[swift-evolution] MemoryLayout for a value

[Xiaodi Wu]

On Thu, Aug 4, 2016 at 2:29 AM, Karl <razielim at gmail.com> wrote:

>
> On 4 Aug 2016, at 06:27, Dave Abrahams via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> on Wed Aug 03 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com
> <http://xiaodi.wu-at-gmail.com/>> wrote:
>
> Why not just MemoryLayout.init(of instance: T), and drop the autoclosure
> magic altogether?
>
>
> My proposal *does* drop the autoclosure magic.  `type(of: x)` is already in
> the standard library (replacing `x.dynamicType`). The reasons not to have:
>
>   MemoryLayout(of: x)
>
> where x is an arbitrary instance, is that it reads and pronounces the
> same as
>
>   MemoryLayout<X>
>
> but has different meaning, and even a different type (which results in
> additional API complexity—the forwarding vars I showed in the [Aside]
> box from my previous post).  Imagine explaining the difference between
> these two in that world:
>
>   MemoryLayout<Int>
>   MemoryLayout(of: Int.self)
>
> The first is a type representing the layout of Int.  The second is an
> instance of that type representing the layout of Int's metatype.
>
>
> It’s confusing because metatypes in Swift are pretty confusing in general:
> Int.self returns Int.Type which is not the same as `type(of: <some Int>)`
> (that would be Int).
>
> If a novice wants to jump in, they’ll have to know that MemoryLayout(of:
> Int.self) would return a MemoryLayout<Int.Type>.
>

Yes, here, I agree Dave is absolutely right. You and Dave have convinced me
that neither `MemoryLayout(of: x)` nor `MemoryLayout.of(x)`, where x is an
instance, would be appropriate.

> The classic sizeofValue evaluated its argument, and in Foundation several
> uses of it actually relied on that side effect. While autoclosures are
> quite clever, in general I think the user expectation is that given
> `a(b(c))` both a and b are invoked, side effects and all.
>
> Note that both type(of:) as it's currently implemented and the old
> dynamicType evaluate its argument/receiver.
>
>
> I didn't realize that, but it's fine.  I'm attached to the use of
> `type(of:)` in this idiom, not to having an autoclosure involved.
>
> No one afaik has ever thought that behavior to be anomalous (though I
> bet we're about to hear some arguments to that effect now).
>
> On Wed, Aug 3, 2016 at 15:46 Dave Abrahams via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> Having seen the effects in the standard library and in other
> code, I'm concerned that we may have made a mistake in removing
> `sizeofValue` et al without providing a replacement.  In the standard
> library, we ended up adding an underscored API that allows
>
>  MemoryLayout._ofInstance(someExpression).size
>
> Where someExpression is an autoclosure, and thus not evaluated.  I
> wanted to bring up the possibility of introducing a replacement as a
> bufix.
>
> I propose that the way to express the above should be:
>
>  MemoryLayout.of(type(of: someExpression)).size
>
> implementable as:
>
>  extension MemoryLayout {
>    @_transparent
>    public
>    static func of(_: T.Type) -> MemoryLayout<T>.Type {
>      return MemoryLayout<T>.self
>    }
>  }
>
> I think this API would solve the concerns I had about confusability that
> led me to advocate dropping the ability to ask for the size of a value.
> The only way to use it is to pass a type and these two expressions have
> equivalent meaning:
>
>    MemoryLayout<Int>
>    MemoryLayout.of(Int.self)
>
> It also has the benefit of isolating the autoclosure magic to type(of:).
>
> ,----[ Aside ]
> | A slightly cleaner use site is possible with a larger API change:
> |
> |   MemoryLayout(type(of: someExpression)).size
> |
> | Which would involve changing MemoryLayout from an `enum` to
> | a `struct` and adding the following:
> |
> |   extension MemoryLayout {
> |     public init(_: T.Type) {}
> |
> |     public var size: Int { return MemoryLayout.size }
> |     public var stride: Int { return MemoryLayout.stride }
> |     public var alignment: Int { return MemoryLayout.alignment }
> |   }
> |
> | However I am concerned that dropping ".of" at the use site is worth the
> | added API complexity.
> `----
>
> Thoughts?
> --
> -Dave
>
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>
> --
> -Dave
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