[swift-evolution] [Draft] Rename Sequence.elementsEqual

[Xiaodi Wu]

Yup.

On Wed, Oct 18, 2017 at 07:28 Ole Begemann <ole at oleb.net> wrote:

> On Tue, Oct 17, 2017, at 20:46, Xiaodi Wu via swift-evolution wrote:
>
> On Tue, Oct 17, 2017 at 12:54 Jonathan Hull <jhull at gbis.com> wrote:
>
> Why was elementsEqual created?  Isn’t it meant to check equality of two
> sequences in a generic context where == isn’t available?
>
>
> No no no no no no no no. That’s precisely why the name is misleading.
> elementsEqual is *not* simply a mixed-type version of ==. Remember that ==
> as implemented on a concrete sequence type *has no obligation* to use
> elementwise comparison using the element’s implementation of ==. This is
> not merely theoretical: [Float].== does *not* do an elementwise comparison
> using Float.==. By contrast, you are guaranteed a true elementwise
> comparison with elementsEqual regardless of how equivalence is defined for
> the sequence.
>
>
> Really? I would have thought an elementwise comparison is exactly what
> [Float].== would do.
>
> I tested this and these are the (very surprising, at least to me) results:
>
> let numbers: [Float] = [1, .nan]
> let copy = numbers
> let sameNumbers: [Float] = [1, .nan]
>
> numbers == copy                     // (1) true (!)
> numbers == sameNumbers              // (2) false (!)
> numbers.elementsEqual(copy)         // (3) false (ok)
> numbers.elementsEqual(sameNumbers)  // (4) false (ok)
>
> So [Float].== returns a different result depending on whether you compare
> two arrays that share the same buffer (1) or two arrays with the same
> contents (2) when the array contains NaN. The reason is that Array.==
> immediately returns true if two arrays share the same buffer
> <https://github.com/apple/swift/blob/master/stdlib/public/core/Arrays.swift.gyb#L2194-L2197>
> .
>
> Even knowing that comparing floats is problematic in the context of NaN
> (and in general), I find this very surprising. It also seems to clash with Michael's
> idea
> <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20171016/040544.html>
> that two substitutable sequences should return true for ==.
>
>
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