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.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="bloop_markdown"><p>Hurray, I cannot wait to get the consistent behavior of <code>open/public protocol</code>s. I’m not sure I could follow the idea behind the proposed <code>closed</code> keyboard/access modifier. It almost felt like <code>closed == final public</code>, am I mistaken something here?</p>
<p>Furthermore, I really would love if the community could revisit how <code>open/public</code> really should behave. When <code>open</code> was implemented and I tried it out without reading the proposal first I bumped into things like <code>open init()</code> which felt really odd. I understand the argumentation from the proposal, but it feels wrong and inconsistent to me.</p>
<p>Here’s how I would have imagined <code>open</code> vs. <code>public</code>. IMHO <code>public</code> should really mean, you cannot subclass, conform or override something in module B from module A.</p>
<p>Modified samples from SE–0117:</p>
<pre><code class="swift">// This class is not subclassable outside of ModuleA.
public class NonSubclassableParentClass {
// This method >is not overridable outside of ModuleA.
public func foo() {}
// This method is not overridable outside of ModuleA because
// its class restricts its access level.
// It is INVALID to declare it as `open`.
public func bar() {}
// The behavior of `final` methods remains unchanged.
public final func baz() {}
}
// This class is subclassable both inside and outside of ModuleA.
open class SubclassableParentClass {
// Designated initializer that is not overridable outside ModuleA
public init()
// Another designated initializer that is overridable outside ModuleA
open init(foo: Int)
// This property is not overridable outside of ModuleA.
public var size : Int
// This method is not overridable outside of ModuleA.
public func foo() {}
// This method is overridable both inside and outside of ModuleA.
open func bar() {}
/// The behavior of a `final` method remains unchanged.
public final func baz() {}
}
/// The behavior of `final` classes remains unchanged.
public final class FinalClass { }
</code></pre>
<pre><code class="swift">/// ModuleB:
import ModuleA
// This is allowed since the superclass is `open`.
class SubclassB : SubclassableParentClass {
// Iff certain conditions are met, the superclass initializers are inherited.
// `init` will stay `public` and won't be overridable.
//
// If the conditions are not met, then `init` is not inherited. That does not
// mean that we can create a new designated `init` that matches it's superclass's
// designated initializer. The behavior should be consistent, like the
// superclass's function `foo` is reserved and not overridable, so is `init`
// reserved in this case and not overridable.
// This is allowed since the superclass's initializer is `open`
override init(foo: Int) {
super.init(foo: foo)
}
init(bar: Int) {
// We could call a super designated initializer from here
super.init()
// or
super.init(foo: bar)
}
// This is invalid because it overrides a method that is
// defined outside of the current module but is not `open'.
override func foo() { }
// This is allowed since the superclass's method is overridable.
// It does not need to be marked `open` because it is defined on
// an `internal` class.
override func bar() { }
}
</code></pre>
<p><code>required</code> should always match the same scope level as the type in which it’s defined. That means if the class is <code>open</code>, than any of it’s <code>required</code> initializers will be <code>open</code> as well.</p>
<p></p></div><div class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div> <br> <div id="bloop_sign_1486600157602554112" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 9. Februar 2017 um 00:49:04, Xiaodi Wu via swift-evolution (<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div><div></div><div>
<title></title>
I agree very much with rationalizing access levels, but I'm not
sure I like this proposal for public vs. closed. How would the
compiler stop me from editing my own code if something is closed?
The answer must be that it can't, so I can't see it as a co-equal
to open but rather simply a statement of intention. Therefore I
think use cases for the proposed behavior of closed would be better
served by annotations and proper semantic versioning.<br>
<br>
As this change didn't seem in scope for Swift 4 phase 1, I've held
off on discussing my own thoughts on access levels. The idea I was
going to propose in phase 2 was to have simply open and public
enums (and protocols). I really think that completes access levels
in a rational way without introducing another keyword.<br>
<div class="gmail_quote">
<div dir="ltr">On Wed, Feb 8, 2017 at 17:05 Matthew Johnson via
swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>>
wrote:<br></div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="word-wrap:break-word" class="gmail_msg">
<div class="gmail_msg">I’ve been thinking a lot about our public
access modifier story lately in the context of both protocols and
enums. I believe we should move further in the direction we
took when introducing the `open` keyword. I have identified
what I think is a promising direction and am interested in feedback
from the community. If community feedback is positive I will
flesh this out into a more complete proposal draft.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">Background and Motivation:</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
In Swift 3 we had an extended debate regarding whether or not to
allow inheritance of public classes by default or to require an
annotation for classes that could be subclassed outside the
module. The decision we reached was to avoid having a default
at all, and instead make `open` an access modifier. The
result is library authors are required to consider the behavior
they wish for each class. Both behaviors are equally
convenient (neither is penalized by requiring an additional
boilerplate-y annotation).
