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

24 Sep 2010

      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,
        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::functionproto::_, 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,

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