[swift-evolution] Constant generic properties in mutable contexts

[Maximilian Hünenberger]

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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