[swift-evolution] SE-0105: Removing Where Clauses from For-In Loops

Xiaodi Wu xiaodi.wu at gmail.com
Fri Jun 24 13:58:01 CDT 2016

On Fri, Jun 24, 2016 at 1:38 PM, Tony Allevato <allevato at google.com> wrote:

> On Fri, Jun 24, 2016 at 11:30 AM Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
>> On Fri, Jun 24, 2016 at 6:37 AM, William Shipley <wjs at mac.com> wrote:
>>> On Jun 23, 2016, at 11:04 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>> Not a practitioner of 80-character line limits, I take it?
>>> I don’t understand why anyone wouldn’t just let Xcode do the wrapping
>>> for most cases. I’ll add newlines if I think it adds to clarity, but in
>>> general I don’t want to code like i’m still on a Wyse WY-50.
>> Of course, to each their own style--I certainly wouldn't want Swift to
>> force everyone to write lines of certain lengths. But 80-character lines is
>> a common style, and I would say that a corollary of "to each their own" is
>> that Swift's grammar should be usable and useful whether or not you adhere
>> to such style choices.
>> If the chief advantage of `where` is that it (quoting someone above)
>> allows one to "understand as much as possible about the control flow of
>> the loop from a single line of code," then we ought perhaps to question its
>> appropriateness when the majority of its benefits [by which I mean, based
>> on your examples and Sean's, more than half of the instances in which it is
>> used] cannot be realized in a very common coding style.
> IMO, we shouldn't be talking about "a single line of code" so much as "a
> single cohesive construct". What's important to me isn't the line length or
> whether the "where" part has to wrap onto another line, it's the number of
> potentially divergent constructs I have to parse mentally in order to
> determine the meaning of the loop.
> With "for-in-where", it's three:
>     for item(1) in someCollection(2) where someCondition(3)
> With the alternatives suggested using if/guard/continue/break, it's more
> than that:
>     for item(1) in someCollection(2) {
>         [if | guard...else](3) someCondition(4) {
>             [continue | break](5)
>         }
>     }
> I have to be more careful now interpreting the meaning of this loop
> because both or either of if/guard or continue/break can invert its
> meaning, where was "for-in-where" is concise and clear.

And if `while` is added along with `where`, all this goes out the window.

> Doing .lazy.filter{...} is slightly cleaner than this, but still suffers
> from the disadvantage that it's very easy for newcomers to leave out the
> "lazy" part and accidentally write poorly performing code, and to the
> argument that "where" is considered confusing by some users, I would argue
> that it's easier to explain how it works than why "lazy" is necessary in
> this context.

Clearly, eagerly filtering results in slower code, but based on the
examples provided above I don't imagine any of them would turn into "poorly
performing" code with eager filtering. In fact, I see some that could run a
little faster with some refactoring along these lines. In any case,
optimizing performance comes after correctness; a user who can't grasp
`lazy` can nonetheless still write useful, correct code.


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