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

24 Sep 2010

      On 09/24/2010 07:35 PM, Eric Niebler wrote:
...
On 9/24/2010 11:15 AM, Roland Bock wrote:
...
On 09/24/2010 04:52 PM, Thomas Heller wrote:
...
you can compute that type information directly in your proto transform:
struct sorted_argument_function:
      proto::and_
      <
         proto::function<fun_terminal, proto::_, proto::_>,
         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...
The proto::and_ has two branches: proto::function and proto::if_.
The proto::if_ is assuming:
1. That there are at least three child nodes
2. That child1 and child2 themselves have a child
3. That the child nodes of child1 and child2 are terminals
The proto::function has only verified the first of these assumptions.
You need to change it to validate the other two as well:
struct function_argument
       : proto::function<proto::terminal<proto::_>, proto::_>
     {};
struct sorted_argument_function
       : proto::and_
         <
             proto::function
             <
                 fun_terminal,
                 function_argument,
                 function_argument
             >,
             proto::if_
             <
                 grammar_less
                 <
                    proto::_value(proto::_child0(proto::_child1))
                  , proto::_value(proto::_child0(proto::_child2))
                 >()
             >
         >
     {};
HTH,
Yes, that helps a lot! Very cool.

Sigh, seeing it, it suddenly seems so obvious, at least after Thomas 
showed the

proto::_value(proto::_child0(proto::_child1))

Thanks and regards,

Roland