[swift-evolution] [Pitch] Renaming sizeof, sizeofValue, strideof, strideofValue

[Matthew Johnson]

> On Jun 2, 2016, at 10:38 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> Well, as I understand it, it's not actually possible to write your own type(of:), so we're going from a "magic" property to a "magic" function at least for now.

No, but you *can* write `func foo<T>(_ t: T)` which accepts any value (you *cannot* write a property that is available for all properties - that would require the ability to write `extension Any`.  This is the distinction I am making.  Of course the implementation is compiler magic no matter how we express it syntactically.  But we can make it *appear* just like it might if the implementation *wasn’t* compiler magic.  That makes it fit into the language better IMO and was the biggest motivator for changing `dynamicType`.

> 
> I'm most alarmed that one implication of the MemoryLayout proposal is loss of the `ofValue` family of functions. These functions don't fit with the design: imagine, what is `MemoryLayout<Double>.size(ofValue: Float(42))`? But the response seems to be that these functions don't seem necessary at all and should be removed. "I don't see a use for it" is an insufficient justification for a feature removal. Looking to other languages, C# has sizeof as a static property but tellingly offers the equivalent of sizeofValue (well, strideofValue) as a function in a different module. Essentially every other C-family language that exposes pointers to the user offers both of and ofValue equivalents. The question is, how does a user with existing code using sizeofValue() migrate to Swift 3? I do not see a viable answer with the MemoryLayout design.

Going with MemoryLayout *does not* mean we would have to give up the value functions if we don’t want to:

struct MemoryLayout<T> {
    init() {}
    init(t: T) { /* throw away the value */ }
    
    // we could omit the static properties and require 
    // writing MemoryLayout<Int>() if we don’t like the duplication
    static let size: Int
    static let spacing: Int
    static let alignment: Int

    let size: Int
    let spacing: Int
    let alignment: Int
}

let size = MemoryLayout<Int>.size
let sizeOfValue = MemoryLayout(42).size

> 
> On Thu, Jun 2, 2016 at 8:03 AM Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
> 
> 
> Sent from my iPad
> 
> On Jun 2, 2016, at 12:27 AM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> On Thu, Jun 2, 2016 at 12:24 AM, Patrick Smith <pgwsmith at gmail.com <mailto:pgwsmith at gmail.com>> wrote:
>> I really like this idea. This IMO is lower level functionality than `type(of:)` (née dynamicType), so I think it makes sense for it to be grouped under its own domain, the MemoryLayout type.
>> 
>> Plus MemoryLayout can be extended with new convenience methods.
>> 
>> I’m fine with those old methods being removed, but I never use them so! Is it the same as calling type(of:) then using that with MemoryLayout? I imagine they could be fixit’d easily, and that they compile down to the same underlying code.
>> 
>> I'm actually souring to the idea. It goes in the diametrically opposite direction from dynamicType. There, something was changed from being property-like to being function-like. Here, Dave's proposal would take something that's a function and turn it into a property. Hmm.
> 
> That's not a fair comparison though.  With dynamicType we removed a "magic" property visible on all types, which isn't something you can write and turned it into a function (which is obviously something you can write).  
> 
> Dave's MemoryLayout creates a new type to bundle together related items which makes their semantic relationship more clear.  It also receives the type via a generic argument rather than a function argument and makes the properties static.  That is more representative of what is actually happening and could help to prevent confusion.  
> 
> If we really need an 'ofValue' option that infers T from a value the properties on MemoryLayout could also be made available as instance properties and it could have an initializer that accepts an instance to T and throws the value away.  However, I'm not at all convinced this is necessary.
> 
>>> On 2 Jun 2016, at 3:05 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> 2. Dave A. and others expressed the opinion that these should probably not be global functions; his preference was for:
>>> 
>>> ```
>>> MemoryLayout<T>.size // currently sizeof()
>>> MemoryLayout<T>.spacing // currently strideof()
>>> MemoryLayout<T>.alignment // currently alignof()
>>> ```
>>> 
>>> 3. Dave A. proposed that sizeofValue(), strideofValue(), and alignofValue() are better off removed altogether. I don't know if people are going to be happy about this idea.
>> 
>> 
> 
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