[swift-evolution] Immutable Structures

[Kevin Lundberg]

I believe Ian is correct in that changing any struct's value in any way creates a whole new struct.
See http://swiftstub.com/635749345 for proof (the didSet property observer fires every time a change happens to the struct).

> On Dec 23, 2015, at 11:00 AM, Lino Rosa via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Reassigning structures will create new ones. Mutating functions and reassigning its properties won't: they mutate it.
> On Wed, Dec 23, 2015 at 10:51 AM Ian Ynda-Hummel <ianynda at gmail.com <mailto:ianynda at gmail.com>> wrote:
> I thought structures were immutable and mutating functions actually just created new structures, thus maintaining the by value semantics.
> On Wed, Dec 23, 2015 at 10:45 AM Lino Rosa via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> I believe the language would be improved by making structures immutable.
> 
> 1) The choice between classes and structures isn’t clear right now. If structures were immutable it would be natural to use them as value objects.
> 
> 2) Refactoring a mutable structure into a class when it’s being passed around multiple threads removes the by-value semantics seamlessly. The resulting mutable class isn’t thread-safe.
> 
> 2.1) Even when passed around a single thread, the resulting class would be passed by reference, so any mutations would have unintended consequences.
> 
> 3) We could probably remove some syntax: `mutating` keyword and variable parameters. Also the `var` keyword before a structure could be used to denote reassignment (not mutability), just as it does with classes.
> 
> Of corse I might not be seeing the whole picture, so please weigh in.
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