> On 20 Dec 2016, at 13:10, Matthew Johnson <[email protected]> wrote:
>
>
>
> Sent from my iPad
>
>> On Dec 20, 2016, at 4:32 AM, Jeremy Pereira via swift-evolution
>> <[email protected]> wrote:
>>
>>
>>> On 20 Dec 2016, at 07:54, Pierre Monod-Broca via swift-evolution
>>> <[email protected]> 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.
>
> The performance hit is likely a bit larger if you *don't* use a mutable
> property and instead create a whole new instance.
How is
let a = SomeStruct()
var b = a
not creating a new instance?
Anyway, the cost depends on how expensive the calculation for the calculated
property is and how often you use it and how well the compiler can optimise
copies of immutable objects.
On the other hand, making a property that is not supposed to change over the
lifetime of the object a let property is self documenting and not to be avoided
IMO.
>
> It might be interesting to think about language solutions to reduce this
> complexity. But in general, the mutability model of Swift's value types is
> an asset and should be embraced, not avoided. That's what a "Swifty"
> solution would do IMO.
Yeah, I really hate it when people say “x is Swifty” or “y is not Swifty”. What
is Swifty or not usually depends on what the person saying it prefers. On the
other hand, most programmers i have come across agree that writing code that is
self documenting is good practice and therefore using let instead of var for
properties that never change over the life time of the object counts in that
respect in my opinion.
>>
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