[swift-evolution] [Pitch] Retiring `where` from for-in loops

[Xiaodi Wu]

On Tue, Jun 14, 2016 at 12:16 PM, David Waite <david at alkaline-solutions.com>
wrote:

> I’m a bit late to this conversation, and I don’t totally understand the
> goal.
>
> There are a *lot* of things you can do in for…in loop with pattern
> matching that also would supposedly go against this interpretation of
> approachability. Pattern matching in general might be considered to go
> against this interpretation.
>
> Is this pitch saying statements such as:
>
> for i in 1..<100 where i%2 == 1 {…}
>
> should be disallowed, while statements like
>
> for case let view? in views { … }
>
> are still approachable enough to warrant being supported in the language?
>

Language design has to weigh many factors simultaneously, I think you'd
agree. The argument, essentially, is that `where` is not approachable *for
the functionality that it provides* (namely, as an alternative for a
trivial `guard...continue` statement). Pattern matching is daunting no
doubt, but it offers functionality not conducive to much simpler syntax.
(Or could it be much simpler? If so, then I would support a proposal to
that effect.)

Put simply, `where` is a less-than-straightforward expression of a very
straightforward concept (filtering an array), whereas pattern matching is
an advanced concept with a commensurately difficult syntax. Others have
brought up generics, for example, but again that's an advanced *concept*;
filtering an array is not.


>
> FWIW, I wouldn’t support removing where based on current arguments without
> either the keyword “where" being eliminated completely from the language,
> and/or adding equivalent intuitive functionality to Sequence with
> same-class performance, e.g. a .where(...) equivalent to .lazy.filter(…).
>
>
I feel bad sending clearly passionate people over to crush another
conversation, but I think you'll find in the Swift repository the
beginnings of some explorations by a certain member of the core team to
rename `.filter()` to `.where()` :D

As to whether certain methods should be lazy or eager by default, that's a
discussion certainly appropriate for this list.


> I’ve known about and used the feature since it was first added to Swift
> (learned via the language book), and don’t fully understand the confusion
> that some developers may have - especially since ‘while’ is already a
> keyword and could have been used if that was the actual semantics.
>

One source of confusion was that `while...where` was supported and had
breaking semantics. Now that's gone with SE-0099. Still, the point is that
`where` is favored by some *because* you don't have to write explicitly
what happens when something doesn't pass the filter, whereas the
counterpoint argument is that not writing explicitly what happens when a
rejected element is encountered *is* the very source of confusion.


>
> -DW
>
> On Jun 14, 2016, at 10:32 AM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> And from the WWDC Platforms SOTU: "Swift is super simple and
> approachable.... It's great as a first language. And in fact, we think this
> is so important that when we designed Swift this was an explicit design
> goal."
>
> I would be absolutely against adding any more sugar to the for loop. In
> that sense, `where` sets a terrible example that certain features of
> sequences deserve contextual sugar. (And before someone points it out
> again, I've already argued why `for...in` holds its own weight, namely
> difficulty of writing a correct `while` replacement and progressive
> disclosure to the learner so that the concept of iterators can be learned
> afterwards.)
>
> In short, I would very much be opposed to adding keywords "for fun."
>
>
>
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