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

Xiaodi Wu xiaodi.wu at gmail.com
Thu Jun 2 10:56:42 CDT 2016

Both good points. I could live with your solution.

On Thu, Jun 2, 2016 at 10:48 AM Matthew Johnson <matthew at anandabits.com>

> 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>
> wrote:
>> Sent from my iPad
>> On Jun 2, 2016, at 12:27 AM, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> On Thu, Jun 2, 2016 at 12:24 AM, Patrick Smith <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> 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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