> On Dec 29, 2015, at 5:44 PM, Dave Abrahams via swift-evolution
> <swift-evolution@swift.org> wrote:
>
>
>> On Dec 29, 2015, at 12:06 PM, Kevin Ballard via swift-evolution
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>
>> I briefly skimmed your proposal, so I apologize if you already addressed
>> this, but it occurs to me that we could support automatic protocol
>> forwarding today on a per-protocol basis simply by declaring a separate
>> protocol that provides default implementations doing the forwarding.
>> Handling of Self return types can then be done by adding a required
>> initializer (or just not implementing that method, so the concrete type is
>> forced to deal with it even though everything else is forwarded).
>>
>> For example, if I want to automatically forward SequenceType to a member, I
>> can do something like
>>
>> protocol SequenceTypeForwarder : SequenceType {
>> typealias ForwardedSequenceType : SequenceType
>>
>> var forwardedSequence : ForwardedSequenceType { get }
>> }
>>
>> extension SequenceTypeForwarder {
>> func generate() -> ForwardedSequenceType.Generator {
>> return forwardedSequence.generate()
>> }
>>
>> func underestimateCount() -> Int {
>> return forwardedSequence.underestimateCount()
>> }
>>
>> func map<T>(@noescape transform:
>> (ForwardedSequenceType.Generator.Element) throws -> T) rethrows -> [T] {
>> return try forwardedSequence.map(transform)
>> }
>>
>> func filter(@noescape includeElement:
>> (ForwardedSequenceType.Generator.Element) throws -> Bool) rethrows ->
>> [ForwardedSequenceType.Generator.Element] {
>> return try forwardedSequence.filter(includeElement)
>> }
>>
>> func forEach(@noescape body: (ForwardedSequenceType.Generator.Element)
>> throws -> Void) rethrows {
>> return try forwardedSequence.forEach(body)
>> }
>>
>> func dropFirst(n: Int) -> ForwardedSequenceType.SubSequence {
>> return forwardedSequence.dropFirst(n)
>> }
>>
>> func dropLast(n: Int) -> ForwardedSequenceType.SubSequence {
>> return forwardedSequence.dropLast(n)
>> }
>>
>> func prefix(maxLength: Int) -> ForwardedSequenceType.SubSequence {
>> return forwardedSequence.prefix(maxLength)
>> }
>>
>> func suffix(maxLength: Int) -> ForwardedSequenceType.SubSequence {
>> return forwardedSequence.suffix(maxLength)
>> }
>>
>> func split(maxSplit: Int, allowEmptySlices: Bool, @noescape isSeparator:
>> (ForwardedSequenceType.Generator.Element) throws -> Bool) rethrows ->
>> [ForwardedSequenceType.SubSequence] {
>> return try forwardedSequence.split(maxSplit, allowEmptySlices:
>> allowEmptySlices, isSeparator: isSeparator)
>> }
>> }
>
> FWIW,
>
> https://github.com/apple/swift/blob/master/stdlib/public/core/SequenceWrapper.swift
>
> <https://github.com/apple/swift/blob/master/stdlib/public/core/SequenceWrapper.swift>
>
> Though I don’t know why we still have this; it’s not used anywhere and should
> probably be removed. I think it was supposed to be part of the new lazy
> sequence/collection subsystem but it was never incorporated.
Dave, thanks for pointing me to the Lazy Collections subsystem. It made for a
really great case study!
A lot of what is happening in there is not directly forwarding related. But I
do think the implementation of the parts that involve forwarding is improved by
using the forwarding mechanism in this proposal. It is more clear and more
robust than the current implementation.
As it turns out, _SequenceWrapperType and the extension to SequenceType in
SequenceWrapper.swift actually are still in use. They contain an
implementation of the forwarding mechanism Kevin Ballard suggested in this
thread. _CollectionWrapperType and the extension to CollectionType are not in
use. LazyCollection uses manual forwarding in the type itself which avoids
some of the drawbacks of the protocol extension approach. Of course this begs
the question of why two different mechanisms are in use and which is actually
preferred.
