[swift-evolution] [Pitch] Add the sign method to the SignedNumberType protocol.

[Haravikk]

> On 23 May 2016, at 20:19, Dany St-Amant via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Challenge accepted… Removed the if/else if, at the cost of double function call to a tri-op:
> 
> extension Bool {
>     func as01<T:SignedNumberType>() -> T { return self ? 1 : 0 }
> }
> 
> extension SignedNumberType {
>     var sign: Self { return (self > 0).as01() - (self < 0).as01() }
> }

I don’t believe this solves the problem; I think you’ve really just moved the branching into the as01() method.

I think the more correct solution would be for all integer types to have a required Bool initialiser in a protocol somewhere, this way you could just do:

	var sign:Self { return Self(boolValue: self > 0) - Self(boolValue: self < 0) }

Since all integer types should be able to do this by just extending the value to fit, rather than testing it. I’m not completely up-to-date on the integer changes that will be in Swift 3, but perhaps conversion from bool will be easier in future, in Swift 2.2 it’s not really treated as a number type for conversion purposes, which is disappointing.

Also apologies for my initial misunderstanding of the need for this sign property, no idea how I managed to get such a mental block as it’s obviously very useful for multiplication when account for sign and a few other cases. Anyway, I’m a +1, though implementation is definitely trickier than it seems it should be, thanks to some of Swift’s number-related quirks!
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