Re: [Boost-users] [proto] Comma Strangeness

Subject: Re: [Boost-users] [proto] Comma Strangeness
From: Manjunath Kudlur (keveman_at_[hidden])
Date: 2010-03-26 21:37:19

> display_expr dumps out an expression without any comma operators.
> It just shows a subscript off "list" with one "index," obj["name3"].attr["b"].
>
> It's the behavior I would expect if the operands of the commas were not
> protoized expressions, but they _should_ be protoized.
>
> The funny thing is, if I write a small testcase just isolating the terminals,
> members, etc. needed to construct the expression, it works as expected.
> But in the context of the (much) larger language, it fails.  However, the
> grammar specification shouldn't have any bearing on what display_expr
> dumps out.
>

I have the following toy program :

#include <iostream>
#include <boost/proto/proto.hpp>

using namespace std;
namespace proto=boost::proto;

struct list_ {};
struct attr_ {};
struct obj_ {};
template<class E> struct obj_expr;

struct obj_domain
  : proto::domain<proto::pod_generator<obj_expr> >
{};

template <class E>
struct obj_expr {
  BOOST_PROTO_BASIC_EXTENDS(E, obj_expr<E>, obj_domain);
  BOOST_PROTO_EXTENDS_SUBSCRIPT();

  BOOST_PROTO_EXTENDS_MEMBERS(
                              ((attr_, attr))
                              )
};

proto::terminal<list_>::type list;
obj_expr<proto::terminal<obj_>::type> obj;

int main()
{
  proto::display_expr(
                      list[obj["name1"].attr["a"],
                           obj["name2"].attr["a"],
                           obj["name3"].attr["b"]]
                     );
  return 0;
}

and I do see the expression getting printed out correctly as follows.
Do you see something different?

subscript(
    terminal(5list_)
  , comma(
        comma(
            subscript(
                member(
                    subscript(
                        terminal(4obj_)
                      , terminal(name1)
                    )
                  , terminal(5attr_)
                )
              , terminal(a)
            )
          , subscript(
                member(
                    subscript(
                        terminal(4obj_)
                      , terminal(name2)
                    )
                  , terminal(5attr_)
                )
              , terminal(a)
            )
        )
      , subscript(
            member(
                subscript(
                    terminal(4obj_)
                  , terminal(name3)
                )
              , terminal(5attr_)
            )
          , terminal(b)
        )
    )
)

Manjunath