Hi, I decided to try to write a small vector library with proto to test it. The following worked without problem (I will use extend further on but at the moment the as_expr are needed) proto::display_expr( proto::as_expr(x)+proto::as_expr(y)*proto::as_expr(z) , std::cout ); I then added struct fastvec_reduction_tag {}; struct sum_ {}; template<typename A> typename proto::result_of::make_expr< fastvec_reduction_tag // , MixedDomain , sum_ const , A const &

::type sum(A const &a) { return proto::make_expr<fastvec_reduction_tag/*, MixedDomain*/>(sum_(), boost::ref(a)); }

Which work well proto::display_expr( sum(x) , std::cout ); And now, I would like to create a "function" dot(x,y) which expand to sum(x*y). I tried: template<typename A, typename B> typename proto::result_of::make_expr< fastvec_reduction_tag // , MixedDomain , sum_ const , BOOST_TYPEOF(boost::ref(proto::as_expr(A())*proto::as_expr(B())))

const & ::type dot(A const &a, B const &b) { return proto::make_expr<fastvec_reduction_tag/*, MixedDomain*/>(sum_(), boost::ref( proto::as_expr(a)*proto::as_expr(b) )); }

Which compile but fail at runtime when I do proto::display_expr( dot(x,y) , std::cout ); (it call << on an invalid vector (type of x and y)). Is there a ways to create the tree for sum(x*y) directly or should I use a transform (create a tree with a dot tag and then transform it with proto)? The page 'users_guide\expression_construction\construction_utils.html' seems very complete, but hard to understand how to put it in practice, perhaps you could link it to some use in the example. Regards, -- Cédric