[swift-users] Why does Array subscript fail at runtime?

[Jeff Kelley]

If I’m not mistaken, the main reason for Swift arrays being unsafe in this way is for performance. Checking each subscript would be expensive in a large loop; it’s much more performant to loop from the array’s start index to its end index.


Jeff Kelley

SlaunchaMan at gmail.com | @SlaunchaMan <https://twitter.com/SlaunchaMan> | jeffkelley.org <http://jeffkelley.org/>
> On Jul 20, 2016, at 9:07 PM, H. Kofi Gumbs via swift-users <swift-users at swift.org> wrote:
> 
> Hello Swift community,
> 
> Here's a philosophy I've struggled with since I started learning Swift. In general, it seems that failable function calls return `Optional`s; however, `Array`s violate this rule by failing fast at runtime. I understand that subscripts can't throw, so the only way to fail fast is to do so at runtime. I also realize that there are many implementations of the `array[safe: index]` that I could choose to use instead. However, I do not understand why the default behavior is still to fail at runtime. Especially given how they are often introduced as a beginner-friendly data structure. Am I missing some language philosophy or major decision?
> 
> Thanks
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