[swift-evolution] Constant generic properties in mutable contexts

[Maximilian Hünenberger]

Oh ok I see. I'm sorry to hold you off more important topics.

- Maximilian

> Am 30.01.2016 um 19:21 schrieb Joe Groff <jgroff at apple.com>:
> 
> This wouldn't change anything with value types. Even if you're not allowed to say:
> 
> array[0].name = "Sam"
> 
> You can still say:
> 
> array[0] = Person(name: "Sam")
> 
> which is equivalent; you're just making it less convenient and harder to optimize. Piecewise mutation of value types is a feature, not a bug, and doesn't have most of the pitfalls of shared mutable reference types.
> 
> -Joe
> 
>> On Jan 30, 2016, at 7:27 AM, Maximilian Hünenberger via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Hi all,
>> 
>> If you have an Array which is declared as mutable variable all contents are implicitly mutable. This is unfortunate especially in case of value types.
>> 
>> Consider this code example:
>> 
>>     struct Person { var name: String }
>> 
>>     var array = [Person(name: "Smith")]
>>     // all persons are implicitly mutable
>>     array[0].name = "Sam"
>> 
>> 
>> So I propose a language feature which addresses this issue:
>> 
>>     var array: [let Person] = [Person(name: "Smith")]
>>     // all persons are immutable
>>     array[0].name = "Sam" // error
>> 
>>     // but still allowing to add and remove persons
>>     array[0] = Person(name: "Sam")
>> 
>> 
>> For clarification: The semantics are the same as if you've wrapped the struct in a class with a "let" property:
>> 
>>     class ConstantWrapper<T> {
>>         let value: T
>>         init(_ value: T) { self.value = value }
>>     }
>> 
>>     var array = [ConstantWrapper(Person(name: "Smith"))]
>>     // all persons are "indirect" immutable
>>     array[0].value.name = "Sam" // error
>> 
>> 
>> This model would allow for more immutability in mutable contexts which ultimately leads to less bugs.
>> 
>> 
>> ##Possible Extensions:
>> 
>> We could also allow a "var" declaration:
>> 
>>     let array: [var Person] = ...
>> 
>> The array models a fixed length array which is highly suggested by some people but you cannot assign a new "Person" to a specific index which is unfortunate. Although this could be solved by tweaking the current model.
>> 
>> 
>> Best regards
>> - Maximilian
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> 
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