I am working on a new draft of the proposal with a greatly expanded motivation
section. I don’t have that completed yet, but I have completed a first pass of
the section on the lazy collection subsystem. I am including the current draft
here. I hope you find it interesting. I am interested in your thoughts on it.
Matthew
Motivation
Delegation is a robust, composition oriented design technique that keeps
interface and implementation inheritance separate. The primary drawback to this
technique is that it requires a lot of manual boilerplate to forward
implemenation to the implementing member. This proposal eliminates the need to
write such boilerplate manually, thus making delegation-based designs much more
convenient and attractive.
This proposal may also serve as the foundation for a future enhancement
allowing a very concise “newtype” declaration. In the meantime, it facilitates
similar functionality, although in a slightly more verbose manner.
Examples
Several examples follow.
The first two show how this proposal could improve how forwarding is
implemented by the lazy collection subsystem of the standard library. This
makes an interesting case study as each example employs a different forwarding
mechanism.
LazySequence
The relevant portion of the current implementation of LazySequence looks like
this (with comments removed and formatting tweaks):
// in SequenceWrapper.swift:
public protocol _SequenceWrapperType {
typealias Base : SequenceType
typealias Generator : GeneratorType = Base.Generator
var _base: Base {get}
}
extension SequenceType
where Self : _SequenceWrapperType, Self.Generator == Self.Base.Generator {
public func generate() -> Base.Generator {
return self._base.generate()
}
public func underestimateCount() -> Int {
return _base.underestimateCount()
}
@warn_unused_result
public func map<T>(
@noescape transform: (Base.Generator.Element) throws -> T
) rethrows -> [T] {
return try _base.map(transform)
}
@warn_unused_result
public func filter(
@noescape includeElement: (Base.Generator.Element) throws -> Bool
) rethrows -> [Base.Generator.Element] {
return try _base.filter(includeElement)
}
public func _customContainsEquatableElement(
element: Base.Generator.Element
) -> Bool? {
return _base._customContainsEquatableElement(element)
}
public func _preprocessingPass<R>(@noescape preprocess: (Self) -> R) -> R? {
return _base._preprocessingPass { _ in preprocess(self) }
}
public func _copyToNativeArrayBuffer()
-> _ContiguousArrayBuffer<Base.Generator.Element> {
return _base._copyToNativeArrayBuffer()
}
public func _initializeTo(ptr: UnsafeMutablePointer<Base.Generator.Element>)
-> UnsafeMutablePointer<Base.Generator.Element> {
return _base._initializeTo(ptr)
}
}
// in LazySequence.swift:
public struct LazySequence<Base : SequenceType>
: LazySequenceType, _SequenceWrapperType {
public init(_ base: Base) {
self._base = base
}
public var _base: Base
public var elements: Base { return _base }
}
LazySequence is using the approach to forwarding mentioned by Kevin Ballard on
the mailing list
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151228/004755.html>in
response to this proposal. This approach has several deficiencies that
directly impact LazySequence:
LazySequence must publicly expose implementation details. Both its _base
property as well as its conformance to _SequenceWrapperType.
The forwarding members must be manually implemented. They are trivial, but
mistakes are still possible. In this case, @warn_unused_result is missing in
some places where it should probably be specified (and would be synthesized
using the approach in this proposal due to its presence in the protocol member
declarations).
It is not immediately apparent that _SequenceWrapperType and the corresponding
extension only provide forwarding members. Even if the name clearly indicates
that it is possible that the code does something different. It is possible for
somebody to come along after the initial implementation and add a new method
that does something other than simple forwarding.
Because the forwarding is implemented via a protocol extension as default
methods it can be overriden by an extension on LazySequence.
Here is an alternative implemented using the current proposal:
// _LazySequenceForwarding redeclares the subset of the members of SequenceType
we wish to forward.
