Sunday, October 19, 2014 [Tweets] [Favorites]

Mutable Collections in Swift

Mike R. Manzano:

How do you create an var that holds an immutable Array? As in a var that you can assign different immutable Arrays to?

BJ Homer:

Because Swift arrays and dictionaries can never be shared, there is no distinction between mutating an existing collection and re-assigning a new collection. The behavior of the code is exactly the same. In either case, the owner’s setter method is called whenever the array is modified.

So to answer the original question, there is no syntax to specify a variable that holds an immutable array because there is nothing that such syntax would add. Swift addresses the issues that made NSArray and NSMutableArray necessary in the first place. If you need a shared array, you can still use the Cocoa types. In every other case, Swift’s solution is safer, simpler, and more concise.

On the whole, I think this is probably a good direction. The downsides would seem to be that the performance model is less clear and that it’s more work to write your own data types as struct-class pairs.

One somewhat common pattern in my Objective-C code is a (often recursive) method that takes a mutable array or dictionary as a parameter and builds it up. You can’t do this with var in Swift because that only lets you modify the collection within the method. However, you can use inout to have Swift “return” the last value to the caller.

This is not the same as passing around an NSMutableArray, though. For example, consider what would happen if there were multiple threads involved. Also, inout only lasts for the duration of the method; the collection cannot (as far as I know) be stashed in another object and then mutated (back in the caller) later.

Update (2014-10-19): Christoffer Lernö responds via Twitter.

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