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

Vladimir.S svabox at gmail.com
Fri Apr 22 12:24:42 CDT 2016


On 22.04.2016 19:46, Dave Abrahams via swift-evolution 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.

I believe the question was regarding the syntax of such construction:
collection[label: Range<Index>]

i.e. the proposal(as I understand) is to have such subsctiptions in Swift 3 :

array[safe: 3] // returns nil if index out of bounds
array[truncate: -1...6] // returns copy/slice of array with indexes 
"truncaed" to correct values i.e. 0...min(array.count-1,6)


>
>> 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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