On Dec 26, 2016, at 12:10, Tony Allevato <[email protected]
<mailto:[email protected]>> wrote:
> On Mon, Dec 26, 2016 at 11:57 AM David Sweeris via swift-evolution
> <[email protected] <mailto:[email protected]>> wrote:
>
> On Dec 26, 2016, at 11:35, Tony Allevato <[email protected]
> <mailto:[email protected]>> 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.
- Dave Sweeris_______________________________________________
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