> On Mar 20, 2017, at 8:26 PM, Charles Srstka via swift-evolution
> <[email protected]> wrote:
>
> the concern with extensions could be easily solved simply by giving
> extensions access to private members of the extended type as long as those
> extensions are in the same module/submodule (or file, if you prefer, but I
> don’t). This would probably eliminate almost all of the use cases for
> fileprivate, as well as freeing us from the sometimes clumsy situation where
> we’re rather artificially forced to use files as a scope, instead enabling us
> to organize code into files as is appropriate for the project.
I think that many advocates of scoped private misunderstand critics' concerns
about extensions.
One of the nice things about file-based `private` is that it allows access from
extensions not only on the same type, but also on *other* types. For example,
suppose I have a Book type and I want to allow them to be added to bookshelves:
class Bookshelf {
// I'm using file-based private here
private(set) var books: [Book]
func addBook(_ book: Book) {
book.bookshelf = self
books.append(book)
}
}
I also want to have a removal method, but—much like `UIView` and `NSView`'s
`removeFromSuperivew`—I want that method to be on `Book`, not `Bookshelf`. With
file-based private, that's no problem—just drop the code in an extension in the
same file:
extension Book {
func removeFromBookshelf() {
guard let shelf = bookshelf else { return }
shelf.remove(at: shelf.books.index(of: self)!)
bookshelf = nil
}
}
When we talk about file-based private being "better for extensions", we're
*not*—or at least not necessarily—talking about it being better for the pattern
of splitting a type into several different extensions to group related code
together. We're talking about the pattern of splitting a *concern* across
several different *types* by extending all of them, taking advantage of Swift's
flexible scoping to encapsulate details even when the type hierarchy doesn't
match the encapsulation boundaries you want.
So allowing `private` members to be seen by extensions anywhere in a project
doesn't address this issue; in fact, it actively and aggressively undermines
it, making it impossible to encapsulate any implementation details at a scope
smaller than a single type. For myself and others like me, it is the exact
opposite of what we want.
--
Brent Royal-Gordon
Architechies
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