[swift-evolution] [Pitch] New collection-based 'repeat' API

[Xiaodi Wu]

On Mon, May 1, 2017 at 10:52 PM, Karl Wagner <razielim at gmail.com> wrote:

>
> On 2 May 2017, at 05:29, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> On Mon, May 1, 2017 at 9:52 PM, Karl Wagner <razielim at gmail.com> wrote:
>
>>
>> > On 2 May 2017, at 04:44, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>> >
>> > Does this gain something by being part of the standard library as
>> opposed to being built on top of it?
>>
>> Well, somebody thought repeatElement<T> was general enough to make part
>> of the standard library. If we’re going to allow repeating a single item as
>> a Collection, we might as well allow generalise it to repeating any
>> Collection in a loop (including a CollectionOfOne, which is the existing
>> use-case).
>>
>
> That doesn't answer the question, though: does the feature you
> propose--repeating any instance of Collection in a loop--gain anything by
> being a part of the standard library rather than an extension to it?
>
> There are _many_ useful algorithms that can be implemented on top of the
> standard library and can be of general use; nonetheless, they aren't a part
> of the standard library. IIUC, it's not because people just haven't had the
> time to flesh it out; rather, it is a deliberate choice to have a small,
> narrowly focused standard library. The philosophy, as I understand it, is
> to make it convenient for users to roll their own conveniences rather than
> providing all the bits and bobs in the library itself.
>
> One of the points of differentiation between standard library and
> Foundation is said to be whether something is necessary to support a core
> language feature, in which case it goes into the standard library. As a
> consequence, there are less commonly used parts of the standard library
> which are there because they support other (decidedly not esoteric) parts
> of the standard library and also happen to have some plausible public uses.
> Taking a quick look into the repository, for instance, `repeatElement` is
> used in the implementation of `UnicodeScalar`. However, just because
> someone determined that `repeatElement` is worth making a public API (since
> it's going to be in the standard library whether or not it's public),
> doesn't _automatically_ mean that a generalization of it should be included
> in the library as well.
>
>
> This seems contradictory. Either the standard library is small and
> deliberately focussed; or it isn’t and it’s okay for random "bits and bobs”
> such as repeatElement to be public just because the standard library
> arbitrarily uses them somewhere.
>

I don't see how. The standard library _is_ small--and it's focused, though
not perhaps on what you think it should be. It's not that only the most
commonly sought-after features go into the standard library; it's that only
such features as are necessary to support the core language go into the
standard library. `repeatElement` isn't randomly part of the library; it's
used to implement strings.

Either “repeat” functionality (in general) is useful and the generalised
> behaviour should be in the standard library, or none of it should be.
>

That doesn't follow at all. It's like saying, either types (in general) are
useful and higher-kinded types should be supported by Swift, or Swift
should be untyped.


> Personally, I usually want to repeat a collection of things far more often
>> than I want to repeat an individual thing. It annoys me that the standard
>> library only provides the one I almost never need.
>>
>
> There are many facilities that the standard library does not provide.
> Heck, we don't even have left padding for strings! (JavaScript reference,
> for those following along.) Is there something about this particular
> feature that makes its not being a part of the standard library uniquely
> problematic?
>
>
> Additionally, it could help remove the top-level “repeatElement” function,
>> which I know irritates some people who would rather us not have any
>> top-level functions in the standard library.
>>
>
> With source stability as a goal, the bar for removal isn't "irritates some
> people." I actively use this function and there's no justification at all
> for breaking it.
>
> Frankly, I cannot see removal of top-level functions simply because they
> are top-level to be in scope essentially ever. So let's subset that out of
> this discussion.
>
>
> Again, this seems contradictory given your comments just a few lines up
> about how spartan the stdlib is supposed to be. Why do you insist that this
> single-purpose function remain,
>

*I'm* not inventing the source stability criteria here. It's been made very
clear that the bar for removing something existing the language now is very
high and not at all based on whether it would be included in the first
place if it were proposed today. There have been several criteria laid out
for such source-breaking changes, and "irritates some people" doesn't meet
that bar. In insisting on preserving source stability, I'm only insisting
that we respect the process laid out for Swift evolution, because that's
the only way this thing can work. Again, I don't come up with the criteria,
but we shouldn't need to litigate the ground rules every time an idea is
discussed.


> and the more broadly-useful, generalised functionality cannot possibly be
> a fit for the standard library?
>

I think, if you read back, I have never insisted that your proposed
functionality is not a fit. I simply asked you a question: does this
generalized feature fit the existing, stated focus that the standard
library doesn't vend every useful function, but only those necessary for
supporting the core language? It appears that _you're_ saying it doesn't,
but that you also don't agree with those criteria. Again, I don't come up
with the criteria. I'm just bringing up that they exist.


> It seems that the reverse is more likely to be true.
>
> As I said, I personally don’t care about what happens to the top-level
> function. This is only a pitch, so I wanted to hear from those people who
> really do want to remove them all before writing up a full proposal.
>
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