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

[Xiaodi Wu]

filter(extractingWhere:)
On Wed, Jun 22, 2016 at 18:53 Dave Abrahams via swift-evolution <
swift-evolution at swift.org> wrote:

>
> 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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> >>
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> >
>
> --
> Dave
>
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