[swift-evolution] [Proposal] Change (Dispatch)Data.withUnsafeBytes to use UnsafeMutableBufferPointer

[Andrew Trick]

> On Dec 27, 2016, at 12:16 AM, Karl via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Looking for feedback before submitting a PR: https://github.com/karwa/swift-evolution/blob/corelibs-unsafebytes/proposals/xxxx-corelibs-unsafebytes.md <https://github.com/karwa/swift-evolution/blob/corelibs-unsafebytes/proposals/xxxx-corelibs-unsafebytes.md>
> 
> —
> 
> Change (Dispatch)Data.withUnsafeBytes to use UnsafeMutableBufferPointer
> 
> Proposal: SE-NNNN <https://github.com/karwa/swift-evolution/blob/corelibs-unsafebytes/proposals/NNNN-filename.md>
> Authors: Karl Wagner <https://github.com/karwa>
> Review Manager: TBD
> Status: Awaiting review
> During the review process, add the following fields as needed:
> 
> Decision Notes: Rationale <https://lists.swift.org/pipermail/swift-evolution/>, Additional Commentary <https://lists.swift.org/pipermail/swift-evolution/>
> Bugs: SR-NNNN <https://bugs.swift.org/browse/SR-NNNN>, SR-MMMM <https://bugs.swift.org/browse/SR-MMMM>
> Previous Revision: 1 <https://github.com/apple/swift-evolution/blob/...commit-ID.../proposals/NNNN-filename.md>
> Previous Proposal: SE-XXXX <https://github.com/karwa/swift-evolution/blob/corelibs-unsafebytes/proposals/XXXX-filename.md>
>  <https://github.com/karwa/swift-evolution/blob/corelibs-unsafebytes/proposals/xxxx-corelibs-unsafebytes.md#introduction>Introduction
> 
> The standard library's Array and ContiguousArray types expose the method withUnsafeBytes, which allows you to view their contents as a contiguous collection of bytes. The core libraries Foundation and Dispatch contain types which wrap some allocated data, but their withUnsafeBytes method only allows you to view the contents as a pointer to a contiguous memory location of a given type.
> 
> Swift-evolution thread: Discussion thread topic for that proposal <https://lists.swift.org/pipermail/swift-evolution/>
>  <https://github.com/karwa/swift-evolution/blob/corelibs-unsafebytes/proposals/xxxx-corelibs-unsafebytes.md#motivation>Motivation
> 
> The current situation makes it awkward to write generic code. Personally, I use the following extension in my projects to sort the naming confusion out:
> 
> protocol ContiguousByteCollection {
>   func withUnsafeBytes<T>(_ body: (UnsafeRawBufferPointer) throws -> T) rethrows -> T
> }
> 
> // stdlib types are fine.
> extension Array: ContiguousByteCollection {}
> extension ArraySlice: ContiguousByteCollection {}
> extension ContiguousArray: ContiguousByteCollection {}
> 
> // corelibs types give us a pointer<T>, should be: { pointer<char>, count }
> #if canImport(Dispatch)
>   import Dispatch
> 
>   extension DispatchData : ContiguousByteCollection {
>     func withUnsafeBytes<T>(_ body: (UnsafeRawBufferPointer) throws -> T) rethrows -> T {
>       return try withUnsafeBytes { try body(UnsafeRawBufferPointer(start: $0, count: count)) }
>     }
>   }
> #endif
> 
> #if canImport(Foundation)
>   import Foundation
> 
>   extension Data : ContiguousByteCollection {
>     func withUnsafeBytes<T>(_ body: (UnsafeRawBufferPointer) throws -> T) rethrows -> T {
>       return try withUnsafeBytes { try body(UnsafeRawBufferPointer(start: $0, count: count)) }
>     }
>   }
> #endif
> Conceptually, the corelibs types are untyped regions of memory, and it would make sense for them to adopt the UnsafeRawBufferPointer model.
> 
>  <https://github.com/karwa/swift-evolution/blob/corelibs-unsafebytes/proposals/xxxx-corelibs-unsafebytes.md#proposed-solution>Proposed solution
> 
> The proposed solution would be to deprecate the current methods on (Dispatch)Data (with 2 generic parameters), and replace them with methods with identical signatures to Array (with 1 generic parameter).
> 
> To be deprecated:
> 
> public func withUnsafeBytes<ResultType, ContentType>(_ body: (UnsafePointer<ContentType>) throws -> ResultType) rethrows -> ResultType
> Replaced with:
> 
> public func withUnsafeBytes<R>(_ body: (UnsafeRawBufferPointer) throws -> R) rethrows -> R
>  <https://github.com/karwa/swift-evolution/blob/corelibs-unsafebytes/proposals/xxxx-corelibs-unsafebytes.md#source-compatibility>
Thanks Karl. Good observation.

I proposed exactly this API along with a few other UnsafeRawBufferPointer compatibility API’s during SE-0183. Look for Tony to follow up on this.

-Andy
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