[swift-evolution] [Review] SE-0088: Modernize libdispatch for Swift 3 naming conventions

[Ben Rimmington]

<https://github.com/apple/swift-evolution/blob/master/proposals/
0088-libdispatch-for-swift3.md>

## Type Names

I was going to suggest unprefixed type names, but having to qualify both
`Dispatch.Data` and `Foundation.Data` would be an issue. If libdispatch
had originally been part of Foundation, you'd need prefixes anyway:

* DispatchQueue
* NotificationQueue
* OperationQueue

## Quality of Service

Should argument labels and type names match those in Foundation?

* qos:        => qualityOfService:
* DispatchQoS => DispatchQualityOfService

Could there be a shared `Swift.QualityOfService` enum?

* QOS_CLASS_DEFAULT = 0x15
* NSQualityOfServiceDefault = -1

The `.unspecified` QoS is not defined in NSQualityOfService.
Would an optional QoS parameter (defaulting to nil) be better?

## Time

The `dispatch_time` function uses a nanoseconds delta, so the operator
overloads taking `seconds: Double` might be ambiguous at the call site.
You could try changing the associated value types of your enum.

```
enum DispatchTimeInterval {
    case      seconds(Double) // changed from `Int`
    case milliseconds(Int64)  // changed from `Int`
    case microseconds(Int64)  // changed from `Int`
    case  nanoseconds(Int64)  // changed from `Int`
}

let _ = DispatchTime.now + 3.5 // ambiguous?
let a = DispatchTime.now +      .seconds(3.5)
let b = DispatchTime.now + .milliseconds(3_500)
let c = DispatchTime.now + .microseconds(3_500_000)
let d = DispatchTime.now +  .nanoseconds(3_500_000_000)
```

Is the `DispatchTime.now` in your example a type property?

## Data

Should the `DispatchData.append` methods have `contentsOf:` labels?
Or could it conform to the RangeReplaceableCollection protocol?

## Queues

My suggestions for the async/sync methods:

* enqueue(...)
* enqueueAndWaitUntilFinished(...)

-- Ben