> How so? It seems that encapsulation is orthogonal to reference/value
> semantics.
Consider this type
public struct Mailbox {
private var _messages: [String]
public mutating func append(message: String) { _messages.append(message)}
public var messages: [String] { return _messages}
public var sortedMessages: [String] { return _messages.sorted() }
}
We subsequently discover in the wild that although there are a variety of
client patterns, certain "important" clients have the particular pattern
1. Add messages up front
2. Use sortedMessages accessor frequently and exclusively
It would improve the important clients without harming other clients to use the
following implementation for our type: if the sorted accessor is used before
the unsorted one, sort the underlying storage, and then serve that until the
type is subsequently mutated (which important clients do not do). This
amortizes the cost of the sort across many calls to the accessor.
Although the underlying storage is "private" and we may consider its
sorted-arity an implementation detail of Mailbox, in fact there is no
straightforward way to get to that implementation from this one. When we chose
value semantics we promised clients that our "encapsulated" values will not
change inside a getter; mutating the "hidden" storage breaks our promise.
I would argue this "encapsulation failure" of the _messages array is actually a
feature, and part of the draw of value types is that they provide these kinds
of guarantees to clients, and we subvert them at our peril. In cases where that
is not desired reference types are available.
> We want to be able to add new stored properties to structs, or change
> existing stored properties to computed properties (and vice versa) without
> breaking binary or source compatibility.
If CGAffineTransform, CGPoint, in_addr_t etc. are changing their members we
have a problem. There are probably structs where this makes more sense, but I
can't immediately think of an example.
On November 20, 2017 at 6:16:55 PM, Slava Pestov (spes...@apple.com) wrote:
On Nov 20, 2017, at 1:58 PM, Drew Crawford via swift-evolution
<swift-evolution@swift.org> wrote:
I'm "weak oppose" on this proposal.
The core premise here is to increase the encapsulation of a struct around its
member variables. But I think the purview of encapsulation is more classes
than structs.
How so? It seems that encapsulation is orthogonal to reference/value semantics.
e.g. a struct leaks information about the mutation of its member variables,
even if those variables are private.
Can you explain what you mean by this?
Structs are the obvious implementation for a named tuple (CGPoint
CGAffineTransform UIColor etc.) where there is inherently a fixed set of
members that are more conveniently accessed directly. Structs and classes have
different purposes and so the argument for consistency with classes is weak.
With regard to the BalancedPair problem, I would prefer to see a "final struct"
or "required init”.
The real reason we want to introduce this language change is to solve some
problems related to resilience. We want to be able to add new stored properties
to structs, or change existing stored properties to computed properties (and
vice versa) without breaking binary or source compatibility. Since
non-delegating initializers expose the exact set of stored properties to the
client module, they break the encapsulation that we need to allow this.
Slava
On November 14, 2017 at 1:31:25 PM, Ted Kremenek (kreme...@apple.com) wrote:
The review of "SE-0189: Restrict Cross-module Struct Initializers" begins now
and runs through November 21, 2017.
The proposal is available here:
https://github.com/apple/swift-evolution/blob/master/proposals/0189-restrict-cross-module-struct-initializers.md
Reviews are an important part of the Swift evolution process. All review
feedback should be sent to the swift-evolution mailing list at:
https://lists.swift.org/mailman/listinfo/swift-evolution
or, if you would like to keep your feedback private, directly to the review
manager.
When replying, please try to keep the proposal link at the top of the message:
Proposal link:
https://github.com/apple/swift-evolution/blob/master/proposals/0189-restrict-cross-module-struct-initializers.md
...
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Other replies
What goes into a review of a proposal?
The goal of the review process is to improve the proposal under review through
constructive criticism and, eventually, determine the direction of Swift.
When reviewing a proposal, here are some questions to consider:
What is your evaluation of the proposal?
Is the problem being addressed significant enough to warrant a change to Swift?
Does this proposal fit well with the feel and direction of Swift?
If you have used other languages or libraries with a similar feature, how do
you feel that this proposal compares to those?
How much effort did you put into your review? A glance, a quick reading, or an
in-depth study?
Thanks,
Ted Kremenek
Review Manager
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