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

[Kevin Ballard]

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.

> 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, 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

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. 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().

-Kevin Ballard