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">A recent thread (<a href="https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170206/031566.html" class="gmail_msg" target="_blank">https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170206/031566.html</a>)
discussed a similar tradeoff regarding whether public enums should
commit to a fixed set of cases by default or not. The current
behavior is that they *do* commit to a fixed set of cases and there
is no option (afaik) to modify that behavior. The Library
Evolution document (<a href="https://github.com/apple/swift/blob/master/docs/LibraryEvolution.rst#enums" class="gmail_msg" target="_blank">https://github.com/apple/swift/blob/master/docs/LibraryEvolution.rst#enums</a>)
suggests a desire to change this before locking down ABI such that
public enums *do not* make this commitment by default, and are
required to opt-in to this behavior using an `@closed`
annotation.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">In the previous discussion I stated a strong
preference that closed enums *not* be penalized with an additional
annotation. This is because I feel pretty strongly that it is
a design smell to: 1) expose cases publicly if consumers of the API
are not expected to switch on them and 2) require users to handle
unknown future cases if they are likely to switch over the cases in
correct use of the API.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">The conclusion I came to in that thread is
that we should adopt the same strategy as we did with classes:
there should not be a default.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">There have also been several discussions
both on the list and via Twitter regarding whether or not we should
allow closed protocols. In a recent Twitter discussion Joe
Groff suggested that we don’t need them because we should use an
enum when there is a fixed set of conforming types. There are
at least two reasons why I still think we *should* add
support for closed protocols.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">As noted above (and in the previous thread
in more detail), if the set of types (cases) isn’t intended to be
fixed (i.e. the library may add new types in the future) an enum is
likely not a good choice. Using a closed protocol discourages
the user from switching and prevents the user from adding
conformances that are not desired.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">Another use case supported by closed
protocols is a design where users are not allowed to conform
directly to a protocol, but instead are required to conform to one
of several protocols which refine the closed protocol. Enums
are not a substitute for this use case. The only option is to
resort to documentation and runtime checks.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">Proposal:</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">This proposal introduces the new access
modifier `closed` as well as clarifying the meaning of `public` and
expanding the use of `open`. This provides consistent
capabilities and semantics across enums, classes and
protocols.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">`open` is the most permissive
modifier. The symbol is visible outside the module and both
users and future versions of the library are allowed to add new
cases, subclasses or conformances. (Note: this proposal does
not introduce user-extensible `open` enums, but provides the syntax
that would be used if they are added to the language)</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">`public` makes the symbol visible without
allowing the user to add new cases, subclasses or
conformances. The library reserves the right to add new
cases, subclasses or conformances in a future version.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">`closed` is the most restrictive
modifier. The symbol is visible publicly with the commitment
that future versions of the library are *also* prohibited from
adding new cases, subclasses or conformances. Additionally,
all cases, subclasses or conformances must be visible outside the
module.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">Note: the `closed` modifier only applies to
*direct* subclasses or conformances. A subclass of a `closed`
class need not be `closed`, in fact it may be `open` if the design
of the library requires that. A class that conforms to a
`closed` protocol also need not be `closed`. It may also be
`open`. Finally, a protocol that refines a `closed` protocol
need not be `closed`. It may also be `open`.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">This proposal is consistent with the
principle that libraries should opt-in to all public API contracts
without taking a position on what that contract should be. It
does this in a way that offers semantically consistent choices for
API contract across classes, enums and protocols. The result
is that the language allows us to choose the best tool for the job
without restricting the designs we might consider because some
kinds of types are limited with respect to the `open`, `public` and
`closed` semantics a design might require.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">Source compatibility:</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">This proposal affects both public enums and
public protocols. The current behavior of enums is equivalent
to a `closed` enum under this proposal and the current behavior of
protocols is equivalent to an `open` protocol under this
proposal. Both changes allow for a simple mechanical
migration, but that may not be sufficient given the source
compatibility promise made for Swift 4. We may need to
identify a multi-release strategy for adopting this proposal.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">Brent Royal-Gordon suggested such a strategy
in a discussion regarding closed protocols on Twitter:</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">* In Swift 4: all unannotated public
protocols receive a warning, possibly with a fix-it to change the
annotation to `open`.</div>
<div class="gmail_msg">* Also in Swift 4: an annotation is
introduced to opt-in to the new `public` behavior. Brent
suggested `@closed`, but as this proposal distinguishes `public`
and `closed` we would need to identify something else. I will
use `@annotation` as a placeholder.</div>
<div class="gmail_msg">* Also In Swift 4: the `closed` modifier is
introduced.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">* In Swift 5 the warning becomes a compiler
error. `public protocol` is not allowed. Users must use
`@annotation public protocol`.</div>
<div class="gmail_msg">* In Swift 6 `public protocol` is allowed
again, now with the new semantics. `@annotation public
protocol` is also allowed, now with a warning and a fix-it to
remove the warning.</div>
<div class="gmail_msg">* In Swift 7 `@annotation public protocol`
is no longer allowed.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg">A similar mult-release strategy would work
for migrating public enums.</div>
<div class="gmail_msg"><br class="gmail_msg"></div>
<div class="gmail_msg"><br class="gmail_msg"></div>
</div>
_______________________________________________<br class="gmail_msg">
swift-evolution mailing list<br class="gmail_msg">
<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a><br class="gmail_msg">
<a href="https://lists.swift.org/mailman/listinfo/swift-evolution" rel="noreferrer" class="gmail_msg" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br class="gmail_msg">
</blockquote>
</div>
_______________________________________________<br>swift-evolution mailing list<br>swift-evolution@swift.org<br>https://lists.swift.org/mailman/listinfo/swift-evolution<br></div></div></span></blockquote></div><div class="bloop_markdown"><p></p></div></body></html>