[swift-evolution] Proposal: Add StaticString.UnicodeScalarView

[Dmitri Gribenko]

Hi Kevin,

Thank you for the proposal.  This is in line with the current design of
String.  One of the Swift 3 goals is a redesign of String, to make the API
easier to use.  I don't think that should block this improvement, since
whatever the new design is, it should provide the UnicodeScalar view
functionality for String, and the same API should be exposed on
StaticString.

On Sun, Dec 6, 2015 at 1:21 AM, Kevin Ballard via swift-evolution <
swift-evolution at swift.org> wrote:

> There's no way to create a substring of `StaticString` that's still typed
> as `StaticString`. This is occasionally desirable, for example if you want
> to extract the filename from `__FILE__` to pass to another API that
> requires a `StaticString`.
>
> I believe the best solution to this is to add a type
> `StaticString.UnicodeScalarView`, similar to `String.UnicodeScalarView`.
> `StaticString` would also be extended with a property `unicodeScalars` and
> two initializers to construct a `StaticString` from a
> `String.UnicodeScalarView`. The full proposed API looks like:
>
> extension StaticString {
>   /// The value of `self` as a collection of [Unicode scalar values](
> http://www.unicode.org/glossary/#unicode_scalar_value).
>   public var unicodeScalars: UnicodeScalarView
>
>   /// Construct the `StaticString` corresponding to the given
>   /// `UnicodeScalarView`.
>   public init(_: UnicodeScalarView)
>
>   /// Construct the `StaticString` corresponding to the given
>   /// `UnicodeScalarView` slice.
>   public init(_: Slice<UnicodeScalarView>)
>
>   /// A collection of [Unicode scalar values](
> http://www.unicode.org/glossary/#unicode_scalar_value) that
>   /// encode a `StaticString`.
>   public struct UnicodeScalarView : CollectionType {
>

UnicodeScalarView should be a slice type, that is,
UnicodeScalarView.SubSequence == UnicodeScalarView.

Then, you will be able to remove the
StaticString.init(Slice<UnicodeScalarView>) initializer.

It should also be CustomStringConvertible, CustomDebugStringConvertible,
with the same behavior as String.

Please also add a CustomReflectable conformance, that extracts the string
value and reflects that.

    init(_: StaticString)
>
>     /// A position in a `StaticString.UnicodeScalarView`.
>     public struct Index : BidirectionalIndexType, Comparable {
>       /// Returns the next consecutive value after `self`.
>       ///
>       /// - Requires: The next value is representable.
>       @warn_unused_result
>       public func successor() -> Index
>
>       /// Returns the previous consecutive value before `self`.
>       ///
>       /// - Requires: The previous value is representable.
>       @warn_unused_result
>       public func predecessor() -> Index
>     }
>
>     /// The position of the first `UnicodeScalar` if the `StaticString` is
>     /// non-empty; identical to `endIndex` otherwise.
>     public var startIndex: Index
>
>     /// The "past the end" position.
>     ///
>     /// `endIndex` is not a valid argument to `subscript`, and is always
>     /// reachable from `startIndex` by zero or more applications of
>     /// `successor()`.
>     public var endIndex: Index
>
>     /// Returns `true` iff `self` is empty.
>     public var isEmpty: Bool
>
>     public subscript(position: Index) -> UnicodeScalar
>   }
> }
>

You omitted == and < APIs for indices.


> An alternative would be to make StaticString itself conform to
> CollectionType, but this is a bad idea for the same reasons that String
> doesn't conform to CollectionType.
>

Agreed.

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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