Re: [Boost-users] [Proto] How to access the grammar of an expression?

24 Sep 2010

      On 09/24/2010 04:52 PM, Thomas Heller wrote:
...
Roland Bock wrote:
...
On 09/24/2010 02:33 PM, Roland Bock wrote:
...
Hi,
ist there a way to access the grammar that was used to construct an
expression?
I have defined a metafunction
template
struct grammar_less: mpl::false_ {};
which has specialization for some grammar combinations.
Now I want to define a function that can take two arguments so that
child0.grammar<  child1.grammar
Any idea how to do that? My test-code is attached.
Regards,
Roland
Hi,
since I know the structure of the types in question, I can circumvent
the problem as shown in the attached code. Not nice, but I could
probably use it.
Regards,
Roland
you can compute that type information directly in your proto transform:
struct sorted_argument_function:
     proto::and_
     <
        proto::function,
        proto::if_
        <
           grammar_less<
              proto::_value(proto::_child0(proto::_child1))
            , proto::_value(proto::_child0(proto::_child2))>()
        >
     >
  {};
OK, admittedly, that looks a lot nicer, but in case I call something like

fun(b(0), 42));

the error messages are just horrible in both solutions...
...
btw: there is a proto specific list: proto@lists.boost.org
Thanks for the hint. I guess I'll join when the next question pops up :-)

Regards,

Roland