Over the past couple of days, the Twitters have discovered some work I'd done
on closure-based setup.
It's clear that a demand is out there and strong for this kind of behavior,
even without implicit `self` as
part of the mix or cascading. In that light, I've put together the following:
https://gist.github.com/erica/96d9c5bb4eaa3ed3b2ff82dc35aa8dae
<https://gist.github.com/erica/96d9c5bb4eaa3ed3b2ff82dc35aa8dae>
If the community demand is this high, I think we should re-consider pushing it
before 3.
Feedback as always welcome, including criticism.
-- E
Introducing with to the Standard Library
Proposal: TBD
Author: Erica Sadun <https://github.com/erica>
Status: TBD
Review manager: TBD
<https://gist.github.com/erica/96d9c5bb4eaa3ed3b2ff82dc35aa8dae#introduction>Introduction
This proposal introduces a with function to the standard library to simplify
the initialization and modification of constants and Foundation-sourced complex
objects.
Swift-evolution thread: What about a VBA style with Statement?
<http://thread.gmane.org/gmane.comp.lang.swift.evolution/14384>
<https://gist.github.com/erica/96d9c5bb4eaa3ed3b2ff82dc35aa8dae#motivation>Motivation
Closure-based initialization enables clean and highly directed set-up in Swift
code. Numerous variations on the theme have been introduced on the Swift
Evolution list and in third party github repositories. Although you can build
solutions natively without functions, current Swift technology has drawbacks:
let questionLabel: UILabel = {
$0.textAlignment = .Center
$0.font = UIFont(name:"DnealianManuscript", size: 72)
$0.text = questionText
$0.numberOfLines = 0
return $0
}(UILabel())
let mySwitch : UISwitch = {
view.addSubview($0)
CenterViewInSuperview($0, horizontal: true, vertical: true)
$0.addTarget(self, action: "action", forControlEvents: .TouchUpInside)
return $0
}(UISwitch())
Assignment must be explicitly typed.
The source item must be postpended to the set-up closure.
The closure must return the item.
This approach is better suited to setting up Foundation objects than modifying
Swift constants. When duplicating and modifying a constant, the closure must
create a var copy and modify that copy.
While the implementation is imperfect, the wins are notable. Code naturally
groups into a clear set-up sequence. The scoped setup avoids clumpy redundant
lines where the same variable is accessed over and over.
let questionLabel = UILabel()
questionLabel.textAlignment = .Center
questionLabel.font = UIFont(name:"DnealianManuscript", size: 72)
questionLabel.text = questionText
questionLabel.numberOfLines = 0
In the case of non-reference types, a constant's fields may be set-up
sequentially without forcing the constant to be a variable.
<https://gist.github.com/erica/96d9c5bb4eaa3ed3b2ff82dc35aa8dae#detailed-design>Detailed
Design
This proposal introduces a with function that enables modification and use of
an instance using positional references. It's not quite as clean as a solution
with implicit self but it offers sufficient utility that a vast swath of Swift
developers have adopted this function in some form or another.
@discardableResult
public func with<T>(_ item: T, update: @noescape (inout T) throws -> Void)
rethrows -> T {
var this = item; try update(&this); return this
}
In use:
struct Person { var name: String, favoriteColor: UIColor }
let john = Person(name: "John", favoriteColor: .blueColor())
let jane = with(john){ $0.name = "Jane" }
print(jane) // Person(name: "Jane", favoriteColor: UIDeviceRGBColorSpace 0 0 1
1)
struct Point { var (x, y) : (Double, Double) }
let p1 = Point(x: 2, y: 3)
let p2 = with(p1){ $0.y = 4 }
print(p1, p2) // Point(x: 2.0, y: 3.0) Point(x: 2.0, y: 4.0)
<https://gist.github.com/erica/96d9c5bb4eaa3ed3b2ff82dc35aa8dae#impact-on-existing-code>Impact
on Existing Code
This proposal is purely additive and has no impact on existing code
<https://gist.github.com/erica/96d9c5bb4eaa3ed3b2ff82dc35aa8dae#alternatives-considered>Alternatives
Considered
Not adopting this proposal_______________________________________________
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