[swift-evolution] Constant generic properties in mutable contexts

[Joe Groff]

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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