[swift-evolution] [swift-evolution-announce] [Review] SE-0159: Fix Private Access Levels

Xiaodi Wu xiaodi.wu at gmail.com
Thu Mar 23 23:12:14 CDT 2017

Or, since many designs for submodules are possible, we can proceed to make
the best decision *now* with respect to access levels, confident that there
will be a good design for submodules whether or not there exist both scoped
and file-based private access. That is to say, any future submodule
proposal would be judged on how well it accommodates desired use cases if
one type of private is removed, and any future design for submodules would
be judged on how well it fits with the current set of access levels without
duplicating functionality with a different syntax if both types of private
are retained.

One very important thing about the evolution process (IMO) is that
decisions made aren't revisited without compelling changes in
circumstances. It is highly unusual that fileprivate vs. private is now
going through this process for a _third_ time. I submit that it is
untenable that every version of Swift should consider a change to the
access modifiers. Given, as some have said above, many different submodule
designs are possible whatever the number of access levels, I would expect
that we would not revisit this topic again for the foreseeable future,
whatever the decision is. That is, the question being asked here is, is it
better for Swift to have both fileprivate and private for all time, or one
file-scoped private for all time?
On Thu, Mar 23, 2017 at 21:48 Charles Srstka via swift-evolution <
swift-evolution at swift.org> wrote:

> On Mar 23, 2017, at 8:35 PM, Matthew Johnson via swift-evolution <
> swift-evolution at swift.org> wrote:
> On Mar 23, 2017, at 8:27 PM, Drew Crawford <drew at sealedabstract.com>
> wrote:
> Sent from my iPhone
> On Mar 23, 2017, at 6:41 PM, David Hart <david at hartbit.com> wrote:
> I have difficulties imagining a submodule proposal that could allow us to
> eliminate fileprivate. Care to give an example?
> The obvious example would be Rust.  Rust has exactly two visibilities, and
> merely one keyword.  By default, members are "private" which is visible
> inside the module (so, like Swift's internal). The "public" keyword is
> similar to Swift.
> The reason this works is that unlike in Swift where a module is something
> like a library or framework (Rust calls those "crates"), in Rust modules in
> are (explicitly) lexically scoped; a "mod myscope {}" module can be created
> for the portion of the file for which the member should be visible and it
> won't be visible outside that scope. Likewise, "fileprivate" can be
> achieved by enclosing the file in a "mod MyFile {}". And like all lexical
> scopes, they can be recursively nested to arbitrary depth to achieve any
> number of visibility behaviors (e.g., declare a module for the first half
> of two files) that would require complex new keywords to achieve in Swift.
>  Finally there are some shortcut features like the ability to infer a
> module structure from the file system.
> This is a good example of what I meant.  There is an extremely broad range
> of possible designs for submodules.  Some of them, such as this example,
> would make it relatively easy to get by without fileprivate.  There are
> also many other possible designs that would not.
> We do not have any idea where Swift will end up yet.  It's not reasonable
> to make any assumptions about what use cases the eventual design might or
> might not address.
> Then why not leave private and fileprivate alone until we do?
> Charles
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