[swift-evolution] [Discussion] "with" statement/method

[Patrick Smith]

I was wondering if all mutating and setters of a method could become a type (Typename.Mutation), say something similar to an enum.
You’d use it like this:
let alice = john mutating [  .firstName = "Alice",  .makeScottishClan]
See more here:https://gist.github.com/BurntCaramel/ba2ce9dfd49595dacce07394de579172
Patrick




On Fri, Apr 22, 2016 at 12:18 AM -0700, "Vladimir.S via swift-evolution" <swift-evolution at swift.org> wrote:










Just wanted to summarize our opinions on this suggestion that we was 
discussing earlier and check if someone is ready to crate an "official" 
proposal for this feature.

There a number of questions regarding this feature we can discuss, for 
example statement vs method, what could be a placeholder of target instance 
inside the scope($0, $, _, .., nothing etc)

The main question, *do you support that we need "with" feature in some way 
in Swift 3.0 out of the box* . And if so, what variant do you prefer.

* this proposal is for *explicit* "with", where it is clear what 
method/property belongs to the "target" instance
* I believe that as such feature is really useful and handy, if it is 
explicit and if it is clearly showing in code what we are doing - we want 
to have this feature as part of language/standard lib rather than 
possibility to use some workaround to implement it
* It is not about saving the space in code. It is about more readable and 
(I insist) more stable(with less errors) code. Much less possibilities for 
copy-paste errors. Wrong code completion suggestion(by editor) can not 
produce error. It is explicit and clear, it has much less noise in code.
* Many of us already implemented and use such "with" construction in some way

There were 2 main suggestions :

1) Introduce "with" statement that can be used in for example in such way:

// set props just after creating
// similar to "if let.. " and "guard let.."
with let questionLabel = UILabel() {
   //set props of created instance here
   // here we can have:
   // $0.prop = value
   // or
   // ..prop = value
   // or
   // .prop = value
   // or
   // _.prop = value
   // or
   // $.prop = value
   // or ?
}
// questionLabel is available here


// works for structures
with var some = SomeStruct() {
   //...
}

// just for some class/structure/enum
with questionLabel {
   // ..
}

probably

with var src = someNamedInstance1,
      let dst = someNamedInstance2 {
    src.propA = dst.propB
    dst.someMethod(src.propC)
    src.someMehtod()
}

or
with someNamedInstance1, someNamedInstance2 {
    $0.propA = $1.propB
    $1.someMethod($0.propC)
    $0.someMehtod()
}


2) Introduce .with method for each(?) class/struct, so we can use out-of-box:

let questionLabel = UILabel().with {
   //set props of created instance here
   $0.prop = value
}

var someStructInstance = SomeStruct().with {target in
   target.prop = value
}

questionLabel.with {label in
   label.prop = value
}

someNamedInstance1.with(someNamedInstance2) {src, dst in
    src.propA = dst.propB
    dst.someMethod(src.propC)
    src.someMehtod()
}

Note that function like this :
func with(item:T, apply:(T)->Void) {  apply(item) }

Produces such kind of problems:
struct A {var x = 1}
let a1 = A() // constant
with (a1) { $0.x = 10 } // this will be compiled without errors/warnings


On 13.04.2016 17:17, Radosław Pietruszewski via swift-evolution wrote:
> It can be (more-or-less) solved in library code today:
>
>     extension NSObjectProtocol {
>         public func with(@noescape fn: Self -> Void) -> Self {
>             fn(self)
>             return self
>         }
>     }
>
>
> This way, you can do, on NSObjects:
>
>     textLabel.with {
>
>     $0.textAlignment = .Left
>
>     $0.textColor = .darkTextColor()
>
>     }
>
>
> I love this pattern.
>
> You can also make it a function to make it work with any value of any kind
> (it will then take form of `with(foo) { …}`).
>
> Ideally, if you could write a universal extension (something like
> `extension Any`), you could just add this behavior, with method syntax, to
> everything.
>
> — Radek
>
>> On 13 Apr 2016, at 15:15, 李海珍 via swift-evolution
>> > wrote:
>>
>> I recently learned some VBA and I found a very conveniently `with` statement.
>>
>> `with` statement can be helpful to set property for UIKit instance.
>>
>> for instance a UILabel instance `textLabel` ,with `with` statement we can
>> set UILabel property like this
>>
>>
>> ```swift
>>
>> with textLabel{
>>
>> .textAlignment= .Left
>>
>> .textColor= UIColor.darkTextColor()
>>
>> .font= UIFont.systemFontOfSize(15)
>>
>> }
>>
>> ```
>>
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>
>
>
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