[swift-evolution] [Accepted with modifications] SE-0045: Add scan, prefix(while:), drop(while:), and unfold to the stdlib

[Dave Abrahams]

on Fri May 06 2016, Kevin Ballard <swift-evolution at swift.org> wrote:

> On Fri, May 6, 2016, at 07:33 PM, Dave Abrahams via swift-evolution wrote:
>> 
>> on Fri May 06 2016, Kevin Ballard <swift-evolution at swift.org> wrote:
>> 
>> > On Fri, May 6, 2016, at 06:05 PM, Dave Abrahams via swift-evolution wrote:
>> >> 
>
>> >> on Fri May 06 2016, Kevin Ballard <swift-evolution at swift.org> wrote:
>> >> 
>> >> > On Fri, May 6, 2016, at 05:31 PM, Kevin Ballard wrote:
>> >> >> On Fri, May 6, 2016, at 05:19 PM, Dave Abrahams via swift-evolution wrote:
>> >
>> >> >> > 
>> >> >> > on Wed May 04 2016, Chris Lattner <swift-evolution at swift.org> wrote:
>> >> >> > 
>> >> >> > > Proposal link: https://github.com/apple/swift-evolution/blob/master/proposals/0045-scan-takewhile-dropwhile.md
>> >> >
>> >> >> > >
>> >> >> > > Hello Swift Community,
>> >> >> > >
>> >> >> > > The review of SE-0045: "Add scan, prefix(while:), drop(while:), and
>> >> >> > > unfold to the stdlib" ran from April 28...May 3, 2016. A subset of the
>> >> >> > > proposal is *accepted, with modifications* for Swift 3.  This proposal
>> >> >> > > included four new additions to the standard library:
>> >> >> > >
>> >> >> > > Sequence.prefix(while:) & Sequence.drop(while:) - These are *accepted* as specified in revision 3 of the proposal.
>> >> >> > 
>> >> >> > I know the review is over and all, but…
>> >> >> > 
>> >> >> > Chris brought to my attention an idea that I liked, and asked me to post
>> >> >> > here about it.  Specifically, the argument label “where:” would probably
>> >> >> > be better than “while:”.  “While” makes it sound to me like it's going
>> >> >> > to take a nullary predicate and be much more active.  But it's hard to
>> >> >> > love
>> >> >> > 
>> >> >> >      s.drop(where: { $0 > 1 })
>> >> >> >      s.drop { $0 > 1 }
>> >> >> > 
>> >> >> > because it makes it sound like a general filtering operation.
>> >> >> > 
>> >> >> > Therefore, I'd much rather see
>> >> >> > 
>> >> >> >        s.droppingPrefix(where: {$0 > 1})
>> >> >> 
>> >> >> Using "where:" has a very high potential for confusion, because
>> >> >> "where" makes it sound like it runs the predicate against every
>> >> >> single element, whereas "while" makes it clear that it stops
>> >> >> evaluating elements once the predicate returns false. Or in other
>> >> >> words, `drop(where: predicate)` looks like it should be equivalent
>> >> >> to `filter({ !predicate($0) })`.
>> >> >
>> >> > Oops, I read your message too fast and missed the fact that you
>> >> > changed the function name too in the very last line.
>> >> 
>> >> And made the exact point that you made :-)
>> >
>> > Indeed. My apologies, I was trying to get through my email too fast and ended up skimming too much.
>> >
>> >> > If we switched to `droppingPrefix(where:)` then we'd need to change `dropFirst(_:)` as well.
>> >> 
>> >> Yes, as I noted in my response to Erica, I would like to see some more design
>> >> coherence in this whole area.  I think
>> >> 
>> >>           xs.dropFirst()  =>  xs.droppingFirst()
>> >>           xs.dropFirst(3) => xs.droppingPrefix(ofMaxLength: 3)
>> >> 
>> >> I'm not wedded to “dropping,” especially because it has other
>> >> connotations.  Dmitri, Max, and I kicked around a bunch of other
>> >> possibilities, e.g.
>> >> 
>> >>           sansPrefix
>> >>           withoutPrefix
>> >
>> > If we do rename this, I'm partial to basing it off of the word "skip",
>> > e.g. skippingPrefix(), since that's similar to "drop" but doesn't have
>> > the connotation of possibly mutating the receiver. That said, having a
>> > first argument label "ofMaxLength:" seems overly verbose. Don't forget
>> > that not everybody has the benefit of Xcode code completion when
>> > writing Swift code.
>> 
>> Me, for example.  It needs to be at least “maxLength,” IMO, for
>> clarity.  “of” clarifies that you're not talking about the maximum
>> length of the result... though I suppose it could be argued that that
>> misinterpretation wouldn't make any sense.
>
> How about "count"? `skippingFirst(count:)` is not too long and it's reasonably descriptive.
>
>> >> Anyway, we can't do all of that by the seat of our pants.  I personally
>> >> wouldn't want to go any further than s.dropPrefix(where: ...)  without a
>> >> new proposal/review.
>> >
>> > Agreed, anything beyond that needs a full review. So back to the
>> > suggestion at hand.
>> >
>> > I wrote up a whole long explanation for why I still think "while" is
>> > the better choice, and then I thought about how this impacts
>> > `prefix(...:)` instead of `drop(...:)`, and in this context "where:"
>> > makes more sense. `prefix(while:)` turns "prefix" from a noun into a
>> > verb, 
>> 
>> Sorta, yeah.
>> 
>> > and the verb meaning of "prefix" is not at all what we're doing with
>> > this operation. But `prefix(where:)` is still a noun and makes
>> > sense. And of course if we have `prefix(where:)` then it's a bit odd
>> > to have `drop(while:)`, so we'd have to go with `drop(where:)` or
>> > `dropPrefix(where:)`. So I guess I've now convinced myself while
>> > writing this email that "where:" is the right choice :D
>> 
>> :D
>> 
>> 
>> > That said, given that we already have `dropFirst(_:)`, maybe we should
>> > go with `dropFirst(where:)` instead of `dropPrefix(where:)`? Otherwise
>> > it's a bit odd to have both "dropFirst" and "dropPrefix" as
>> > names. 
>> 
>> Agreed, that might make a more reasonable interim stopping point.
>> 
>> > Admittedly, this same argument applies to the existing `drop(while:)`
>> > name too, and I don't have a great explanation for that beyond the
>> > fact that I had a vague idea that dropFirst(_:) might be renamed to
>> > drop(first:), though that can't actually happen because having both
>> > dropFirst() and drop(first:) is hugely confusing, and it doesn't make
>> > sense to rename dropFirst() to drop().
>> 
>> Ya just lost me in those twisty turns, sorry.
>
> What I meant was that `drop(while:)` doesn't match the existing
> `dropFirst(_:)` name either (just as `dropPrefix` wouldn't), and my
> reasons for proposing `drop(while:)` instead of `dropFirst(while:)`
> were not well-founded.

Okay, well I'm afraid I've lost the thread a bit here.  IIRC, we made
some good progress toward a consensus in this discussion, though.  It
would be awesome if you could pull it together into a proposal or at
least a summary of the current status.  I think it's still reasonable to
imagine cleaning this area up for Swift 3.

-- 
Dave