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

Erica Sadun erica at ericasadun.com
Thu Aug 17 20:06:11 CDT 2017


> 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 <mailto: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.

-- E

>> On Aug 17, 2017, at 3:53 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 
>> This is, I would argue, much too limiting in the way of semantics and not at all required by “map”. It’s unclear to me how _any_ result with reference semantics or any function with side effects could be used in a way that comports with that definition.
>> 
>> On the other hand, just as y = 0x is a function, { _ in Foo() } is a closure that very much does project from a domain to a range. I’m not sure I understand what wins are to be had by having “collect {}” as a synonym for “map { _ in }”.
>> 
>> On Thu, Aug 17, 2017 at 16:01 Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> On Aug 17, 2017, at 12:04 PM, Max Moiseev <moiseev at apple.com <mailto:moiseev at apple.com>> wrote:
>>> 
>>> 
>>>> On Aug 17, 2017, at 10:05 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> Also, for those of you here who haven't heard my previous rant on the subject, I dislike using map for generating values that don't depend on transforming a domain to a range. (It has been argued that `_ in` is mapping from `Void`, but I still dislike it immensely)
>>> 
>>> Can you please elaborate why (or maybe point me at the rant)? 
>> 
>> 
>> Summary:
>> 
>> . Since this application is a generator and not a transformative function, `map` is a misfit to usage semantics. It breaks the contract that map means to project from a domain to a range via a function. More languages conventionally use `collect` than `map` to collect n applications of a generator closure
>> 
>> -- E
>> 
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