On Fri, Feb 16, 2018 at 2:28 PM, Steven Watanabe via Boost < boost@lists.boost.org> wrote:
AMDG
On 02/16/2018 01:09 PM, Zach Laine via Boost wrote:
I'm don't understand what that would mean, exactly. What you can write though is:
template <typename Tag, typename... T> auto operator()(Tag, T &&... t)
Is that different from the intended use case above?
There's one annoying problem with this, which is that it can match an expression transform, with Tag as the Expression and T as an empty parameter pack.
Very true, and very annoying. I tend to explicitly cover the possible arities. They are unary, binary, ternary for if_else, and N for call. I'm certainly open to something that makes transforms easier to write, if you or Peter have ideas. In this case, something like the expression_tag<> scheme Peter recommended helps with this particular ambiguity: template <typename Tag, typename... T> auto operator()(expression_tag<Tag>, T &&... t) { // and it also seems to help with writing targeted transforms in C++17: if (yap::to_kind<Tag>() == yap::expr_kind::terminal) { return yap::transform(yap::as_expr<my_expr>(std::forward<T>(t))); } else { return yap::make_expression<yap::to_kind<my_expr, Tag>()>(std::forward<T>(t)...); } } Zach