On Wed, Oct 25, 2017 at 12:06 PM, David Zarzycki <d...@znu.io> wrote:
> > > On Oct 25, 2017, at 12:01, David Sweeris <daveswee...@mac.com> wrote: > > > On Oct 25, 2017, at 5:29 AM, David Zarzycki via swift-dev < > swift-dev@swift.org> wrote: > > > > On Oct 25, 2017, at 02:34, Xiaodi Wu via swift-dev <swift-dev@swift.org> > wrote: > > Please see earlier replies summarizing core team members' persuasive > arguments as to why not multiple protocol hierarchies like this. > > > Hi Xiaodi, > > The above line is does it help your argument. Even if a person was > motivated to search through the entire mailing list archives looking for > said arguments, they won’t be able to guess which arguments you found to be > persuasive (and who was sufficiently “core” at the time, no less). Can you > please provide URLs into the archives? Is that a big ask? > > > If I'm (now) reading this correctly, he put the argument itself at the end > of his earlier reply to you: > > > Right, to which I replied that I wasn’t proposing the Rust model or your > similar “MaybeEquatable” model. > The other Dave was proposing such a thing, and that was my reply to him. You're right that you are proposing something different. > I was proposing something different where Float and Int are both Equatable > and Substitutable, but neither Equatable nor Substitutable inherit from the > other. The former is mathematical and the latter is not (which allows it to > deal with NaN payloads, ±0, etc). Generic algorithms care mostly if not > completely about mathematics, while generic containers care mostly if not > completely about substitutability. They can live alongside each other and > get along peacefully/sanely. And if people need to care about both, then at > least they have an out. > The issue with this is similar to that in my reply earlier about bitwise comparison of floating-point values. Yes, you can propose some total ordering over floating-point values totally divorced from `==`, but I'm quite certain that people who invoke `sort()` on an array of floating-point values don't simply want *some* deterministic order, but rather an actual increasing order of numeric values. Likewise, when someone asks if an array contains a floating-point value (say, `10.0 as Decimal`), they generally want to know if *any* representation of that value exists. The point is that _what kind of substitutability_ matters, and the kind that people will expect for floating-point values is the very mathematical substitutability that is supposed to be guaranteed by Equatable, which simply does not accommodate NaN.
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