// The protocol is an implementation detail and is marked private.
private protocol _LazySequenceForwarding {
typealias Generator : GeneratorType
@warn_unused_result
func generate() -> Generator
@warn_unused_result
func underestimateCount() -> Int
@warn_unused_result
func map<T>(
@noescape transform: (Generator.Element) throws -> T
) rethrows -> [T]
@warn_unused_result
func filter(
@noescape includeElement: (Generator.Element) throws -> Bool
) rethrows -> [Generator.Element]
@warn_unused_result
func _customContainsEquatableElement(
element: Generator.Element
) -> Bool?
func _copyToNativeArrayBuffer() -> _ContiguousArrayBuffer<Generator.Element>
func _initializeTo(ptr: UnsafeMutablePointer<Generator.Element>)
-> UnsafeMutablePointer<Generator.Element>
}
public struct LazySequence<Base : SequenceType> : LazySequenceType {
public init(_ base: Base) {
self._base = base
}
// NOTE: _base is now internal
internal var _base: Base
public var elements: Base { return _base }
public forward _LazySequenceForwarding to _base
// The current proposal does not currently support forwarding
// of members with nontrivial Self requirements.
// Because of this _preprocessingPass is forwarded manually.
// A future enhancement may be able to support automatic
// forwarding of protocols with some or all kinds of
// nontrivial Self requirements.
public func _preprocessingPass<R>(@noescape preprocess: (Self) -> R) -> R? {
return _base._preprocessingPass { _ in preprocess(self) }
}
}
This example takes advantage of a very important aspect of the design of this
proposal. Neither Base nor LazySequence are required to conform to
_LazySequenceForwarding. The only requirement is that Base contains the members
specified in _LazySequenceForwarding as they will be used in the synthesized
forwarding implementations.
The relaxed requirement is crucial to the application of the protocol
forwarding feature in this implementation. We cannot conform Base to
_LazySequenceForwarding. If it were possible to conform one protocol to another
we could conform SequenceType to _LazySequenceForwarding, however it is
doubtful that we would want that conformance. Despite this, it is clear to the
compiler that Base does contain the necessary members for forwarding as it
conforms to LazySequence which also declares all of the necessary members.
This implementation is more robust and more clear:
We no longer leak any implementation details.
There is no chance of making a mistake in the implementation of the forwarded
members. It is possible that a mistake could be made in the member declarations
in _LazySequenceForwarding. However, if a mistake is made there a compiler
error will result.
The set of forwarded methods is immediately clear, with the exception of
_preprocessingPass because of its nontrivial Self requirement. Removing the
limitation on nontrivial Self requirements is a highly desired improvement to
this proposal or future enhancement to this feature.
The forwarded members cannot be overriden in an extension on LazySequence. If
somebody attempts to do so it will result in an ambiguous use error at call
sites.
LazyCollection
The relevant portion of the current implementation of LazyCollection looks like
this (with comments removed and formatting tweaks):
// in LazyCollection.swift:
public struct LazyCollection<Base : CollectionType>
: LazyCollectionType {
public typealias Elements = Base
public var elements: Elements { return _base }
public typealias Index = Base.Index
public init(_ base: Base) {
self._base = base
}
internal var _base: Base
}
extension LazyCollection : SequenceType {
public func generate() -> Base.Generator { return _base.generate() }
public func underestimateCount() -> Int { return _base.underestimateCount() }
public func _copyToNativeArrayBuffer()
-> _ContiguousArrayBuffer<Base.Generator.Element> {
return _base._copyToNativeArrayBuffer()
}
public func _initializeTo(
ptr: UnsafeMutablePointer<Base.Generator.Element>
) -> UnsafeMutablePointer<Base.Generator.Element> {
return _base._initializeTo(ptr)
}
public func _customContainsEquatableElement(
element: Base.Generator.Element
) -> Bool? {
return _base._customContainsEquatableElement(element)
}
}
extension LazyCollection : CollectionType {
public var startIndex: Base.Index {
return _base.startIndex
}
public var endIndex: Base.Index {
return _base.endIndex
}
public subscript(position: Base.Index) -> Base.Generator.Element {
return _base[position]
}
public subscript(bounds: Range<Index>) -> LazyCollection<Slice<Base>> {
return Slice(base: _base, bounds: bounds).lazy
}
public var isEmpty: Bool {
return _base.isEmpty
}
public var count: Index.Distance {
return _base.count
}
public func _customIndexOfEquatableElement(
element: Base.Generator.Element
) -> Index?? {
return _base._customIndexOfEquatableElement(element)
}
public var first: Base.Generator.Element? {
return _base.first
}
}
LazyCollection is using direct manual implementations of forwarding methods. It
corresponds exactly to implementations that would be synthesized by the
compiler under this proposal. This approach avoids some of the problems with
the first approach:
It does not leak implementation details. This is good!
