On Sat, Sep 9, 2017 at 07:51 Brent Royal-Gordon <br...@architechies.com> wrote:
> On Sep 8, 2017, at 6:03 PM, Xiaodi Wu via swift-evolution < > swift-evolution@swift.org> wrote: > > For any open protocol (i.e., a protocol for which the universe of possible > conforming types cannot be enumerated a priori by the protocol designer) > worthy of being a protocol by the Swift standard ("what useful thing can > you do with such a protocol that you could not without?"), any sufficiently > interesting requirement (i.e., one for which user ergonomics would > measurably benefit from a default implementation) either cannot have a > universally guaranteed correct implementation or has an implementation > which is also going to be the most performant one (which can therefore be a > non-overridable protocol extension method rather than an overridable > protocol requirement with a default implementation). > > > Counter-example: `index(of:)`, or rather, the underscored requirement > underlying `index(of:)`. The "loop over all indices and return the first > whose element matches" default implementation is universally guaranteed to > be correct, but a collection like `Set` or `SortedArray` can provide an > implementation which is more performant than the default. > Don't get me started on Swift's handling of equality and arrays with NaN. _customIndexOfEquatable, if I'm not mistaken, is a part of that whole tangle of performance optimizations which gleefully refuse to acknowledge Equatable's semantic peephole for some values of a type being unordered with respect to everything else. In a world where this trade-off between performance and correctness had not been taken, I don't imagine that it would be possible to make the protocol extension method 'index(of:)' any more performant than 'index(where: { $0 == $1 })'.
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