[swift-evolution] "with" operator a la O'Caml?

[Andy Chou]

Thanks Erica, I wasn't aware of that proposal. If I'm reading it right, the proposed 'with' function won't work for let-constants in structures, e.g.:

struct Person {
    let name: String
    let address: String
}

@discardableResult
public func with<T>(_ item: T, update: (inout T) throws -> Void) rethrows -> T {
    var this = item
    try update(&this)
    return this
}

let john = Person(name: "John", address: "1 battery st")
let jane: Person = with(john) { $0.name = "Jane" }   // Cannot assign to property: 'name' is a 'let' constant

Andy

> On Dec 19, 2016, at 11:44 AM, Erica Sadun <erica at ericasadun.com> wrote:
> 
> https://github.com/apple/swift-evolution/pull/346 <https://github.com/apple/swift-evolution/pull/346>
> 
> Be aware that there's a bug that's being worked on:
> 
> https://bugs.swift.org/browse/SR-2773 <https://bugs.swift.org/browse/SR-2773>
> 
> -- E
> 
> 
>> On Dec 19, 2016, at 12:40 PM, Andy Chou via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Of course. Thanks for pointing out the obvious solution. This preserves the immutability of the struct and doesn't require O(n^2) code for structs with large numbers of fields. 
>> 
>> I was thinking of a generic solution - perhaps something like a synthetic initializer that does what your solution does. But that may be overkill given how relatively easy it is to do this per struct...
>> 
>> On the other hand a generic solution would encourage using immutable structs. I wasted too much time trying to solve this, I suspect others would just give up and use var, or even classes. 
>> 
>> Andy
>> 
>> On Dec 19, 2016, at 10:43 AM, Nick Keets <nick.keets at gmail.com <mailto:nick.keets at gmail.com>> wrote:
>> 
>>> You are probably asking for a generic solution, but for a specific struct you can implement it like this:
>>> 
>>> extension Person {
>>>     func with(name: String? = nil, address: String? = nil, phone: String? = nil) -> Person {
>>>         let name = name ?? self.name
>>>         let address = address ?? self.address
>>>         let phone = phone ?? self.phone
>>>         return Person(name: name, address: address, phone: phone)
>>>     }
>>> }
>>> 
>>> 
>>> On 19 Dec 2016, 20:28 +0200, Andy Chou via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>, wrote:
>>>> I like that structs are value types in Swift, this encourages the use of immutable data. O'Caml has an operator "with" that allows for copying an existing struct with a change to one field. I looked at Lenses for this functionality and it seems like a lot to digest for something so simple. I also tried to implement this using a constructor, or a function, and it was not obvious how to do so without a lot of code duplication.
>>>> 
>>>> What's I'm looking for is something like this (not necessarily with this syntax):
>>>> 
>>>> struct Person {
>>>> let name: String
>>>> let address: String
>>>> let phone: String
>>>> }
>>>> 
>>>> func f() {
>>>> let andy = Person(name: "Andy", address: "1 Battery St., San Francisco, CA", phone: "1234567")
>>>> let chris = andy.with(name: "Chris")
>>>> let dave = andy.with(address: "50 Townsend St., San Francisco, CA")
>>>> }
>>>> 
>>>> I tried to implement a "with" function like this but default arguments cannot reference properties of self. Same problem trying to do this in a constructor.
>>>> 
>>>> Obviously it's possible to create an entirely new Person specifying the values from an existing Person, but this is very tedious with structures with many properties.
>>>> 
>>>> Anyone taken a look at this before? Any suggestions?
>>>> 
>>>> Andy
>>>> 
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> 

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