From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (from majordomo@localhost) by pauillac.inria.fr (8.7.6/8.7.3) id WAA04494; Sat, 14 Aug 2004 22:47:43 +0200 (MET DST)
X-Authentication-Warning: pauillac.inria.fr: majordomo set sender to owner-caml-list@pauillac.inria.fr using -f
Received: from concorde.inria.fr (concorde.inria.fr [192.93.2.39]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id WAA06274 for <caml-list@pauillac.inria.fr>; Sat, 14 Aug 2004 22:47:42 +0200 (MET DST)
Received: from herd.plethora.net (herd.plethora.net [205.166.146.1])
	by concorde.inria.fr (8.12.10/8.12.10) with ESMTP id i7EKlZRM011209
	for <caml-list@inria.fr>; Sat, 14 Aug 2004 22:47:38 +0200
Received: from bhurt.plethora.net (bhurt.plethora.net [205.166.146.49])
	by herd.plethora.net (8.11.6/8.10.1) with ESMTP id i7EKl7J18642;
	Sat, 14 Aug 2004 15:47:07 -0500 (CDT)
Date: Sat, 14 Aug 2004 15:55:03 -0500 (CDT)
From: Brian Hurt <bhurt@spnz.org>
X-X-Sender: bhurt@localhost.localdomain
To: Jon Harrop <jon@jdh30.plus.com>
cc: Ocaml Mailing List <caml-list@inria.fr>
Subject: Re: [Caml-list] CFG's and OCaml
In-Reply-To: <200408142119.11234.jon@jdh30.plus.com>
Message-ID: <Pine.LNX.4.44.0408141548490.4282-100000@localhost.localdomain>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
X-Miltered: at concorde with ID 411E7A67.000 by Joe's j-chkmail (http://j-chkmail.ensmp.fr)!
X-Loop: caml-list@inria.fr
X-Spam: no; 0.00; caml-list:01 2004:99 disallow:01 'a-:01 'a-:01 parses:01 bool:01 evaluates:01 ocaml:01 ocaml:01 syntax:02 ast:02 enforce:02 comparison:02 wrote:03 
Sender: owner-caml-list@pauillac.inria.fr
Precedence: bulk

On Sat, 14 Aug 2004, Jon Harrop wrote:

> On Saturday 14 August 2004 04:33, Brian Hurt wrote:
> > > 3. If so, is the fact that most languages disallow "a<b<c" due to this?
> > 
> > No.  "a<b<c" is parsed the same way as "a+b+c".
> 
> Sorry, I should have been more specific. With left- or right- or 
> non-associative, commuting, 'a->'a->'a operators (like + and *) you can get 
> away with parsing that way, e.g. "a+b+c" as:
> 
> either  (a+b)+c  or  a+(b+c)
> 
> But you can't do this with comparison 'a->'a->bool operators because it forces 
> you to deviate from conventional mathematical meaning, e.g. you get a type 
> error in OCaml on the "3" in "1<2<3" because it parses as "(1<2)<3" which 
> evaluates to "true<3" which just doesn't make any sense.
> 

The syntax of a language doesn't enforce a given meaning on the language 
being parsed.  "Colorless green ideas sleep furiously" is a syntactically 
correct English sentence, even if it is utterly meaningless.

The AST  of a<b<c has to be one of two ways:

       <                <
      / \              / \
    a    <      or    <   c
        / \          / \
       b   c        a   b

i.e. a < (b < c) or (a < b) < c.  What the meaning of these two 
expressions are is entirely up to the compiler- more spefically, up to the 
parts which are not lex or yacc based.

Although this does bring up one interesting question- is a<b<c 
syntactically different than (a<b)<c?  Generally, languages want to 
consider "extra" parenthesis to be harmless.

-- 
"Usenet is like a herd of performing elephants with diarrhea -- massive,
difficult to redirect, awe-inspiring, entertaining, and a source of
mind-boggling amounts of excrement when you least expect it."
                                - Gene Spafford 
Brian

-------------------
To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners