[swift-evolution] Allowing mutable members in structs

[Radosław Pietruszewski]

> Would it make sense to allow mutable properties in structs, which could be mutated even by non-mutating methods?

This would break the whole mutability model of structs. The whole idea is: structs are always immutable, and you can mark things as “mutable”, but this can only happen if a struct is assigned to a mutable variable.

— Radek

> On 11 Mar 2016, at 11:38, Dan Raviv via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Would it make sense to allow mutable properties in structs, which could be mutated even by non-mutating methods?
> 
> I've noticed this could be useful when implementing a non-mutating protocol method. The method semantically doesn't change the state of the protocol's implementor, and therefore shouldn't be declared as mutating. However, *some* implementations might need to change some internal state for implementing the functionality. Making specific struct properties mutable for such a case could make sense.
> 
> Alternatively, Swift could allow implementing a non-mutating protocol method as a mutating method in the implementor. Currently, this doesn't seem to be  allowed; Swift doesn't recognize the mutating method as a match for the non-mutating protocol method.
> 
> Cheers,
> Dan
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