[swift-evolution] Proposal: Python's indexing and slicing

[Dave Abrahams]

> On Dec 19, 2015, at 8:27 PM, Dennis Lysenko <dennis.s.lysenko at gmail.com> wrote:
> 
> Dave, perhaps we could use "^" as an anchor point for the start index and $ as the anchor point for the end index? It's familiar to anyone who knows a bit of regex, and all vim users. My main worry would be ^ is already infix xor operator.

We could.  

Downsides:
• it's a needless additional sigil 
• it requires a language extension to express x[^ + 3] unless we fake it with prefix operators ^- and ^+.


> 
> On Fri, Dec 18, 2015 at 5:43 PM Paul Ossenbruggen via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> I would like to avoid what you currently have to do for iterating a subcontainer. 
> 
> for a in container[0..container.count-4] {
> 	// do something. 
> }
> 
> The slicing syntax would certainly help in these common situations. Maybe there are easy ways that I am not aware of. 
> 
> - Paul
> 
>> On Dec 18, 2015, at 2:39 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> 
>>> On Dec 18, 2015, at 1:46 PM, Joe Groff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> 
>>>> On Dec 18, 2015, at 4:42 AM, Amir Michail via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> Examples:
>>>> 
>>>> >>> l=[1,2,3,4,5]
>>>> >>> l[-1]
>>>> 5
>>>> >>> l[-2]
>>>> 4
>>>> >>> l[2:4]
>>>> [3, 4]
>>>> >>> l[2:]
>>>> [3, 4, 5]
>>>> >>> l[-2:]
>>>> [4, 5]
>>>> >>> l[:3]
>>>> [1, 2, 3]
>>>> >>> l[::2]
>>>> [1, 3, 5]
>>>> >>> l[::]
>>>> [1, 2, 3, 4, 5]
>>> 
>>> Accepting negative indices is problematic for two reasons: it imposes runtime overhead in the index operation to check the sign of the index; also, it masks fencepost errors, since if you do foo[m-n] and n is accidentally greater than m, you'll quietly load the wrong element instead of trapping. I'd prefer something like D's `$-n` syntax for explicitly annotating end-relative indexes.
>> 
>> Yes, we already have facilities to do most of what Python can do here, but one major problem IMO is that the “language” of slicing is so non-uniform: we have [a..<b], dropFirst, dropLast, prefix, and suffix.  Introducing “$” for this purpose could make it all hang together and also eliminate the “why does it have to be so hard to look at the 2nd character of a string?!” problem.  That is, use the identifier “$” (yes, that’s an identifier in Swift) to denote the beginning-or-end of a collection.  Thus,
>> 
>>   c[c.startIndex.advancedBy(3)] =>	c[$+3]        // Python: c[3]
>>   c[c.endIndex.advancedBy(-3)] =>	c[$-3]        // Python: c[-3]
>>   c.dropFirst(3)  =>			c[$+3...]     // Python: c[3:]
>>   c.dropLast(3) =>			c[..<$-3]     // Python: c[:-3]
>>   c.prefix(3) =>			c[..<$+3]     // Python: c[:3]
>>   c.suffix(3) => 			c[$-3...]     // Python: c[-3:]
>>    
>> It even has the nice connotation that, “this might be a little more expen$ive than plain indexing” (which it might, for non-random-access collections).  I think the syntax is still a bit heavy, not least because of “..<“ and “...”, but the direction has potential. 
>> 
>>  I haven’t had the time to really experiment with a design like this; the community might be able to help by prototyping and using some alternatives.  You can do all of this outside the standard library with extensions.
>> 
>> -Dave
>> 
>> 
>> 
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-Dave



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