Re: [boost] [proto] grammar compile error

Subject: Re: [boost] [proto] grammar compile error
From: Eric Niebler (eric_at_[hidden])
Date: 2009-07-04 12:57:53

Felipe Magno de Almeida wrote:
> Hello,
> 
> When I have a grammar like this:
> 
> struct create_table_grammar
>   : boost::proto::shift_left
>     <
>       boost::proto::shift_left
>       <
>         boost::proto::terminal<tags::create>
>         , boost::proto::terminal<tags::table>
>       >
>    , boost::proto::terminal<const char*>
>    >
> {};
> 
> And this expression:
> 
> create << table << "lala";
> Where create and table are terminals with the respective tags.
> 
> I have this compile error:
> examples/test1.cpp: In function ‘int main()’:
> examples/test1.cpp:12: error: no match for ‘operator<<’ in
> ‘sql::keywords::create << sql::keywords::table’

Right, because the expression "create << table" doesn't match the 
grammar you've defined above.

> If I don't inner expressions, then (create << table) works. But I
> can't define a more restricted grammar this way.
> I want create << table to work only when preceded with another
> specific expression.
> I wanted create << table to be enabled, but to disable everything else
> except (create << table) << "";
> 
> Is this possible?

Not directly. You will have to allow "create << table" in your grammar 
(possibly by not specifying any grammar when defining your domain, or 
else by specifying a loose one), and then define a /separate/ grammar 
that validates fully formed expressions for correctness. Presumably 
somewhere in your code there is a place where these expressions get 
evaluated. That's the place to ensure that "create << table" is always 
followed by a string literal. For that, you would use BOOST_MPL_ASSERT 
and proto::matches.

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com