[swift-dev] C-style For Loops
dabrahams at apple.com
Fri Feb 19 20:52:06 CST 2016
Sorry to have left this so long; I'm just catching up.
on Sat Dec 19 2015, Brent Royal-Gordon <brent-AT-architechies.com> wrote:
>> I personally liked the original free-function syntax—"stride(from:
> -50, to: 50, by: 9)". When we got protocol extensions, we decided
> we'd prefer methods everywhere except in a few special cases, and this
> clearly falls outside those criteria.
> I don't know about that—in `stride(from: -50, to: 50, by: 9)`, I don't
> see any particular reason to privilege the `from` over the `to`.
Me neither! I'm just saying, the criteria we used to decide which
things could remain non-methods would not have allowed us to keep stride
this way. We would need to have agreement on some principle that would
allow this form.
> I mean, I suppose `from` always starts the seqeunce whereas `to`
> doesn't always appear in it, but they still feel like peers which
> equally define the operation as a whole.
Yes, but consider a.appendingContentsOf(b) or (sorry for bringing this
up) x.union(y). I, at least, don't think those should be free functions.
I think the reason stride feels different is that it's weird for a
method on a scalar value to produce something with higher
> This has definitely always bothered me about Swift 2.
> Incidentally, I just thought of another issue with using negative
> values with normal intervals to walk backwards: What does
> `(10..<20).by(-2)` mean? A naive implementation would probably give
> you this:
> [20, 18, 16, 14, 12, 10]
> It *should* mean this, which would not be useful terribly often:
> [18, 16, 14, 12, 10]
Great point, to say nothing of (10..<19).by(-2)! (does it start at 18 or 19)?
I think it *should* mean something useful... but I don't know why
you think one is more useful than the other.
Certainly, (10..<19).by(2).reversed() is unambiguous. I think it would
have a nice symmetry if you could say that:
r.by(-x).reversed() == r.by(x)
r.by(x).reversed() == r.by(-x)
Would those be useful semantics in your eyes?
> Meanwhile, you really want to be able to get this, but the types and
> operators for an open-start interval don't currently exist:
> [20, 18, 16, 14, 12]
I don't understand why “you really want to be able to get this” from (10..<20).by(-2).
Wouldn't (12...20).by(-2) be a reasonable way to get there?
> I wonder if intervals might need another look, and maybe a redesign to
> make them more general.
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