most Lisp forms return a value, and you are looking 
at an exception.  Although functions may return zero values (via 
VALUES), we're looking at something that isn't a function; GO is a 
special operator.  Some special operators, of course, produce values, 
but some don't.  The syntax for "|" (I had to dig for it, but I was 
curious too) is documented in �1.4.4.20.4.2 Unconditional Transfer of 
Control in the ``Syntax Section''[1] which states (notice that it 
specifically uses the term "operators"):


""Some operators perform an unconditional transfer of control, and so 
never have any return values. Such operators are notated using a 
notation such as the following:

F a b c =>|""


As Kaz mentioned, there's no way for anything to consume the result of 
GO, even if it did produce a result.  A simpler example using assignment 
shows that in (setf *x* (go end)), the value of *x* doesn't get changes 
at all:


CL-USER> (defparameter *x* 'x)
*X*

CL-USER > (tagbody
              (setf *x* (go end))
              end)
NIL

CL-USER > *x*
X


Although CL isn't a pure functional language, there are similarities and 
CL can often be used purely and functionally.  It would be interesting 
to see how other pure and not-so-pure functional-ish languages handle 
these sorts of situations.  Another good thing to compare is condition 
handling.  And sure enough, of five restart functions in CL (ABORT, 
CONTINUE, MUFFLE-WARNING, STORE-VALUE and USE-VALUE), two (ABORT and 
CONTINUE) use the "=>|" notation [2].

//JT

[1] http://www.lispworks.com/documentation/HyperSpec/Body/01_ddtdb.htm
[2] http://www.lispworks.com/documentation/HyperSpec/Body/f_abortc.htm