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

[Dave Abrahams]

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