[swift-evolution] Trial balloon: Ensure that String always contains valid Unicode

[Kevin Ballard]

That kind of lazy checking of arrays is used pretty rarely (since, as
you say, it only occurs with an `as!` expression). But doing lazy
checking of strings would end up having to check *every* string that
comes from ObjC (which, in a Swift app that uses Cocoa frameworks, is
likely to be most strings the app works with).

-Kevin Ballard

On Mon, Jan 4, 2016, at 02:41 PM, Félix Cloutier wrote:
> There are precedents for lazily checking for validity after bridging.
> Using `array as! [T]` on a NSArray without generics  fails lazily if
> you access an object that's not a T.
>
> Félix
>
>> Le 4 janv. 2016 à 14:59:47, Dmitri Gribenko via swift-evolution <swift-
>> evolution at swift.org> a écrit :
>>
>> On Mon, Jan 4, 2016 at 9:37 PM, Kevin Ballard via swift-evolution <swift-
>> evolution at swift.org> wrote:
>>> __
>>> I agree in principle that it would be great if String could enforce
>>> that it's always valid.
>>>
>>> But unfortunately, in practice, there's no way to do that without
>>> making it expensive to bridge from Obj-C. Because, as you've
>>> demonstrated, you can create NSStrings that contain things that
>>> aren't actually valid unicode sequences, every single bridge from an
>>> NSString to a String would have to be checked for validity. Not only
>>> that, but it's not clear what the behavior would be if an invalid
>>> string is found, since these bridges are unconditional - would Swift
>>> panic? Would it silently replace the invalid sequence with U+FFFD?
>>> Or something else entirely? But the question doesn't really matter,
>>> because turning these bridges from O(1) into O(N) would be an
>>> unacceptable performance penalty anyway.
>>
>> Currently String replaces invalid sequences with U+FFFD lazily during
>> access, but there are corner cases related to Objective-C bridging
>> that can still leak invalid Unicode.
>>
>> Dmitri
>>
>> --
>> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
>> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
>>
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