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

Erica Sadun erica at ericasadun.com
Mon Dec 19 13:44:37 CST 2016

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:


-- E

> On Dec 19, 2016, at 12:40 PM, Andy Chou via swift-evolution <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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>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
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