Steven Watanabe wrote:
Markus Werle wrote:
"User" is a very ambiguous word for proto. Actually most "users" of proto will be "library writers" seeking to boost (sic!) their productivity. So even the nitty-gritty details matter. I reject any "do-not-mind-it's-only-deep-inside" arguments here. Everybody writing a compile time algorithm acting on proto structures for whatever reason will be affected by this "detail" and the implications might only become clear later, when somebody is in need to do some compile-time-magic.
From Daixtrose I know that the number of template arguments can be reciprocal to readability, maintainability and extensibility.
The expr<> type doesn't show up in code, unless you're doing something a little weird. Can you say why you think an extra defaulted template parameter on a type which doesn't show up in user code affects readability, maintainability or extensibility?
You can (and should) write expr<terminal_tag, args0<int> > rather than expr<terminal_tag, args0<int>, 0>.
I agree it's worth considering these issues, but have a look at Proto's examples. The expr<> type is not mentioned once in any of them. The advanced features of Proto make stating expression types in code unnecessary.
Library writers are not somehow special in this respect. When I said "user" I meant anyone who is not the author or maintainer of proto.The only problems that I can see are that mpl::apply<expr<mpl::_, mpl::_>, terminal_tag, args0<int> > won't work and that mpl::quote2<expr> will fail.
That's a legitimate concern. How important do you consider these use cases? -- Eric Niebler Boost Consulting www.boost-consulting.com