Design note: Namespaces

Q: Should namespaces be their own standalone feature, as in Cpp1?

A: Maybe, but I'm going to see if we can do better using metaclass functions and partial types.

I intend to initially implement namespace as a standalone feature, as it is in today's syntax, just so that there's something to use short-term even before I add support for classes.

But I'm curious whether a namespace metaclass function will work. The idea is that, like interface and base_class and value and such are specializations of the general type, perhaps namespace could be expressed like that as well. One benefit I hope this might bring to users would be able to naturally use type features, notably that private access could be a nice replacement for the ::detail subnamespace idiom. See P0707 section 3.11 for a discussion. There are other potential smaller simplification benefits too, such as slightly simpler name lookup (when the only scopes are types, functions, and blocks) and one less kind of special-case feature in the language.

However, there is one main differences between a namespace and those other kinds of restricted types like interface and value: A namespace can be reopened. So to make it possible to express a namespace as a restricted type would require that types themselves support partial types that can be reopened. But that's interesting because, as shown on that C# doc page, partial types are a generally useful feature, not something I'd add only to be able to use them to express namespaces. For example, partial types are known to be useful for frameworks that rely on source generation to produce part of the code for a type, by allowing such generated code to be emitted as a self-contained unit that's a partial type, rather than as a brittle and easily-disturbed chunk of generated code inside the main manually-written type, usually with comments like "please don't change any of the code inside these comments!!" around it... experience has found these are error-prone in practice, and when the programmer disturbs code inside the comments (or even near the comments) it's easy to confuse the code generator so that they can't update the code, or can't round-trip the code.

We'll see how it goes!

While we're talking about namespaces, here's a related question...

Q: Should namespaces be the same as modules, or orthogonal as in Cpp1?

A: Orthogonal.

This is a reasonable question to ask, since some languages try to make them the same, and I asked it too... for now I've been convinced by the modules experts that making them orthogonal is useful. For example, a namespace may be contributed to by multiple modules... consider that when you can legally specialize a library template for your own type (including when that is allowed for std templates), you naturally need to put the specialization in the original namespace.