Good point.
Pierre
> Le 20 déc. 2016 à 11:32, Jeremy Pereira <jeremy.j.pere...@googlemail.com> a
> écrit :
>
>
>> On 20 Dec 2016, at 07:54, Pierre Monod-Broca via swift-evolution
>> <swift-evolution@swift.org> wrote:
>>
>> But for a struct to be immutable, you don't need all its properties to be
>> let. (I guess that's Derrick's point)
>
> Yes you do. Consider
>
> struct Person: Hashable
> {
> let firstName: String
> let lastName: String
> let hashValue: Int
>
> init(firstName: String, lastName: String)
> {
> self.firstName = firstName
> self.lastName = lastName
> self.hashValue = firstName.hashValue ^ lastName.hashValue
> }
> }
>
> func == (l: Person, r: Person) -> Bool
> {
> return l.firstName == r.firstName && l.lastName == r.lastName
> }
>
> Pretend that the hash value is quite expensive to calculate so I only want to
> do it once. With the above code, this is fine but if I change the lets to
> vars (e.g. var firstName: String), I am open to
>
> let chris = Person(firstName: “Chris”, lastName: “Lattner”) // Immutable
>
> var andy = chris
> andy.firstName = “Andy”
>
> andy.hashValue // Gives the wrong answer unless you are exceptionally lucky.
>
> I’ve used hashValue, but the same argument would apply to any computed
> property where you might want to cache the computed value. As soon as you
> make any of the properties that the computed property depends on `var`, you
> have to add code that invalidates the cached value which is a performance and
> a complexity hit for your struct.
>
>
>
>>
>> ´´´
>> struct Person { /* . . . var properties . . . */ }
>> let john = Person() // john is completely immutable
>> ´´´
>>
>> If a struct has var properties, you can do a mutable copy, change it, assign
>> it to a new let, and you take advantage of immutability again.
>>
>> ´´´
>> let jim = { var copy = john
>> copy.firstname = "Jim"
>> return copy
>> }()
>>
>> func with<T>(_ original: T, transform: (inout T) -> ()) -> T {
>> var copy = original
>> transform(©)
>> return copy
>> }
>>
>> let jane = with(john) { $0.firstname = "Jane" }
>> // jane is also immutable
>> ´´´
>> (I didn't check this compiles)
>>
>> Pierre
>>
>>> Le 19 déc. 2016 à 23:08, Andy Chou via swift-evolution
>>> <swift-evolution@swift.org> a écrit :
>>>
>>> Value semantics help reduce the issues around mutability, but they don't go
>>> away completely. I would like to create structs that are completely
>>> immutable after construction. Turning the properties into vars
>>> unfortunately loses this idea.
>>>
>>> The proposed 'with' function doesn't construct new instances, which means
>>> the let constants are already set. Nick's solution works, as it's basically
>>> a copy constructor that allows for changes while the new object is
>>> constructed. But it needs to be created for each struct. Which I'm fine
>>> with :)
>>>
>>> Andy
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>
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