[swift-evolution] [Accepted] SE-0172: One-sided Ranges

[Haravikk]

> On 25 Apr 2017, at 17:47, Douglas Gregor <dgregor at apple.com> wrote:
> 
>> 
>> On Apr 25, 2017, at 9:20 AM, Tony Allevato <tony.allevato at gmail.com <mailto:tony.allevato at gmail.com>> wrote:
>> 
>> 
>> On Tue, Apr 25, 2017 at 8:55 AM Haravikk via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> While it's good that this was accepted I still feel there could be some more discussion about whether the operators should be prefix/postfix or instead involve a more explicit declaration of one-sidedness.
>> 
>> I still would very much prefer that the operator declarations were binary and take Void as a second argument, this way there is very explicit indication that one-sidedness was requested, rather than potentially accidental; assuming many subscripts and methods that take currently closed ranges will be updated to also take one-sided ranges, the distinction is very important as a omitting one of the values could represent a mistake, and result in one-sided behaviour with unintended consequences.
>> 
>> With a Void "open" argument this would look like:
>> 
>> func ... <T:Comparable>(lhs:T, rhs:Void) -> RangeFrom { ... }
>> func ... <T:Comparable>(lhs:Void, rhs:T) -> RangeTo { ... }
>> 
>> let rangeFrom = 5 ... ()
>> let rangeTo = () ... 5
>> 
>> Not the prettiest with the Void brackets, however, if we could in future get underscore as another alias for Void we could refine this into:
>> 
>> Why would it make sense to have `_` as an alias for Void?
>>  
>> 
>> let rangeFrom = 5 ... _
>> let rangeTo = _ ... 5
>> 
>> This would have consistency with other uses of underscore that are used to indicate that something is being ignored on purpose, which I think fits this proposal very well. It would also leave the prefix/postfix ellipsis operator free for use on something else with less chance of creating ambiguity.
>> 
>> I disagree that this proposed use of the underscore is consistent with other uses.
>> 
>> The underscore is used as a pattern matching construct that means "I don't care what actual value goes here; match and allow anything and throw it away." But in the range case, not only is this not a pattern, but you also *do* care very much what value is used there—not any arbitrary value is acceptable. The value you want is the least or greatest possible value for that range.
> 
> Right; “_” is a placeholder for “don’t care”; it shouldn’t have different semantic meanings in different contexts.

"Don't care" is little different to "omit", which is what is would be happening in the case of declaring a one-sided range. It's not a radically different meaning in different contexts, it's just slightly widening what it means.

>> Regardless, these ideas were brought up during the proposal's discussion and it's probably safe to assume that the core team considered them but felt they weren't a good solution.
> 
> Yes, the core team did consider the ideas around using the binary operators rather than introducing the new prefix/postfix operators, and felt that the proposal provided the clearest Swift code.

Can you please qualify this further? How is this clearly a one-sided range rather than a mistake:

	let values = someArray[5...] // This one I wanted to be a one-sided range
	let values = someArray[5...] // Scrolled up for correct endpoint to use, but forgot to fill it in

I just don't think that we should throw away the property whereby a range is incomplete until a second value or something else is given. If not an underscore then something else would be fine, even just the brackets that we have now to represent Void; omitting arguments is fine in methods and functions where it's still clear, but I just don't think that this is the case here, as this isn't a unique new operator, it's an overload of an existing one that does essentially the same thing but differently.

The only similar case that springs to mind is that we an exclamation both for negation, and force-unwrapping, but they're used at different ends to an argument, so it's not similar in practice.
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