Generics, the Swift ABI and you

SWIFT GENERICS, THE ABI AND YOU
Swift Austin
May 3rd, 2017

THIS IS A *BIG* TOPIC
And I’m not a compiler expert, and Swift’s
implementation is in ﬂux.
(Plus it’s been a long week already)
If Reality disagrees, it wins.

THIS IS NOT
SUPPOSED TO BE A
LECTURE
*Please* interrupt if you have questions,
opinions, disagreements,etc.
This will be much better as a discussion.

QUICK INTRODUCTION
• Generic functions can work with any type.
• They help reduce boilerplate and duplication
• (From https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/
Generics.html)
func swapTwoValues<T>(_ a: inout T, _ b: inout T) {
let temporaryA = a
a = b
b = temporaryA
}

QUICK HISTORY (IN SWIFT)
Slide from WWDC 2016 Session 402
Generics were new (more or less) in Swift 3

ASSOCIATEDTYPES
Mostly the same thing, but for Protocols
Slide from WWDC 2016 Session 419

EXAMPLE:ARRAY
func makeIterator() -> IndexingIterator<Array<(String, String)>>

EXAMPLE: UNSAFE POINTERS
reqDispatchData = requestBody.withUnsafeBytes { (ptr: UnsafePointer<UInt8>) ->
DispatchData in
DispatchData(bytes: UnsafeBufferPointer<UInt8>(start: ptr,
count: requestBody.count))
}

EXAMPLE:WEAK WRAPPER
class Weak<T: AnyObject> {
weak var value : T?
init (value: T) {
self.value = value
}
}
extension Array where Element:Weak<AnyObject> {
mutating func reap () {
self = self.filter { nil != $0.value }
}
}
From: http://stackoverﬂow.com/a/24128121

ASIDE:WHAT’S ABI?
Application Binary Interface: It’s what makes a language “all grown up”

WHAT DOES THAT HAVE TO DO WITH
GENERICS?
• Chris Lattner:“…including some of the most important generics
features needed in order to lock down the ABI of the standard
library.As such, the generics and ABI stability goals will roll into a
future release of Swift, where I expect them to be the highest priority
features to get done.”
• From http://ericasadun.com/2016/05/16/winding-down-swift-3-0-
abi-stability-deferred/

SO, WHAT ARE WE STILL MISSING?
• Variadic Generics
• Generic value parameters
• Extensions of structural types
• Currently, only nominal types (classes, structs, enums, protocols) can be extended. One could imagine extending
structural types--particularly tuple types--to allow them to, e.g., conform to protocols
public func zip<... Sequences : SequenceType>(... sequences: Sequences...)
-> ZipSequence<Sequences...> {
return ZipSequence(sequences...)
} struct MultiArray<T, let Dimensions: Int> {
// specify the number of dimensions to the array
subscript (indices: Int...) -> T {
get {
require(indices.count == Dimensions)
// ...
}
}

SE-0148 GENERIC SUBSCRIPTS
https://github.com/apple/swift-evolution/blob/master/proposals/0148-generic-
subscripts.md
extension Dictionary {
subscript<Indices: Sequence>(indices: Indices) -> [Iterator.Element] where
Indices.Iterator.Element == Index {
// ...
}
}

SE-0142 PERMIT WHERE CLAUSES TO CONSTRAIN ASSOCIATED
TYPES
SE-0143 CONDITIONAL CONFORMANCES
https://github.com/apple/swift-evolution/blob/master/proposals/0142-associated-types-constraints.md
https://github.com/apple/swift-evolution/blob/master/proposals/0143-conditional-conformances.md
extension Array: Equatable where Element: Equatable {
static func ==(lhs: Array<Element>, rhs: Array<Element>) -> Bool { ... }
}
protocol Sequence {
associatedtype Iterator : IteratorProtocol
associatedtype SubSequence : Sequence where SubSequence.Iterator.Element == Iterator.El
...
}

RESOURCES:
• https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md
• https://github.com/apple/swift/blob/master/docs/Generics.rst
• https://github.com/apple/swift/blob/master/docs/ABIStabilityManifesto.md
• https://developer.apple.com/library/content/documentation/Swift/Conceptual/
Swift_Programming_Language/Generics.html
• https://www.natashatherobot.com/swift-protocols-with-associated-types/

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