Add accessor with bounds check to Array

[jarod (Jarod Long)]

@Jens

But I think values[safe: index] is as bad as values[checked: index] . The term "safe" is used to describe the language ( Swift makes it easy to write software that is incredibly fast and safe by design. ), hence Swift's regular array subscript has been designed to be safe (by trapping if its out of bounds check fails), and it's also used consistently afaics in the language for things like UnsafePointer , unsafeBitCast , unsafeDowncast , withUnsafeBytes , etc, just like the term "(un)checked" is consistently used, in eg:

let a = Range(uncheckedBounds: (lower: 123, upper: -1))
print(a) // 123..<-1

It would be unfortunate to introduce additional meanings of the words "(un)safe" and "(un)checked" in the language when there currently afaik is only one.

Safety can be thought of in tiers -- trapping is safer than reaching into out-of-bounds memory, but returning nil is even safer than trapping, requiring the programmer to consciously deal with the optionality at the call site instead of implicitly crashing. Swift.org - About Swift explicitly references optionals as a safety feature, so I don't feel that it's a stretch to call this the safer version of the API.