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

Xiaodi Wu xiaodi.wu at gmail.com
Thu Aug 17 19:56:47 CDT 2017


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() }
```

-- E
>
>
> On Aug 17, 2017, at 3:53 PM, Xiaodi Wu <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> wrote:
>
>>
>> On Aug 17, 2017, at 12:04 PM, Max Moiseev <moiseev at apple.com> wrote:
>>
>>
>> On Aug 17, 2017, at 10:05 AM, Erica Sadun via swift-evolution <
>> 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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>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>
>
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