Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!ima!johnl
From: johnl@ima.UUCP
Newsgroups: mod.compilers
Subject: YACC for dynamic grammars
Message-ID: <458@ima.UUCP>
Date: Mon, 26-Jan-87 10:27:28 EST
Article-I.D.: ima.458
Posted: Mon Jan 26 10:27:28 1987
Date-Received: Tue, 27-Jan-87 03:49:29 EST
Sender: johnl@ima.UUCP
Lines: 30
Approved: compilers@ima.UUCP

Is it possible to use YACC to write a parser for a simple language
whose gramatic may dynamically change, namely defining new operators?
In this interpreter, new operators (unary or binary) can be added
by the user dynamically, e.g. define the binary operator ++
with a certain precedence and then the system can recognize a ++ b.
I have no experiences with YACC, I see it in the following way:
YACC generates some tables which correspond to the grammar.
If a new operator is added to the grammar, it should be possible
to dynamically update the tables in order to recognize the new item.
If this is not possible, is there any other way?

--Micha

ARPA:		micha%ecrcvax.UUCP@seismo.CSS.gov
		micha%ecrcvax.UUCP%Germany.CSNET@CSNET-RELAY
CSNET:		micha%ecrcvax.UUCP@Germany.CSNET
UUCP:		unido!ecrcvax!micha
[I doubt it -- the calculation that produces the parsing tables from the input
grammar is pretty complicated.  If you only allow people to define new
operators that have the same precedence as existing ones, your job is pretty
easy since you haven't really changed the syntax, just added new flavors of
existing tokens from the point of view of the parser.  If you get more general
than that, you can add ambiguties and you probably need the full extremely slow
glory of Earley's algorithm, which is a long way from the efficient LALR
approach of yacc.  -John]
--
Send compilers articles to ima!compilers or, in a pinch, to Levine@YALE.EDU
Plausible paths are { ihnp4 | decvax | cbosgd | harvard | yale | cca}!ima
Please send responses to the originator of the message -- I cannot forward
mail accidentally sent back to compilers.  Meta-mail to ima!compilers-request