[swift-evolution] [Pre-proposal] Enforcing Correct use of Array Indices

Dave Abrahams dabrahams at apple.com
Mon Oct 3 10:36:23 CDT 2016

on Mon Oct 03 2016, Haravikk <swift-evolution at swift.org> wrote:

>> On 30 Sep 2016, at 13:10, Dave Abrahams via swift-evolution
>> <swift-evolution at swift.org> wrote:
>> on Thu Sep 29 2016, Haravikk
>> <swift-evolution at swift.org
>> <mailto:swift-evolution at swift.org>>
>> wrote:
>>> So the issue of Array index safety came up on the discussion for the
>>> .indexed() proposal. Now of course there's nothing wrong with arrays
>>> using integers as indices as such, but it does mean that indices can
>>> be manipulated outside of the array, which somewhat defeats the idea
>>> of the indexing model requiring them to be passed back to the parent
>>> collection for manipulation.
>>> In a few types of my own I've avoided this problem by doing the
>>> following:
>>> public struct MyMaskedIndex : Comparable { fileprivate var raw:Int }
>>> // Comparable conformance omitted
>>> public struct MyType : Collection {
>>>    // Lots of stuff omitted
>>>    public func distance(from start:MyMaskedIndex, to
>>> end:MyMaskedIndex) -> Int {
>>>        return end.raw - start.raw;
>>>    }
>>> }
>>> In essence MaskedIndex is still just an Int, and should optimise as
>>> such, but in development the use of a type like this ensures that
>>> indices from my collection can't be manipulated externally, which
>>> enables the type-checker to conveniently prevent any mistakes I might
>>> make. It's a handy pattern for other things like hashed values,
>>> ensuring I can't use unhashed values of the same type by accident and
>>> so-on.
>> Yeah, but it doesn't really ensure you won't use an invalid index.
>> Among many other things, you can always reset the collection to an empty
>> state and all the old indices become invalid with respect to it.
>>> I just wanted to raise the topic to see what other people thought
>>> about the idea of doing something similar for Array and any other
>>> types that use integer types directly as indices? For convenience it
>>> is still possible to have a public initialiser on the masking type(s),
>>> so that custom values can be used, but by using MaskedIndex(raw:) it
>>> should be much more obvious what's happening.
>> Believe me, we considered this when doing the Array design.  Being able
>> to index an Array with Ints is pretty fundamental to its usability and
>> adoptability, and wrapping the index doesn't buy any real safety.
> It certainly doesn't solve all the problems, but then it really is
> just intended to avoid the simple mistake of manipulating the index
> externally; like I say, the wrapped value should optimise away (since
> all it is is an Int in reality), and if it's given a public
> constructor then a person can still do external manipulation if they
> must, they just have to be explicit about it.
> The idea is literally just to let the type-checker catch some simple,
> but easy to make, errors that purpose-made index types like
> DictionaryIndex aren't vulnerable to.

Yes, I understand.  You asked what “people thought;” I was saying we
thought about it and explaining the rationale for our choice.  The
benefits don't seem to outweigh the costs.


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