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

Karl Wagner razielim at gmail.com
Fri Apr 14 12:00:17 CDT 2017

> I'd actually say the #1 reason not to add this feature is that a lot of developers don't seem to understand this, and they're likely to use the feature to make their code try to continue in the face of programmer error instead of trapping like it properly should. A program in an inconsistent state is dangerous; best to stop it quickly before it does some damage.)

Right, so I think the reason is actually that a lot of developers don’t understand what an Array is. There are two use-cases for an Array:

1) As a string of items, don’t care about the length. The maximum prior knowledge you can have is that the order may or may not be significant. This includes operations like iteration, mapping, reducing and filtering.
2) As a string of items of specific length. You have prior knowledge about what you expect to find at each location. This includes operations like random-access subscripting, which is what we’re talking about.

Basically, the interesting part of a statement such as “let someValue = myArray[2]” is: why index 2? What’s so special about that element; why couldn't someValue be the item at any index N instead? It’s because we know to expect something of special significance at index 2.

In that case, the only time myArray[2] will fail is when your prior knowledge breaks down. The type-system has no way to encode and check for the length of an Array, and that has allowed somebody to pass in a bad value. So what to do?

A) If you absolutely require a value for myArray[2]: Insert a precondition check.
B) If you can still continue without myArray[2]: Check the length of the Array. Your logic will be branching anyway in this case, to account for the value (and subsequent values) being/not being present.

Personally, the only valid use-case I can think of is when you want to initialise an Array’s elements out-of-order - i.e., you want to set a value for myArray[2] despite myArray[0] and [1] not being populated. In that case, it would be better to have some kind of SparseArray type, and for us to have a proper API for unsafe initialisation of stdlib types. 

- Karl

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