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

[Dave Abrahams]

on Mon Apr 11 2016, Haravikk <swift-evolution at swift.org> 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? 

That's exactly the question.  We've considered doing it both ways.

> 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> 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:
>
>     let a = [1,2,3]
>     let b = 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