From mboxrd@z Thu Jan  1 00:00:00 1970
Path: news.gmane.org!not-for-mail
From: Taylan Ulrich Bayirli/Kammer <taylanbayirli@gmail.com>
Newsgroups: gmane.lisp.guile.user
Subject: Re: Why is an empty lambda body not allowed?
Date: Sat, 05 Jul 2014 01:01:39 +0300
Message-ID: <871tu0eqbg.fsf@taylan.uni.cx>
References: <CAOOWqiqJqjxR65yVALkqPT4WnyXz60svN-zQ4qt4de9rWFUMJA@mail.gmail.com>
	<877g3tdkvs.fsf@taylan.uni.cx>
	<CAOOWqiqD9qUqFi1gvQU+EE4eEjWOijMV-F=0fZGReDucbN7bAw@mail.gmail.com>
NNTP-Posting-Host: plane.gmane.org
Mime-Version: 1.0
Content-Type: text/plain
X-Trace: ger.gmane.org 1404511373 14869 80.91.229.3 (4 Jul 2014 22:02:53 GMT)
X-Complaints-To: usenet@ger.gmane.org
NNTP-Posting-Date: Fri, 4 Jul 2014 22:02:53 +0000 (UTC)
Cc: "guile-user@gnu.org" <guile-user@gnu.org>
To: Tobias Brandt <tob.brandt@gmail.com>
Original-X-From: guile-user-bounces+guile-user=m.gmane.org@gnu.org Sat Jul 05 00:02:46 2014
Return-path: <guile-user-bounces+guile-user=m.gmane.org@gnu.org>
Envelope-to: guile-user@m.gmane.org
Original-Received: from lists.gnu.org ([208.118.235.17])
	by plane.gmane.org with esmtp (Exim 4.69)
	(envelope-from <guile-user-bounces+guile-user=m.gmane.org@gnu.org>)
	id 1X3BZ8-0004v1-6a
	for guile-user@m.gmane.org; Sat, 05 Jul 2014 00:02:46 +0200
Original-Received: from localhost ([::1]:38213 helo=lists.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <guile-user-bounces+guile-user=m.gmane.org@gnu.org>)
	id 1X3BZ5-0005oN-Dm
	for guile-user@m.gmane.org; Fri, 04 Jul 2014 18:02:43 -0400
Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:52735)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <taylanbayirli@gmail.com>) id 1X3BYn-0005o7-VT
	for guile-user@gnu.org; Fri, 04 Jul 2014 18:02:35 -0400
Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <taylanbayirli@gmail.com>) id 1X3BYe-0004J7-Tn
	for guile-user@gnu.org; Fri, 04 Jul 2014 18:02:25 -0400
Original-Received: from mail-we0-x22d.google.com ([2a00:1450:400c:c03::22d]:62305)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <taylanbayirli@gmail.com>) id 1X3BYd-0004Ie-Ln
	for guile-user@gnu.org; Fri, 04 Jul 2014 18:02:16 -0400
Original-Received: by mail-we0-f173.google.com with SMTP id t60so2144944wes.18
	for <guile-user@gnu.org>; Fri, 04 Jul 2014 15:02:13 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113;
	h=from:to:cc:subject:references:date:in-reply-to:message-id
	:user-agent:mime-version:content-type;
	bh=FdoMHd0kNLoZKgHE2w2RCXbVlMobZPrnZH6qbRil+iU=;
	b=P/MyyWir6pWVujvkSFumn9C+aQA1AeDl52Rm1qLkrMdVkn9mVt43bHO0eIC6Zfxfe6
	N8EEgM+wGOOJyX7Nw+vb0G3xwLJzuu8KTo7UxULyhBm85mQSVfJQZn7bxqxKY23QqOuV
	dSxOPJkbYLmIAo4b1EUUr1KZS4fW2RJztzLnjnQzI5ZdzDlI2jNwgluB636am7sPRms0
	jP9B+3RJQuB+oAVoJVjZEkXkpmgMLSwOYVgQkjO15vLSlGWMWovUTKrzSpbyTr2TBUNu
	TXONrFgMe0zTX1FE00xs5oGJub2noY7IIVw8OarCB83E//bRE4InVgDmcBtDx0lh3MbD
	OPKA==
X-Received: by 10.194.1.164 with SMTP id 4mr14847523wjn.17.1404511333771;
	Fri, 04 Jul 2014 15:02:13 -0700 (PDT)
Original-Received: from taylan.uni.cx ([85.100.205.21])
	by mx.google.com with ESMTPSA id ft2sm74634307wib.1.2014.07.04.15.02.12
	for <multiple recipients>
	(version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
	Fri, 04 Jul 2014 15:02:13 -0700 (PDT)
In-Reply-To: <CAOOWqiqD9qUqFi1gvQU+EE4eEjWOijMV-F=0fZGReDucbN7bAw@mail.gmail.com>
	(Tobias Brandt's message of "Fri, 4 Jul 2014 21:14:43 +0200")
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)
X-detected-operating-system: by eggs.gnu.org: Error: Malformed IPv6 address
	(bad octet value).
X-Received-From: 2a00:1450:400c:c03::22d
X-BeenThere: guile-user@gnu.org
X-Mailman-Version: 2.1.14
Precedence: list
List-Id: General Guile related discussions <guile-user.gnu.org>
List-Unsubscribe: <https://lists.gnu.org/mailman/options/guile-user>,
	<mailto:guile-user-request@gnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/archive/html/guile-user>
List-Post: <mailto:guile-user@gnu.org>
List-Help: <mailto:guile-user-request@gnu.org?subject=help>
List-Subscribe: <https://lists.gnu.org/mailman/listinfo/guile-user>,
	<mailto:guile-user-request@gnu.org?subject=subscribe>
Errors-To: guile-user-bounces+guile-user=m.gmane.org@gnu.org
Original-Sender: guile-user-bounces+guile-user=m.gmane.org@gnu.org
Xref: news.gmane.org gmane.lisp.guile.user:11324
Archived-At: <http://permalink.gmane.org/gmane.lisp.guile.user/11324>

