I keep running myself into this issue when I try using some of my generic
classes built on top of Protocols.
I have a TimeSeries protocol which has an Underlying (Float, Double, ..
whatever) and also a Self requirement because you can return a sub series.
There’s more to the protocol but this is enough for example
public protocol TimeSeries
{
typealias Underlying
mutating func addValue( value : Underlying, tick : Int )
func subSeriesFromTick( tick : Int )->Self
}
I have two concrete implementers of this protocol, a FullTimeSeries which is a
memory pig and a CompressedTimeSeries which usually isn't
public struct FullTimeSeries<T> : TimeSeries
{
public typealias Underlying = T
// implementation
}
public struct CompressedTimeSeries<T> : TimeSeries
{
public typealias Underlying = T
// implementation
}
These work just fine. Now I want a UIView which shows a TimeSeries, in this
case actually a TimeSeries of Floats, of course the code below doesn’t work
public class TimeSeriesView : UIView
{
public typelias Underlying = Float
public var timeSeries : TimeSeries
}
"Protocol 'TimeSeries' can only be used as a generic constraint because it has
Self or associated type requirements”, my least favourite error message which
always pops up just after I’ve done all the work and just before I get to use
the fruits of my labour. I never see it coming.
The typealias in the above code doesn’t do anything of course, it was just me
having an attempt.
I want to say that timeSeries is any TimeSeries with an underlying type of
Float, but you can’t. I wanted to have a generic UIView I could just stuff a
TimeSeries of Floats into and it would show it.
The best I can come up with is to parameterise on T, which is a TimeSeries
public class TimeSeriesView<T:TimeSeries>
{
public typealias Underlying = Float
public var timeSeries : T
}
now I have to create the correct TimeSeriesView<FullTimeSeries<Float>> or
TimeSeriesView<CompressedTimeSeries<Float>>, which wasn’t really what I wanted
to do.
There’s still no clever way around this right, this is still how Generics and
Protocols work together, there’s nothing in Swift 2.0 which helps you work
around this, no way to use Protocol extensions to do it? I thought (I’ve tried
this before) of making a FloatTimeSeries protocol which only deals with Floats
and has no Self or associated type issues which basically has all the same
methods of TimeSeries but with ‘Float’ specifically instead of Underlying and
then using a protocol extension like this
extension FullTimeSeries : FloatTimeSeries where T:Float
{
}
but Swift has you covered on that one too .. "Extension of type
'FullTimeSeries' with constraints cannot have an inheritance clause”
Nothing from last years WWDC I missed here? You still can’t do this, right?
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