CODEWITHSUNDEEP
swiftswift-extensionsswift2swift-protocols
Travis Griggs
Travis Griggs

Reputation: 22252

Protocol Extension type Self does not match initializer types

Let's say for academic purposes, I would prefer an expression like

someInt.asDouble

rather than the stock

Double(someInt)

Since all of the various Swift integer types conform to the IntegerType protocol AND because there seems to be a Double() initializer that fits every kind of those int types, I thought I could something like:

extension IntegerType {
    var asDouble:Double {
        return Double(self)
    }
}

This does not work. And I would like to know why? I would like to know if there is some magic that would make it work? A where maybe? Or something to do with the self reference?

The error I get in the console of the playground I tried in this reads:

Playground execution failed: /var/folders/2k/6y8rslzn1m95gjpg534j7v8jzr03tz/T/./lldb/41416/playground37.swift:31:10: error: cannot invoke initializer for type 'Double' with an argument list of type '(Self)'
                return Double(self)
                       ^
/var/folders/2k/6y8rslzn1m95gjpg534j7v8jzr03tz/T/./lldb/41416/playground37.swift:31:10: note: overloads for 'Double' exist with these partially matching parameter lists: (Double), (UInt8), (Int8), (UInt16), (Int16), (UInt32), (Int32), (UInt64), (Int64), (UInt), (Int), (Float), (Float80), (String), (CGFloat), (NSNumber)
                return Double(self)

Upvotes: 2

Views: 185

Answers (1)

Rob Napier
Rob Napier

Reputation: 299275

All IntegerType types can be promoted to IntMax, and IntMax can be converted to Double.

extension IntegerType {
    var asDouble: Double {
        return Double(self.toIntMax())
    }
}

As you suggest, this is not a particularly useful thing to do (Double(x) is the correct syntax), but it may be useful for other problems.

I would continue stdlib's pattern if I were going to create this. The pattern is toX() rather than asX:

extension IntegerType {
    func toDouble() -> Double {
        return Double(self.toIntMax())
    }
}

See also toOpaque() and toUIntMax(). as suggests a simple reinterpretation of the type without copying, and a var suggests that it's a property of this value. That's not what's happening. You're actually converting it to a different value, stored in a different format, that may not even be equal. (For large integers, there may be rounding errors such that x != Int(x.toDouble()).)

It's worth also reading Trying to extend IntegerType (and FloatingPointType); Why can't all Int types be converted to NSTimeInterval, which discusses the problem further, along with why similar features can be dangerous unless done with great care.

Upvotes: 3

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