[swift-evolution] [Pitch] Adding safety to arrays

[Paweł Wojtkowiak]

I think the subscript index should not be optional - you want 2nd array element, 3rd, etc., not a "nil" element - this makes no sense to me. Also it feels like some strange optional behavior compared to the normal usage, so -1 from me on this idea.

I think that there can be situations where you'd like to have an optional value from array, e.g. Checking for index of some element before then increasing it by one to get the next one. It would be more convenient and look more clearer to have one optional binding in if/guard than checking against the count and then handling the element. It also feels natural - there either can be an element at given subscript, or there might not be - the first thing it brings to mind is optionals, also it feels familiar from dictionaries.

I think we should preserve the unsafe version for performance reasons but allow some optional access too, so it feels more natural when reading the code. I saw several implementations on stack overflow and the one with "safe:" argument name seemed quite popular. I think it would add complexity to the language to add another syntax/keyword/strange use cases (like optional index), so I'd stay with the argument label. Maybe it's not perfectly pretty, but doesn't harm the language in any way and its intentions are clear.

This is of course something everyone could write in their own extension, but I think adding it to the standard lib would make me happy. I already had some cases where I'd like to use it, not sure about others, but this is still up to the discussion.

Wysłane z iPhone'a

Dnia 13.04.2017 o godz. 18:18 David Sweeris <davesweeris at mac.com> napisał(a):

> 
>> On Apr 13, 2017, at 08:53, Josh Parmenter <jparmenter at vectorform.com> wrote:
>> 
>> This seems inconsistent to me. 2 is 2... 2 itself is not optional. You wouldn't expect 2 to be unwrapped.
> 
> Correct. I think the idea was that "2?" would get converted to an `Optional<Int>`, which would then call a subscript that took an `Index?` instead of `Index`. The trailing "?" doesn't do that, but for some reason I thought it might.
> 
> IMHO, the semantics are clear(ish) in context, but it feels slightly odd for "?" to have two opposite behaviors depending on whether the argument is or isn't an Optional.
> 
> In any case, given that we're discussing a "safe subscript", we need a way to differentiate the safe subscript from the unsafe subscript. The only two ways to do that are to either add an argument label or change the argument type, and IMHO the best alternate argument type is `Index?`, since it's pretty light-weight, already part of the stdlib, and already familiar to Swift developers.
> 
> Someone already said it was a bad idea, though, so I'm rethinking my support.
> 
> - Dave Sweeris 
> 
>