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Subject: Re: [boost] Yap's formal review is starting now!
From: Zach Laine (whatwasthataddress_at_[hidden])
Date: 2018-02-16 20:45:00

On Fri, Feb 16, 2018 at 2:28 PM, Steven Watanabe via Boost <
boost_at_[hidden]> wrote:

> 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,


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