[swift-evolution] [Pitch] Improve `init(repeating:count)`

[Xiaodi Wu]

On Thu, Aug 17, 2017 at 8:06 PM, Erica Sadun <erica at ericasadun.com> wrote:

>
> On Aug 17, 2017, at 6:56 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> On Thu, Aug 17, 2017 at 7:51 PM, Erica Sadun <erica at ericasadun.com> wrote:
>
>> What people are doing is taking a real set of values (1, 2, 3, 4, 5, for
>> example), then discarding them via `_ in`, which is different from `Void ->
>> T` or `f(x) = 0 * x`. The domain could just as easily be (Foo(), "b", 💩,
>>  UIColor.red, { x: Int in x^x }). There are too many semantic shifts away
>> from "I would like to collect the execution of this closure n times" for it
>> to sit comfortably.
>>
>
> What arguments might help to alleviate this discomfort? Clearly, functions
> exist that can map this delightfully heterogeneous domain to some sort of
> range that the user wants. Would you feel better if we wrote instead the
> following?
>
> ```
> repeatElement((), count: 5).map { UIView() }
> ```
>
>
> My favorite solution is the array initializer. Something along the lines
> of `Array<T>(count n: Int, generator: () -> T)`. I'm not sure it _quite_
> reaches standard library but I think it is a solid way to say "produce a
> collection with a generator run n times". It's a common  task. I was asking
> around about this, and found that a lot of us who work with both macOS and
> iOS and want to stress test interfaces do this very often. Other use cases
> include "give me n random numbers", "give me n records from this database",
> etc. along similar lines.
>
> The difference between this and the current `Array(repeating:count:)`
> initializer is switching the arguments and using a trailing closure  (or an
> autoclosure) rather than a set value. That API was designed without the
> possibility that you might want to repeat a generator, so there's a bit of
> linguistic turbulence.
>

...but, again, why would such an API be superior to `map`, which is already
widely used and clear? My point above was that, if your problem with using
`(0..<5).map { _ in UIView() }` is the discarding of a set of values (which
I argue is semantically unproblematic in the first place, since it's still
mapping from a domain to a range), then `repeatElement((), count: 5).map {
UIView() }` produces the same result without violating your stipulated
semantics.
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