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From Humberto Sanchez II <hsanche...@yahoo.com>
Subject Re: [gosh] On the sad tale of BNF and optional semicolons
Date Tue, 21 Feb 2012 17:38:15 GMT
That was what I was going to say.  May cause problems with legacy code
that accidentally left them out.  However, it is trivial to fix.

_________________________________________________





On 2/21/12 11:35 AM, "Ariel Jakobovits" <arieljake@yahoo.com> wrote:

>> 3.    Make semicolons mandatory.
>+1
> 
>Ariel Jakobovits
>Email: arieljake@yahoo.com
>Phone: 650-690-2213
>Fax: 650-641-0031
>Cell: 650-823-8699
>
>
>________________________________
> From: David Arno <david@davidarno.org>
>To: flex-dev@incubator.apache.org
>Sent: Tuesday, February 21, 2012 9:27 AM
>Subject: [gosh] On the sad tale of BNF and optional semicolons
> 
>I have spent a bit of time over the last few days trying to define a BNF
>grammar for AS3. As "Left Right" predicted, it's looking like this just
>isn't possible. The problem is such a trivial little thing too: optional
>semicolons.
>
>To illustrate the problem, let me give an example piece of AS3 BNF:
>
>imports = import | imports import
>import = 'import' type_reference ';'
>
>As I'm sure there are many here who are unfamiliar with BNF, I'll explain
>what the above means. The first line defines a symbol: imports, which is
>defined as either being import, or a recursive reference to itself,
>followed
>by import. In other words, a language consisting of one or more import
>symbols is a language that matches the imports symbol. Next, I define what
>the import symbol is, it being an "import" keyword token, followed by yet
>another symbol, type_reference (which I haven't included here) and
>finally a
>semicolon. This snippet then covers import collections, such as:
>
>       import flash.display.DisplayObject;
>       import flash.events.Event;
>       import flash.events.MouseEvent;
>
>However, there is a problem. AS3 doesn't mandate that I specify those
>semicolons at the end of the lines. AS3 supports implied semicolons: in
>certain circumstances, the newline character is good enough to tell the
>compiler that the end of a statement has been reached, so a semicolon
>isn't
>required. Unfortunately, such a concept cannot be handled by BNF. If end
>of
>line characters are significant within the grammar, then within BNF, they
>must be explicitly referenced. As a result, the BNF becomes really
>complex:
>
>imports = import | imports import
>import = 'import' type_reference import_terminator
>    | 'import' unimportant_newlines type_reference statement_terminator
>unimportant_newlines = '\n' | unimportant_newlines '\n'
>statement_terminator = ';'
>    | ';' unimportant_newlines
>    | '\n'
>    | '\n' unimportant_newlines
>    | unimportant_newlines ';'
>    | unimportant_newlines ';' unimportant_newlines
>
>Not only does this become unreadable (remember, unimportant_newlines will
>appear absolutely everywhere in the BNF where whitespace is allowed in the
>code, but any tool that generates a parser from BNF-like definitions will
>complain of conflicts as newline characters can apply to multiple rules at
>any one time and so the parser has to guess which one to use.
>
>As far as I can see, we have three choices:
>
>1.     Hand craft a parser that can handle optional semicolons, rather
>than
>using a BNF-based one. I really don't want to do this as it requires us to
>pick one language for the compiler , it's harder to maintain and takes
>longer to write.
>2.     Hand craft a lexical analyser that knows about optional semicolons
>and inserts missing ones into the token stream passed to the parser. I
>have
>to confess I've no idea at this stage how feasible this would be, and it
>has
>the same issues of language-specificity and complexity as the previous
>option.
>3.     Make semicolons mandatory.
>
>The purpose of this email is to gauge people's reactions to option 3. If
>we
>created a compiler that mandated semicolons, would this cause problems for
>anyone? Is it an idea we can consider, or is it a complete no-no?
>
>Thoughts and opinions please people.
>
>David.



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