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

[Xiaodi Wu]

The performance of stdlib methods is not fixed in stone.

And a language feature being undocumented wouldn't explain why the entire
stdlib uses it only three times :)
On Tue, Jun 14, 2016 at 1:42 AM Charlie Monroe via swift-evolution <
swift-evolution at swift.org> wrote:

> I used to do low latency java for trading systems... the kind of coding
> where we would go out of our way to avoid ANY intraday gc activity (yes it
> can be done, even for parsing xml files). So we cared about a lot of
> things... But when you look at the numbers above on a 4_000_000 iterations
> loop and say the differential matters? I say you are probably using the
> wrong tools to write your code in the first place, and you should be using
> accelerate.
>
>
> Yes, I say it does matter. Of course, 4 million iterations is a fictional
> example that rarely occurrs on its own, but gives you an idea that there's
> more going on behind the scenes other than pure iteration.
>
> You're looking at it from a point where one app does one thing and the
> loop will have just a few iterations. Try to look at it from a point where
> the entire OS, all the processes and kernel  are written in Swift (might be
> distant future, but Swift is heading that way, isn't it?).
>
> In such case if each for-loop takes a few extra instruction, then - again
> - yes, it matters. I know we're not talking about all for-loops, just those
> filtering the sequence - which seems not that common of a case - but my
> point is valid, that Swift should provide easily-reachable means to be a
> "good citizen". Removing it will lead developers to use .filter(_:)
> instead, in order to save lines of code and additional typing.
>
> While many searches in open source code found a minimum usage of
> for-in-where, I think this is not due to it being confusing, but just not
> well known, otherwise it would be used a lot more. Most developers that
> have prior programming experience will only skim through the Language Guide
> itself, which doesn't mention that `where` can be used in for loops (!!!),
> or even while loops and there isn't a single example where it would be used.
>
> Both
>
> https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/ControlFlow.html
> and the ePub download on
> https://swift.org/documentation/#the-swift-programming-language
>
> Maybe I'm missing something, but the language guide only mentions `where`
> for the switch-case scenario. So the argument (which floated around here)
> that this is not being actively used is kind of moot since how could anyone
> be using it since it's not properly documented and no one who doesn't
> closely watch release notes can possibly know about this.
>
>
>
> As for the '
>
>
> I've previously noted that if/guard-continue come in really close
> speed-wise, which makes them candidates for a fix-it in case `where` is
> indeed removed.
>
> My response here was solely to Jean-Daniel's note that he mustn't forget
> to include the lazy accessor, pointing out that even the lazy accessor is
> slower than using an inline check.
>
>
>
> --
>
> Brent Royal-Gordon
>
> Architechies
>
>
>
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