[swift-evolution] Stdlib closure argument labels and parameter names

[Dave Abrahams]

on Wed Jun 22 2016, Xiaodi Wu <swift-evolution at swift.org> wrote:

> I'll duly oblige with some pushback on `suchThat`. I get that you're trying
> to clarify whether filter retains or gets rid of elements that match the
> predicate, but I don't think "filter such that" expresses this idea at all.
>
> Comparing to "filter where," "filter such that" is equally susceptible to
> misinterpretation that you are filtering to remove elements that are
> matched. For example: "find me some apples, filtering such that are
> bruised."

Hahaha, that's a very different interpretation of “such” that I hadn't
considered!  OK, suppose it was “soEach:” ?

    let primes = xs.filter(soEach: isPrime)

> I'd suggest that if you want to be perfectly clear, you'd need something
> like `filter(keepingWhere:)`.

    let primes = xs.filter(keepingWhere: isPrime)

A slight problem is that filter is nonmutating, so all elements are
“kept.”  But maybe that's just Dave being overly concerned with unlikely
misinterpretations at the cost of “naturalness.”

Further thoughts?

> On Wed, Jun 22, 2016 at 18:33 Dave Abrahams via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>>
>> on Tue Jun 21 2016, Dave Abrahams <swift-evolution at swift.org> wrote:
>>
>> > on Mon Jun 20 2016, Brent Royal-Gordon <swift-evolution at swift.org>
>> wrote:
>> >
>> >>> A couple of weeks ago we started to notice that we had some
>> poorly-named
>> >>> closure parameters and argument labels in the standard library, so we
>> >>> did a complete audit of the standard library's APIs and came up with a
>> >>> preliminary proposal for changes, which we applied in a branch and you
>> >>> can review in https://github.com/apple/swift/pull/2981.  Let's please
>> >>> carry on further discussion here rather than in the pull request,
>> though.
>> >>
>> >> In general, I like this; `orderingBy` is a particularly nice
>> >> improvement over the old `isOrderedBefore` convention.
>> >
>> > I don't really love the use of “by”, FWIW, but I thought `orderingWith`
>> > was more confusable (ordering A with B might swap A and B, whereas the
>> > parameter is a closure).  It could be argued, though, that I am being
>> > overly concerned with unlikely misinterpretations, at the cost of
>> > “naturalness”—a known weakness of mine ;-).  Anyway, as ever I'm open to
>> > discussion on this.
>> >
>> >> A few specific comments about things I don't like:
>> >>
>> >> * In `map` and `flatMap`, I'm not sure how much `transform` buys us
>> >>   over `elementTransform`.
>> >
>> > I think you mean the converse.  And I agree that `elementTransform`
>> > is probably not an improvement over `transform`.
>>
>> ...and I've gone back to `transform` in my PR.
>>
>> >> * In general, I'm not a fan of most of the changes away from `where`
>> >> labels.
>> >
>> > The only such changes I can find are in
>> >
>> https://github.com/apple/swift/pull/2981/commits/3418eede88d724ad23731fe8f412f51e03cf5106
>> >
>> > Note that part of this change was to make all filter closures
>> > consistent; in the main `filter` API there was no label at all.
>> > However, we felt that there's a real clarity problem with the polarity
>> > of the argument (we talk about “filtering things out” but the closure
>> > indicates which elements to keep).  And we couldn't find a “where”-based
>> > name that began to clarify it.
>> >
>> > I will argue that even changing to “suchThat,” as in the PR, does not
>> > sufficiently clarify the closure's polarity, and the only true fix for
>> > filter is to use a different base name (some have suggested “select,”
>> > and I have other ideas), but that is out of scope for this particular
>> > set of changes.  So if the community is happier with a “where” label
>> > here I can live with it.  I do think “suchThat” is marginally clearer.
>>
>> I have not received any further pushback on “suchThat,” so I've left it
>> alone.
>>
>> >
>> >> Those are a nice, straightforward convention applied broadly across
>> >> the Sequence APIs. (Yes, I criticized `where` as a method name in
>> >> another thread, but I don't think `where` is a problem when there's a
>> >> function base name to give it context.) When they don't work, that's
>> >> usually because of a less-than-ideal base name. I'm not saying that
>> >> *all* base names that aren't compatible with `where` should be
>> >> changed, but rather that if `where` is not enough, that's an API
>> >> smell.
>> >>
>> >> * In particular, `elementWhere` is not a good label for the same
>> >> reason that `removeElement` is not a good name. Session 403 last week
>> >> actually talked about this between roughly minutes 8 and 11. (I'm sure
>> >> you know about its content; you probably saw it before we did.)
>> >
>> > Yes I do, and I think you misinterpreted the message in that session.
>> > There's nothing wrong with repeating type information when it's
>> > necessary for clarity or fluency at the use-site.  In the case of
>> > `contains(elementWhere:)`, it's there for fluency:
>> >
>> >        customers.contains(where: isSingle)
