[swift-users] [swift-evolution] How does "Sequence.joined" work?

Taylor Swift kelvin13ma at gmail.com
Tue Aug 8 23:50:32 CDT 2017

On Wed, Aug 9, 2017 at 12:29 AM, Félix Cloutier via swift-evolution <
swift-evolution at swift.org> wrote:

> Yes, exactly. An Array<T> is a struct wrapper for a reference type
> representing storage. Mutating functions first check if they own the only
> reference to the storage using isKnownUniquelyReferenced
> <https://developer.apple.com/documentation/swift/2429905-isknownuniquelyreferenced>.
> If not, they make a fresh copy before applying the mutating operation.
> There's no difference for `let` arrays. Access control is enforced at
> compile-time through Array's design: the compiler will prevent you from
> calling `mutating` functions on `let` structs, and Array is careful to not
> expose functionality that could modify its storage outside of `mutating`
> functions.
> There is no secret. Anyone could implement the same thing only using
> publicly available and documented compiler features. In fact, it's been
> done already for some very powerful collections
> <https://github.com/lorentey/BTree>.

This isn’t entirely true. That BTree module readme seems to contain a lot
of unsubstantiated hyperbole. It’s possible to implement a classic
red-black tree in Swift that performs better than a sorted Array, down to
about *n* = 1,500 items, not *n* = *100,000* items as it claims. (Actually,
heap allocators these days are good enough that performance is on par with
Array all the way down to *n* = 1.) Red-Black trees are slow when
*distributed* as packages because of the crossmodule optimization boundary.
(This also means the BTree module is much slower than Array for most
reasonable *n*.) It’s possible to write modules using compiler attributes
that mitigate this slowdown (reclaiming over 50% of lost performance) but
it’s hacky and forces you to design your libraries like the standard
library (meaning: ugly underscored properties everywhere and everything is
public). And these features aren’t “publicly available” or documented at
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