Tobias Brandt <tob.brandt@gmail.com> writes:

> I don't really care what (lambda ()) evaluates to as long as it's
> consistent with the rest of the language. But what rubs me the wrong
> way is that
>
> (define x (begin))
> (lambda () x)
>
> is not the same as
>
> (lambda () (begin))
>
> because one of the strength of functional programming is referential
> transparency. Therefore I would expect the definition of x (which has
> no side effects) to be substitutable for x itself. This is apparently
> not the case as one (begin) has a different meaning than the other
> because of its context.

I agree it's annoying.  However it only breaches referential
transparency on a superficial, textual level, or not at all depending on
your definition.  In your example, the right-hand side of the `define'
is (grammatically) an <expression> whereas the other form is in a <body>
position, so it's not so that you're exchanging two <expression>s.  It's
rather so that a valid <expression> (in Guile Scheme at least, see
below) happens to also be a valid <body> (which however leads to an
error later on).

By the way, it comes out the expression-begin is actually specified (in
R5 through 7RS) to take at least one operand, so in your example the
(define x (begin)) is unspecified in first place.

OTOH, a <body> is defined as "<definition>* <sequence>", where
<sequence> is "<command>* <expression>", meaning that in a <body> that
is just "(begin)", the (begin) is an <expression>.  In RnRS, (begin) is
actually an invalid <expression>, but since Guile allows it in general,
one would expect it to work here too.  (Guile's "excuse" here could be
that it uses a wholely different grammar that is compatible with the
RnRS one, instead of one constructed through incremental extensions to
it.)

I now wish Guile simply didn't allow (begin) as a valid expression. :-)

While this is just a nitpick, I'll file a bug report for it.

> As another data point, this seems to be fine:
>
> (use-modules (oop goops))
> (define-method (foo))
>
> So, GOOPS doesn't mind empty method bodies. However
>
> (define-method (foo) (begin))
>
> fails. Not very consistent either, I'm afraid.

I just checked and `define-method' (deferring to the auxilliary macro
`method') special-cases zero-form method bodies, simply giving them a
body of (if #f #f), so it introduces the asymmetry intentionally.  Dunno
why, but changing it could break backwards compatibility.  OTOH it's
nasty that the second failes when the first doesn't, so I'll file an
additional report for that.

> Cheers,
> Tobias

Taylan