>> >
>> > doesn't read as well as:
>> >
>> >        customers.contains(elementWhere: isSingle)
>> >
>> > The point is not to imagine that every argument should be preceded by
>> > a noun, and repetition of type information is often the result of
>> > trying to do that.
>> >
>> >> * I like `separatedWhere` on `split`, but I think the Equatable
>> >> version needs a similar renaming.
>> >
>> > That's a nice thought; I think it's arguably out-of-scope here, though.
>> >
>> >> Perhaps `separatedBy`?  `separatedOn`? The usual opposite of `where`,
>> >> `of`, doesn't work here. (Alternatively, `separatedWhere` could be
>> >> `separatorWhere` instead, but that's not quite as elegant.)
>> >
>> > I'd want to consider variations of `separatingAt` or `onSeparator` or
>> > `atSeparator` too... which makes me thing “separatedWhere” might not be
>> > as good as “separatingWhere” for the closure version.
>> >
>> >> * I'm very uncomfortable with the amount of weight
>> >> `accumulatingResultBy` adds to `reduce`. `combinedBy` seems perfectly
>> >> cromulent to me. I'm even more concerned by your suggestion in the
>> >> pull request body of
>> >> `accumulating(startingFrom:combiningBy:)`. `reduce` is a subtle and
>> >> slightly confusing operation; adding more words to its call sites will
>> >> not solve that problem. If you want to invent a new name from whole
>> >> cloth, I would probably use something like `combining(with
>> >> initialResult: T, by nextResult: (T, Element) -> T)`. (For that
>> >> matter, while we're working in this area, `sequence(first:next:)`
>> >> could use a similar coat of paint.)
>> >
>> > As with `filter(suchThat:`, `reduce(accumulatingResultBy:` is attempting
>> > to solve with an argument label what IMO is a grave weakness in clarity
>> > of the base name.  If you read the documentation for `reduce`, you'll
>> > see that it's all about accumulating a result, and if you consider that
>> > its current signature often leads to O(N^2) behavior and we are thinking
>> > about adding an overload that takes its “accumulator” inout, the
>> > arguments for avoiding the name “accumulate” get progressively weaker.
>> > But as noted earlier, changing base names is out-of-scope for this
>> > proposal.  As with “filter,” I could live with leaving this alone,
>> > though I do believe “accumulatingResultBy:” is a real improvement in
>> > clarity.
>>
>> ...but I think it's overly specific at the expense of smoothness.  So
>> I've removed `Result` from that name.
>>
>> >> * I agree with the comment on GitHub that `invoke` should be
>> >> `execute`.
>> >
>> > Why?  Rationales help.
>> >
>> >> If you see a distinction between the two cases on the number of
>> >> arguments, I would then suggest `passTo` as the label on these
>> >> methods: `views.forEach(passTo: addSubview)`,
>> >> `withUnsafeBufferPointer(&bytes, passTo: Data.init(buffer:))`.
>> >
>> > Those are intriguing ideas, but that direction tends to suggest this
>> > would be better:
>> >
>> >   views.passEach(to: addSubview)
>> >   passUnsafeBufferPointer(to: Data.init(buffer:))
>> >
>> > ...until you pass a trailing closure:
>> >
>> >   views.passEach { addSubView($0) }
>> >   passUnsafeBufferPointer { Data.init(buffer:$0) }
>> >
>> > (note: withUnsafeBufferPointer takes only one argument, a closure).
>> >
>> >>
>> >> * It's a little odd that you're using `comparingBy` for `Equatable`
>> >> and `orderingBy` for `Comparable`. Did you judge `equatingBy` to be
>> >> too awkward?
>> >
>> > Yes, and because it's not “equating,” which would mean using equality
>> > (==) it's “testing equivalence” with respect to the predicate.
>> >
>> >> Perhaps the real problem is that `Equatable` ought to be `Comparable`
>> >> and `Comparable` ought to be `Orderable`?
>> >
>> > I don't think so, personally, but regardless I consider such a change
>> > out-of-scope for this proposal.
>> >
>> >> Or maybe `comparingBy` should just be something more general, like
>> >> `matchingBy`? That would make perfectly sensible but slightly odd use
>> >> cases like this one read better:
>> >>
>> >>      let isAnIdiot = luggageCombination.starts(with: [1, 2, 3, 4,
>> >> 5], matchingBy: <=) // Matches [1,2,3,4,5], but also [1,1,1,1,1],
>> >> [1,2,3,2,1], etc.
>> >
>> > That would not be legal, as <= is not an equivalence relation.  You
>> > could think about redefining the meaning of `starts(with:` to not
>> > require an equivalence relation, but that's something I'm not confident
>> > *I* know how to do meaningfully, and regardless is again out-of-scope.
>> >
>> >> Very soon (hopefully), I will be posting an early draft of a proposal
>> >> renaming the various first/last/prefix/suffix/etc. APIs. I believe the
>> >> only place it touches on your proposal is in
>> >> `starts(with:isEquivalent:)`, but I think your changes to the second
>> >> parameter label can be easily incorporated into what I'm doing.
>> >
>> > Great!
>>
>> I'm going to write up the proposal ASAP based on the current PR unless I
>> get more feedback.
>>
>> Thanks,
>>
>> --
>> Dave
>>
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-- 
Dave