[swift-evolution] [Proposal] Safer half-open range operator
Vladimir.S
svabox at gmail.com
Sun Apr 24 01:10:00 CDT 2016
We already have this feature(to append labels for substiption), so I
believe it is possible to implement this proposal:
class A {
subscript(safe range: Range<Int>) -> [Int] {
get { return [1,2,3] } set { print(newValue) }
}
subscript(truncate range: Range<Int>) -> [Int] {
get { return [1,2,3] } set { print(newValue) }
}
}
var a = A()
var arr = a[safe: 0...10]
print(arr)
arr = a[truncate: 0...10]
print(arr)
On 23.04.2016 12:25, Luis Henrique B. Sousa via swift-evolution wrote:
> No, I got the half-joke on the python-like example. :-)
>
> I meant the label as part of the brackets content, right before the range
> itself. E.g. [truncate: Range<Index>]
> where "truncate" is the label I'm referring to.
>
> Thanks
>
> - Luis
>
> On Friday, April 22, 2016, Dave Abrahams via swift-evolution
> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>
>
> on Fri Apr 22 2016, "Luis Henrique B. Sousa via swift-evolution"
> <swift-evolution at swift.org> wrote:
>
> > is this syntax reasonably simple to implement?
>
> If you mean a syntax that allows 0..<-2, it's implementable but I'd be
> opposed to it. You'd have to introduce a new overload of ..< that
> produced something other than a Range or CountableRange, because those
> have a precondition that the LHS is <= the RHS.
>
> > Or is there another solution that would work with less impact in terms
> > of design? I mean the subscript with a label on it,
> > i.e. collection[label: Range<Index>]
>
> I'm sure there are lots of other possibilities :-)
>
> >
> > It's been a while since the last feedback, so I'm doing some rewriting
> > on this proposal and still considering to submit it for review.
> >
> > - Luis
> >
> > On Wed, Apr 13, 2016 at 10:29 PM, Dave Abrahams via swift-evolution
> > <swift-evolution at swift.org> wrote:
> >
> > on Wed Apr 13 2016, Maximilian Hünenberger
> > <swift-evolution at swift.org> wrote:
> >
> > > Should this new operator form a new range? How can this range
> know about
> > the
> > > array's indices?
> > >
> > > A while ago there was a proposal (unfortunately it was not
> discussed
> > enough)
> > > which introduced safe array indexing:
> > >
> > > array[safe: 3] // returns nil if index out of bounds
> >
> > Wrong label, but I wouldn't be opposed to adding such an operator for
> > all Collections.
> >
> > > So another way to handle this issue would be to make another
> subscript
> > like:
> > >
> > > array[truncate: -1...6]
> >
> > That approach makes sense too. But then do we add
> >
> > x[python: 0..<-2] // all but the last two elements?
> >
> > ;^)
> >
> > > Best regards
> > > - Maximilian
> > >
> > > Am 12.04.2016 um 01:21 schrieb Luis Henrique B. Sousa via
> swift-evolution
> > > <swift-evolution at swift.org>:
> > >
> > > The idea of having a new operator following the principles of
> overflow
> > > operators looks great. Two distinct operators doing implicit and
> > explicitly
> > > might really be a good way to go; it would be concise and
> wouldn't look
> > like
> > > some magic happened behind the scenes. I'd like to hear more
> opinions
> > about
> > > it.
> > >
> > > > what we'll have in case a[-1 &..< 5]? should this raise error
> or become
> > [0
> > > ..< 3] ? I think, the latter.
> > > I agree here, I'd choose the latter.
> > >
> > > From my perspective, the behaviour I'm proposing is what a
> considerable
> > > number of users expect, especially if coming from other
> languages that
> > > follow that path. Of course I'm not comparing languages here, but
> > > considering the Swift principles of being a safer language, in
> my opinion
> > > we'd rather have a partial slice than a crash in execution time
> (when the
> > > user is not totally aware of it).
> > >
> > > Many thanks for all your additions so far. It's really good to
> see that
> > > these things are not set in stone yet.
> > >
> > > - Luis
> > >
> > > On Apr 11, 2016 4:21 PM, "Vladimir.S via swift-evolution"
> > > <swift-evolution at swift.org> wrote:
> > >
> > > +1 for the idea "in general". But I also think that explicit is
> better
> > than
> > > implicit, especially if we deal with possible errors. Just like
> we work
> > > in Swift with integer overflow : '+' will generate run time
> error, but
> > > saying &+ we point Swift that we know what we do.
> > >
> > > but.. what we'll have in case a[-1 &..< 5]? should this raise
> error or
> > > become [0 ..< 3] ? I think, the latter.
> > >
> > > On 11.04.2016 17:02, Haravikk via swift-evolution wrote:
> > >
> > > I like the idea in theory, but the question is; is it really
> safer to
> > > return a result that the developer may not have wanted, versus an
> > > error
> > > indicating that a mistake may have been made? I wonder if perhaps
> > > there
> > > could be an alternative, such as a variation of the operator like
> > > so:
> > >
> > > let b = a [0 &..< 5]// Equivalent to let b = a[0 ..< min(5,
> > > a.endIndex)],
> > > becomes let b = a[0 ..< 3]
> > >
> > > I’m just not sure that we can assume that an array index out of
> > > range error
> > > is okay without some kind of indication from the developer, as
> > > otherwise we
> > > could end up returning a partial slice, which could end up causing
> > > an error
> > > elsewhere where the size of the slice is assumed to be 5 but isn’t.
> > >
> > > On 11 Apr 2016, at 13:23, Luis Henrique B. Sousa via
> > > swift-evolution
> > > <swift-evolution at swift.org
> > > <mailto:swift-evolution at swift.org>>
> > > wrote:
> > >
> > > This proposal seeks to provide a safer ..< (aka half-open range
> > > operator)
> > > in order to avoid **Array index out of range** errors in
> > > execution time.
> > >
> > > Here is my first draft for this proposal:
> > >
> >
> https://github.com/luish/swift-evolution/blob/half-open-range-operator/proposals/nnnn-safer-half-open-range-operator.md
> >
> > >
> > > In short, doing that in Swift causes a runtime error:
> > >
> > > leta =[1,2,3]
> > > letb =a[0..<5]
> > > print(b)
> > >
> > > > Error running code:
> > > > fatal error: Array index out of range
> > >
> > > The proposed solution is to slice the array returning all
> > > elements that
> > > are below the half-open operator, even though the number of
> > > elements is
> > > lesser than the ending of the half-open operator. So the example
> > > above
> > > would return [1,2,3].
> > > We can see this very behaviour in other languages, such as
> > > Python and
> > > Ruby as shown in the proposal draft.
> > >
> > > This would eliminate the need for verifications on the array
> > > size before
> > > slicing it -- and consequently runtime errors in cases when the
> > > programmer didn't.
> > >
> > > Viewing that it is my very first proposal, any feedback will be
> > > helpful.
> > >
> > > Thanks!
> > >
> > > Luis Henrique Borges
> > > @luishborges
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> > --
> > Dave
> >
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> --
> Dave
>
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>
>
> --
>
>
> ---
>
> *Luís Henrique Borges*
> iOS Developer at IBM <http://ibm.com>
> Dublin, Ireland - luish.github.com <http://luish.github.com>
>
>
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