The forwarded members cannot be overriden.
Unfortunately it still has some drawbacks:
It is still possible to make mistakes in the manual forwarding implementations.
The set of forwarded methods is even less clear than under the first approach
as they are now potentially interspersed with custom, nontrivial member
implementations, such as subscript(bounds: Range<Index>) ->
LazyCollection<Slice<Base>> in this example.
This approach requires reimplementing the forwarded members in every type which
forwards them and is therefore less scalable than the first approach and this
proposal. This may not matter for LazyCollection but it may well matter in
other cases.
One intersting difference to note between LazySequence and LazyCollection is
that LazySequence forwards three members which LazyCollection does not: map,
filter, and _preprocessingPass. It is unclear whether this difference is
intentional or not.
This difference is particularly interesting in the case of _preprocessingPass.
LazyCollectionappears to be using the default implementation for CollectionType
in Collection.swift, which results in _base._preprocessingPass not getting
called. It is not apparent why this behavior would be correct for
LazyCollection and not for LazySequence.
I wonder if the difference in forwarded members is partly due to the fact that
the set of forwarded members is not as clear as it could be.
Here is an alternate approach implemented using the current proposal. It
assumes that the same SequenceType members that are forwarded by LazySequence
should also be forwarded by LazyCollection, allowing us to reuse the
_LazySequenceForwarding protocol declared in the first example.
// _LazyCollectionForwarding redeclares the subset of the members of Indexable
and CollectionType we wish to forward.
// The protocol is an implementation detail and is marked private.
private protocol _LazyCollectionForwarding: _LazySequenceForwarding {
typealias Index : ForwardIndexType
var startIndex: Index {get}
var endIndex: Index {get}
typealias _Element
subscript(position: Index) -> _Element {get}
var isEmpty: Bool { get }
var count: Index.Distance { get }
var first: Generator.Element? { get }
@warn_unused_result
func _customIndexOfEquatableElement(element: Generator.Element) -> Index??
}
public struct LazyCollection<Base : CollectionType>
: LazyCollectionType {
public typealias Elements = Base
public var elements: Elements { return _base }
public init(_ base: Base) {
self._base = base
}
internal var _base: Base
public forward _LazyCollectionForwarding to _base
// It may be the case that LazyCollection should forward _preprocessingPass
// in the same fashion that LazySequence uses, which cannot yet be automated
// under the current proposal.
}
extension LazyCollection : CollectionType {
// This implementation is nontrivial and thus not forwarded
public subscript(bounds: Range<Index>) -> LazyCollection<Slice<Base>> {
return Slice(base: _base, bounds: bounds).lazy
}
}
This approach to forwarding does not exhibit any of the issues with the manual
approach and only takes about half as much code now that we are able to reuse
the previous declaration of _LazySequenceForwarding.
NOTE: LazyMapCollection in Map.swift uses the same manual forwarding approach
as LazyCollection to forward a handful of members and would therefore also be a
candidate for adopting the new forwarding mechanism as well.
>
>> With this protocol declared, I can then say something like
>>
>> struct Foo {
>> var ary: [Int]
>> }
>>
>> extension Foo : SequenceTypeForwarder {
>> var forwardedSequence: [Int] { return ary }
>> }
>>
>> and my struct Foo now automatically implements SequenceType by forwarding to
>> its variable `ary`.
>>
>> The downside to this is it needs to be manually declared for each protocol.
>> But I wager that most protocols actually aren't really amenable to
>> forwarding anyway.
>>
>> -Kevin Ballard
>>
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
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