[swift-evolution] [Pitch] Add the DefaultConstructible protocol to the standard library

Xiaodi Wu xiaodi.wu at gmail.com
Mon Dec 26 15:46:11 CST 2016

This sounds like what UnsafeMutableRawBufferPointer was designed for, no?

On Mon, Dec 26, 2016 at 4:39 PM, Tony Allevato via swift-evolution <
swift-evolution at swift.org> wrote:

> On Mon, Dec 26, 2016 at 1:19 PM David Sweeris <davesweeris at mac.com> wrote:
>> On Dec 26, 2016, at 12:10, Tony Allevato <tony.allevato at gmail.com> wrote:
>> On Mon, Dec 26, 2016 at 11:57 AM David Sweeris via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> On Dec 26, 2016, at 11:35, Tony Allevato <allevato at google.com> wrote:
>> Mathematically, identities are associated with (type, operation) pairs,
>> not types alone.
>> This conversation has put me in the column of "numeric types shouldn't
>> have default initializers at all", personally.
>> I'd agree, except sometimes you need a T, *any* T, for when you want to
>> create a "pre-sized" array for stuffing results into by index:
>> for i in ... {
>>     a[i] = ...
>> }
>> Simply saying "var a =[T](); a.reserveCapacity()" doesn't cut it because
>> it'll still crash if you try to store anything in a[i] without somehow
>> putting at least i+1 elements in the array first.
>> Array already has init(repeating:count:) that puts the responsibility of
>> choosing the default value at the call site. If someone were writing a
>> generic algorithm around this, then why not just propagate that
>> responsibility out to its call site as well? That way, the algorithm isn't
>> making any assumptions about what the "default" value is or even if one
>> exists, and it doesn't impose additional requirements on the element type.
>> For example, the user could get the default from a static factory method,
>> an instance method on another object, or something else entirely.
>> Yeah, that's what I would use… The "filled out" example would be:
>> extension Array {
>>     public func pmap<T: DefaultInitable> (transform: (Element) -> T) -> [
>> T] {
>>         var result = Array<T>(repeating: T(), count: self.count) //Pick
>> a T... any T...
>>         for i in self.indices {
>>             result[i] = whateverTheConcurrentExectutionSyntaxIs(self[i],
>> transform)
>>         }
>>         return result
>>     }
>> }
>> var thisCouldTakeAWhile = Array((0...10000)).pmap {
>>     someReallySlowFunction($0)
>> }
>> At least I *think* that’d work... I haven’t tried it yet... Anyway,
>> without some way (*any* way) of getting an instance of T to fill in the
>> `result` array, it becomes much trickier to keep track of all the
>> concurrently-calculated transformed values. In this case, the semantics of
>> `T()` are fairly irrelevant because the semantics of the *overall
>> statement* is just to work around a language limitation (Swift not
>> having separate allocation and initialization phases), which doesn’t have
>> anything to do with the semantics of the initial value that got passed as
>> the `repeating` argument.
> This looks like it's abusing T() to stand in for nil, though. Before the
> result[i] assignment executes, result[i] shouldn't conceptually have a
> value—you're putting a default in there because the implementation requires
> it. T() as a placeholder is just as valid/invalid as any other value in T's
> space.
> It's a square-peg-in-a-round-hole problem—design-wise, an array of
> optionals would be a better fit, but you rightly don't want to return that
> from the function, and you don't want to bear the cost of converting the
> array of optionals to an array of non-optionals after the computation is
> complete. That makes complete sense, but perhaps *that* is the problem that
> should be addressed, instead of trying to use T() to avoid it? Or
> alternatively, a parallel map operation should return an array of futures
> (or a collection type that itself is a future).
>> - Dave Sweeris
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