Is SwiftUI part of the Swift language or not?

[ole (Ole Begemann)]

@Rod_Brown

My general understanding is that if the @ is followed by an upper case character (eg “ViewBuilder”) these features are defined in user or framework code. In this case, it’s SwiftUI framework providing the implementation of the ViewBuilder builder functionality. When it is followed by a lower case, this is a sign of a language-defined feature.

That’s my understanding as well. Generalizing the pattern of allowing user-defined "attributes" starting with @ has been discussed under the term "custom attributes" on the forum. As far as I know it hasn't been formalized yet, but here's a post from @Douglas_Gregor that summarizes the intent:

Using types for attributes, and their initializers to build values, means that we can build on existing infrastructure for (e.g.) runtime inspection (use as? dynamic casting), any future static-reflection facilities (which of course have to work with user-defined types and values thereof), and simplifies the problem space.

I don't think that we should allow just any type to be used as an attribute, though: we should require that types be marked with a (built-in!) attribute to indicate that the types themselves can be used as custom attributes.

I'd suggest that we reserve all attribute names starting with a lowercase letter or an underscore for the compiler; that fits well with Swift's API design guidelines suggesting that type names start with an uppercase letter.