From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=0.4 required=5.0 tests=BAYES_00,FORGED_MUA_MOZILLA
	autolearn=no autolearn_force=no version=3.4.4
X-Google-Thread: 103376,5412c98a3943e746
X-Google-NewGroupId: yes
X-Google-Attributes: gida07f3367d7,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Received: by 10.68.74.201 with SMTP id w9mr5988464pbv.0.1331230223716;
        Thu, 08 Mar 2012 10:10:23 -0800 (PST)
Path: 
 h9ni3260pbe.0!nntp.google.com!news1.google.com!news4.google.com!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Verified compilers?
Date: Thu, 08 Mar 2012 20:10:21 +0200
Organization: Tidorum Ltd
Message-ID: <9rsb0eFqfrU1@mid.individual.net>
References: <op.v91cleyiule2fv@douda-yannick> <jj3p5v$2tp$1@munin.nbi.dk>
 <9207716.776.1331054644462.JavaMail.geo-discussion-forums@ynaz38>
 <nhzeozam35it$.13mzx8ap4cmdc$.dlg@40tude.net>
 <op.wascesqrule2fv@douda-yannick>
 <4edda5mav3cf$.149pbgyxl1wx5.dlg@40tude.net>
 <9rplcgF5a2U1@mid.individual.net>
 <1psd0g0womgxi.1sle7ol12x3d5.dlg@40tude.net> <jj8v9j$b8g$1@munin.nbi.dk>
 <1d576lv10h3ax$.lkh58yfbl5dy$.dlg@40tude.net>
Mime-Version: 1.0
X-Trace: individual.net bD2MaCZZG7VxyMAaoVR8dQKUHrU6hDxjYmsV1NcMmDKnMfHJSv
Cancel-Lock: sha1:pw29kRcUpgV6BoOVeFWwkfs9qUQ=
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6;
 rv:10.0.2) Gecko/20120216 Thunderbird/10.0.2
In-Reply-To: <1d576lv10h3ax$.lkh58yfbl5dy$.dlg@40tude.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Date: 2012-03-08T20:10:21+02:00
List-Id: <comp.lang.ada>

On 12-03-08 11:25 , Dmitry A. Kazakov wrote:
> On Wed, 7 Mar 2012 18:42:23 -0600, Randy Brukardt wrote:
>
>> As far as LR parsing is concerned, that's the most general practical parsing
>> scheme. There are many legitmate BNFs that cannot be parsed using recursive
>> descent.
>
> There are also ambiguous BNFs, which means that BNF is not so formal as it
> may appear.

Ambiguity has nothing to do with formality.

If you think of a grammar only as defining a language (a set of 
strings), ambiguity is irrelevant; it only means that some sentence can 
be derived in more than one way from the start symbol, but that has no 
effect on the language, as a set of strings.

Ambiguity is only harmful when you want to use the grammar to parse a 
sentence into a parse/derivation tree, and then want to use the tree for 
something, for example to evaluate the meaning of the sentence.

If you define (as one always does) the meaning of a sentence as a 
mathematical function of the sentence's parse tree, and if (because of 
ambiguity) the sentence has several parse trees, the meaning also 
becomes ambiguous -- unless you can prove that the "meaning function" 
has the same value for all the parse trees for the same sentence.

> The point is that I don't care about that or about restricting
> it to a certain class etc. Because there is no value in such activity at
> all. BNF is good for *informal* description of the language syntax.

There is nothing informal about BNF, if it is used correctly.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .