[Mono-list] I want to get in on this.

[Jay Freeman (saurik)]

Miguel:

Well, maybe I'm just doing this wrong, but I decided to try this and it
doesn't work.  I even made it easy on it, my test has EXPR and var_list be
tokens rather than complex expressions.  I copied this from a terminal
window, so the tabs aren't right, but here is the base of my test:

<file name="vb.y">
%token FOR VAR EXPR SOMETHING TO NEXT var_list

%%

body:
        /* nothing */
        | statement body
        ;

statement:
        for_statement
        | SOMETHING
        ;

for_statement:
        FOR VAR '=' EXPR TO EXPR body opt_next
        ;

opt_next:
        /* nothing */
        | NEXT var_list
        ;
</file>

I am pretty sure I got this right.  This yields 5 shift/reduce conflicts.
If you put a defined deliniator around "body" in for_staement, such as '('
and ')', you can get it to work.  VB has statements that have to end in '\n'
or be separated by ':', so that helps a little, but I can't come up with a
good organization of separators that makes it both work _and_ not require
that there be two separators somewhere (example, if you are willing to
always have two enters at the end of for statements, that could work, but I
bet when you start adding more types even that would only be a temporary
hack).

Note that if you remove the empty expansion of opt_next (thereby requiring
separate Next statements) it works fine.  If need be I can do more research
and attempt to explain exactly _why_ this doesn't work, but for now all I
can really do is state that it doesn't.

Sincerely,
Jay Freeman (saurik)
saurik@saurik.com

----- Original Message -----
From: "Miguel de Icaza" <miguel@ximian.com>
To: "Jay Freeman (saurik)" <saurik@saurik.com>
Cc: "mono-list" <mono-list@ximian.com>
Sent: Friday, July 13, 2001 12:20 AM
Subject: Re: [Mono-list] I want to get in on this.


...
>
> Of course it can be parsed by yacc.
>
> The grammar is not deterministic, and that is what you use the actions
> for.
>
> Indeed, you can not use yacc to build an error checking C# parser, nor
> a Java one.  You need to delay those actions to the `actions'.  Just
> like you would with the grammar I showed you for the for next.
>
> Miguel.