[swift-evolution] [Proposal] Safer half-open range operator

Luis Henrique B. Sousa lshsousa at gmail.com
Sun Apr 24 04:38:11 CDT 2016


Many thanks, Vladimir, I wasn't familiar with this yet. :-)

- Luis

On Sun, Apr 24, 2016 at 7:10 AM, Vladimir.S via swift-evolution <
swift-evolution at swift.org> wrote:

> 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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