make_expression() generates a type more complicated than expected

[rgrover]

make_expression<boost::yap::expression, expr_kind::terminal>(1);

generates a type which an a terminal within a terminal: const boost::yap::expression< boost::yap::expr_kind::terminal, boost::hana::tuple<boost::yap::expression<boost::yap::expr_kind::terminal, boost::hana::tuple<int>>>>

This is more complicated than the expected type:

const boost::yap::expression<boost::yap::expr_kind::terminal, boost::hana::tuple<int> >

The above is also the type correctly returned by

expression<expr_kind::terminal, boost::hana::tuple<int>>{1};

[tzlaine]

You got what you asked for. :) You asked YAP to make you an expression of an arbitrary kind, in this case a terminal, whose subexpressions are just integer "1". But integer 1 is not a YAP expression, so YAP wraps it in a terminal first. Imagine you had instead called:

auto x = make_expression<boost::yap::expression, boost::yap::expr_kind::unary_plus>(1);

The type of x is:

const boost::yap::expression< boost::yap::expr_kind::unary_plus, boost::hana::tuple<boost::yap::expression<boost::yap::expr_kind::terminal, boost::hana::tuple>>>

... which is the same pattern of return you got with your original call, just with the outer "terminal" replaced with "unary_plus".

I think you meant to use make_terminal instead. Here are the types MPL reports for these two expressions:

boost::mpl::print<decltype(boost::yap::make_expression<
                           boost::yap::expression,
                           boost::yap::expr_kind::terminal>(1))>
    f;
(void)f;

boost::mpl::print<decltype(
    boost::yap::make_terminal<boost::yap::expression>(1))>
    e;
(void)e;

/home/tzlaine/boost_1_66_0/boost/mpl/print.hpp: In instantiation of ‘struct boost::mpl::print<boost::yap::expression<(boost::yap::expr_kind)1, boost::hana::tuple<boost::yap::expression<(boost::yap::expr_kind)1, boost::hana::tuple > > > >’: /home/tzlaine/yap/example/calc3.cpp:103:9: required from here /home/tzlaine/boost_1_66_0/boost/mpl/print.hpp:64:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] sizeof(T) > -1

/home/tzlaine/boost_1_66_0/boost/mpl/print.hpp: In instantiation of ‘struct boost::mpl::print<boost::yap::expression<(boost::yap::expr_kind)1, boost::hana::tuple<int> > >’:
/home/tzlaine/yap/example/calc3.cpp:108:9:   required from here
/home/tzlaine/boost_1_66_0/boost/mpl/print.hpp:64:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

[rgrover]

Thanks for this explanation. Could be useful to add to the documentation.

By the say, the trick about using boost::mpl::print doesn't work for me (at least with clang-5). It is a useful tool to have. How do you trigger your type-printing error/warning messages?

[tzlaine]

I think you need -Wall. Another option is to do MPL_ASSERT:

        BOOST_MPL_ASSERT((std::is_same<
                          decltype(yap::deref(std::move(ref))),
                          term<double> &>));

Which produces very nice, very verbose output for what the two non-same types are.