Changes to the Swift Standard Library in 1.1 beta 3

The biggest deal in this latest beta is the documentation. There are 2,745 new lines of /// comments in beta 3, and even pre-existing documentation for most items has been revised.

That doesn’t mean there aren’t any functional changes to the library, though. Here’s a rundown of what I could find:

  • As mentioned in the release notes, the various literal convertible protocols are now implemented as inits rather than static functions.
  • The ArrayBoundType protocol is gone, and the various integer types that conformed to it no longer do.
  • The CharacterLiteralConvertible protocol is gone. Character never appeared to conform to it – it conformed to ExtendedGraphemeClusterLiteralConvertible which remains.
  • The StringElementType protocol is gone, and UInt8 and UInt16 no longer conform to it. The UTF16.copy method, that relied on it to do some pointer-based manipulation, is also gone.
  • Dictionary’s initializer that took a minimumCapacity argument no longer has a default for that minimum capacity. It’s interesting that this worked previously, since if you did the same thing with your own type (i.e. gave it both an init() and an init(arg: Int = 0)) you’d get a compiler error when you tried to actually use init().
  • In extending RandomAccessIndexType, integers now use their Distance typealias for Int rather than straight Int.
  • The static from() methods on integers are all gone.
  • There’s now an EnumerateSequence that returns an EnumerateGenerator, and enumerate() returns that rather than the generator.
  • The various integer types no longer have initializers from Builtin.SomeType.
  • The _CocoaArrayType protocol (as used to initialize an Array from a Cocoa array) has been renamed to _SwiftNSArrayRequiredOverridesType.
  • Numerous _CocoaXXX and _SwiftXXX protocols have acquired an @objc attribute.
  • _PrintableNSObjectType (which wasn’t explicitly implemented by anything in the std lib) is gone.
  • The AssertStringType and StaticStringType protocols are gone, as has AssertString. StaticString still exists and is clearly described as a static string that can be known at compile-time.
  • assert and precondition are much simplified with those string types removed, leaving one version each that just uses String for it’s message parameter. assertionFailure, preconditionFailure and fatalError all take a String for their message now as well, though they still take StaticString for the filename argument as described in the Swift blog’s article.
  • There are two new sort functions that operate on a ContiguousArray (one for arrays of comparable elements, and one that takes a comparison predicate).
  • But there are two fewer sorted functions. The ones that take a MutableCollectionType, and return one, are gone. Seems a shame, though it didn’t really make sense for them to take a mutable type given they didn’t need to mutate it. Maybe they’ll be replaced with versions that return ExtensibleCollectionType.
  • There’s a new unsafeDowncast that is equivalent to x as T but with the safeties removed – to be used only when using as is causing performance problems.
  • Raise a glass for the snarky “Haskell’s fmap, which was mis-named” comment, which is now replaced by a very straight-laced description of what actually does.

In keeping with the documenting theme, there are a lot of argument name changes here as well. Continuing a trend seen in previous betas, specific, descriptive argument names are preferred over more generic ones. This is probably a good Swift style tip for your own code, especially if you’re also writing libraries.

Some examples:

  • Naming arguments in protocols instead of just using ‘_‘ e.g. func distanceTo(other: Self) instead of func distanceTo(_: Self) (there’s no compulsion to use the same names for your method arguments that the protocol does but you probably should)
  • Avoiding just naming the variable after the type (e.g. sequence: Sequence), For example, Array.extend has renamed its argument to newElements.
  • Replacing newValues with newElements in various places, presumably because of the unintended Swift implications of the term “values”.
  • Avoiding i or v such as subscript(position: Index) instead of subscript(i: Index), and init(_ other : Float) instead of init(_ v : Float).
  • Descriptive names for predicate arguments, such as isOrderedBefore rather than just pred.

Rather than me regurgitate the comments here, I’d suggest option-clicking import Swift and reading through it in full. There’s lots of informative stuff in there. Major types like Array and String have long paragraphs in front of them with some good clarifications. Many things are explained that you’d otherwise only have picked up on by watching the developer forums like a hawk.1

Here are a few items of specific interest:

The comment above GeneratorType has been revised. The suggestion that if using a sequence multiple times the algorithm “should probably require CollectionType, since CollectionType implies multi-pass” is gone. Instead it states that “any code that uses multiple generators (or for ... in loops) over a single sequence should have static knowledge that the specific sequence is multi-pass”. The most common case of having that static knowledge being that you got the sequence from a collection I guess.

Above Slice we find “Warning: Long-term storage of Slice instances is discouraged“. It makes it pretty clear Slice is mainly there to help pass around subranges of arrays, rather than being a standalone type in its own right. They give you the value type behaviour you’d get from creating a new array (i.e. if a value in the original array is changed, the value in the slice won‘t) while allowing you to get the performance benefits of copy-on-write for a sub-range of an existing array.

Several of the _SomeProto versions of protocols now have an explicit comment along the lines of the previously implied “this protocol is an implementation detail of SomeProto; do not use it directly”. I’m still not sure why these protocols are written in this way, possibly as a workaround for some compiler issues? If you have an idea, let me know.

edit: a post on the dev forum confirms it’s a temporary workaround for a compiler limitations related to generics, though doesn’t get specific about what limitation. Thanks to @anatomisation and @ivicamil for the link.

The reason for ContiguousArray’s existence is finally made clear. It’s there specifically for better performance when holding references to classes (not value types). There’s also a comment above the various collection withUnsafeBufferPointer methods suggesting you could use them when the optimizer fails to eliminate bounds checks automatically, in a performance-vs-safety trade-off.

Chris Lattner clarified on the dev forum that although this version is a GM, that’s in order to allow you to use it to submit apps to the mac store rather than because this is the final shipping version. We’ll likely see more changes to Xcode 6.1 before that, and with it maybe further changes to the standard lib.

  1. Or reading this blog, of course. Hopefully the comments don’t get too helpful, what would I write about then? 

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