Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!tank!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.misc
Subject: Re: Language Design, or: is C's Grammar Context Free
Keywords: BNF, Grammars, Silliness
Message-ID: <16591@mimsy.UUCP>
Date: 28 Mar 89 18:39:32 GMT
References: <5200040@m.cs.uiuc.edu> <12443@watdragon.waterloo.edu> <12778@watdragon.waterloo.edu>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 37

>In article <16559@mimsy.UUCP> I included the throwaway% line:
>>... At least, this is the only explanation I can see that we never
>			    ^^^^^^^^^^^^^^^^^^^^^^^^^^
>>argue whether Pascal's grammar is context free.
-----
% cf. Webster: `throw.away \'thro--*-.wa-\ n : a handbill or circular
  distributed free'; applied to writing (especially comic writing), it
  refers to a line with similar worth.
-----

In article <12778@watdragon.waterloo.edu> akwright@watdragon.waterloo.edu
(Andrew K. Wright) writes:
>The difference I see is that a C grammar that accepts undeclared typedef
>names destroys more of the structure of the grammar than a Pascal
>grammar that accepts undeclared variable names.  Consider the following
>C expression:
>
>	x = (a)(b);
>
>If "a" is a typedef name, then the right side is a cast.  If "a" is
>not a typedef name, then the right side is a function call.
>I dont believe similar situations exist for Pascal (corrections welcome).

I am not sure what you mean by `destroys more of the structure of the
grammar'.  It is ambiguous, and in particular it is an ambiguity of an
uncomfortable sort, quite unlike the shift/reduce ambiguity for if/then/else
which everyone resolves by taking the shift.  Yacc permits this reduce/
reduce conflict, but resolves it by taking the first grammar rule; we
want the parser to take the *appropriate* grammar rule, which is sometimes
the first and sometimes not.  So yacc unaided will not produce a
deterministic parser that does what we want.

BUT all this applies to constructing a deterministic LALR(1) parser,
which is not at all the same thing as having a context free grammar!
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris