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

Karl Wagner razielim at gmail.com
Mon May 1 22:52:59 CDT 2017

> 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 <mailto:razielim at gmail.com>> wrote:
> > On 2 May 2017, at 04:44, Xiaodi Wu <xiaodi.wu at gmail.com <mailto: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.

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

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

- Karl

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