[swift-evolution] Type-based ‘private’ access within a file

[Douglas Gregor]

Hello Swift Community,

In rejecting SE-0159 <https://github.com/apple/swift-evolution/blob/master/proposals/0159-fix-private-access-levels.md>, the core team described a potential direction we would like to investigate for “private” access control that admits a limited form of type-based access control within files. The core team is seeking some discussion here and a motivated volunteer to put together a proposal along these lines for review in the Swift 4 time-frame (i.e., very soon). To be clear, the core team it’s sure this is the right direction to go… but it appears promising and we would *love* to be able to settle the access-control issue.

The design, specifically, is that a “private” member declared within a type “X” or an extension thereof would be accessible from:

	* An extension of “X” in the same file
	* The definition of “X”, if it occurs in the same file
	* A nested type (or extension thereof) of one of the above that occurs in the same file

This design has a number of apparent benefits:
	+ “private” becomes the right default for “less than whole module” visibility, and aligns well with Swift coding style that divides a type’s definition into a number of extensions.
	+ “fileprivate” remains for existing use cases, but now it’s use it more rare, which has several advantages:
		+ It fits well with the "progressive disclosure” philosophy behind Swift: you can use public/internal/private for a while before encountering and having to learn about “fileprivate”   (note: we thought this was going to be true of SE-0025 <https://github.com/apple/swift-evolution/blob/master/proposals/0025-scoped-access-level.md>, but we were clearly wrong)
		+ When “fileprivate” occurs, it means there’s some interesting coupling between different types in the same file. That makes fileprivate a useful alert to the reader rather than, potentially, something that we routinely use and overlook so that we can separate implementations into extensions.
	+ “private” is more closely aligned with other programming languages that use type-based access control, which can help programmers just coming to Swift. When they reach for “private”, they’re likely to get something similar to what they expect—with a little Swift twist due to Swift’s heavy use of extensions.
	+ Loosening the access restrictions on “private” is unlikely to break existing code.

There are likely some drawbacks:
	- Developers using patterns that depend on the existing lexically-scoped access control of “private” may find this new interpretation of “private” to be insufficiently strict
	- Swift’s access control would go from “entirely lexical” to “partly lexical and partly type-based”, which can be viewed as being more complicated

Thoughts? Volunteer?

	- Doug
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