Thanks Erica, I wasn't aware of that Swift evolution proposal. If I'm reading
it right, this wouldn't work with structs with let-variables...? Here's what I
get with this example:
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" } // Test.swift:24:41:
Cannot assign to property: 'name' is a 'let' constant
Andy
> On Dec 19, 2016, at 11:44 AM, Erica Sadun <[email protected]> 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
>> <[email protected] <mailto:[email protected]>> 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 <[email protected]
>> <mailto:[email protected]>> 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
>>> <[email protected] <mailto:[email protected]>>, 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
>>>>
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> [email protected] <mailto:[email protected]>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
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