[swift-evolution] [Pitch] `Int.init(ObjectIdentifier)` and `UInt.init(ObjectIdentifier)` should have a `bitPattern:` label

[Charlie Monroe]

+1. Agreed. Otherwise, it's kind of a cryptic initializer.

> On Jul 15, 2016, at 3:54 PM, Arnold Schwaighofer via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This is probably uncontroversial.
> 
> PR: https://github.com/apple/swift-evolution/pull/434
> 
> 
> 
>> On Jul 13, 2016, at 4:39 PM, Arnold Schwaighofer via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Int.init(ObjectIdentifier) and UInt.init(ObjectIdentifier) should have a 'bitPattern:’ label to make it clear at the use site that we interpret the value as a bit pattern.
>> 
>> In Swift we have ObjectIdentifier values which uniquely identify a class instance or metatype. They are implemented as a struct which holds the value of the reference to the instance or metatype as a raw pointer.
>> 
>> /// A unique identifier for a class instance or metatype.
>> public struct ObjectIdentifier : Hashable, Comparable {
>>   internal let _value: Builtin.RawPointer
>>   ...
>> }
>> 
>> 
>> We have constructors for Int and Uint that capture this value. These constructors don’t have an argument label.
>> 
>> extension UInt {
>>   /// Create a `UInt` that captures the full value of `objectID`.
>>   public init(_ objectID: ObjectIdentifier) {
>>     self.init(Builtin.ptrtoint_Word(objectID._value))
>>   }
>> }
>> 
>> extension Int {
>>   /// Create an `Int` that captures the full value of `objectID`.
>>   public init(_ objectID: ObjectIdentifier) {
>>     self.init(bitPattern: UInt(objectID))
>>   }
>> }
>> 
>> This proposals suggest adding a label ‘bitPattern’ to the constructor: Int.init(bitPattern: ObjectIdentifier).
>> 
>> extension UInt {
>>   /// Create a `UInt` that captures the full value of `objectID`.
>>   public init(bitPattern objectID: ObjectIdentifier) {
>>     self.init(Builtin.ptrtoint_Word(objectID._value))
>>   }
>> }
>> 
>> extension Int {
>>   /// Create an `Int` that captures the full value of `objectID`.
>>   public init(bitPattern objectID: ObjectIdentifier) {
>>     self.init(bitPattern: UInt(objectID))
>>   }
>> }
>> 
>> 
>> Motivation
>> ==========
>> 
>> Adding a label ‘bitPattern’ to the constructors makes it clear that we interpret the pointer value as a bit pattern at the use site. It is similar to what we do in other APIs, for example in "UInt(bitPattern: UnsafePointer<Void>(value)))"
>> 
>> Impact on existing code
>> =======================
>> 
>> Existing code will have to add the bitPattern label.
>> 
>> 
>> Alternatives
>> ============
>> 
>> Leave as is.
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