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

[Adam Nemecek]

It's not for branching based on the value, it's for calculating
mathematical functions with the sign retrieved from the value. So for the
same reason, no it should not be an enum. It should be the same type as the
type it's called on. It's that way in Haskell as well

http://hackage.haskell.org/package/base-4.9.0.0/docs/Prelude.html#v:signum

This function is basically in every language

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sign

https://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#signum(double)

http://en.cppreference.com/w/cpp/numeric/math/signbit

https://golang.org/pkg/math/#Signbit

https://msdn.microsoft.com/en-us/library/system.math.sign(v=vs.110).aspx

It's used a bunch e.g. in dsp but also in mathematics

https://www.quora.com/What-are-the-real-life-applications-of-Signum-function

This quote somewhat summarizes it "So signum shows up in many places where
discontinuous jumps must be written in closed form."



On Mon, May 23, 2016 at 12:29 AM, Haravikk <swift-evolution at haravikk.me>
wrote:

> Could you give an example of this method’s usage? Surely your value is
> either positive, negative or zero already, so this method doesn’t return
> anything more useful.
>
> In other words, anywhere that I might do this:
>
> if myValue.sign > 0 { … }
>
> I could just as easily do:
>
> if myValue > 0 { … }
>
> To the same end result surely? Unless I’m missing something it seems
> redundant.
>
> If there is a use-case for this, would it make more sense to have the
> return type as an enum with cases for Positive, Negative and Zero?
>
> On 22 May 2016, at 08:07, Adam Nemecek via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Howdy,
> I think that the SignedNumberType should implement a method called sign
> that will return -1 for negative numbers, 0 for 0 and 1 for positive
> numbers. This is similar to the signum method in e.g. Java and similarly
> called methods in other languages.
>
> The implementation is fairly straight forward
>
> extension SignedNumberType {
>   var sign: Self {
>     if self == 0 {
>       return 0
>     }
>     else if self > 0 {
>       return 1
>     }
>     return -1
>   }
> }
>
> I was trying to implement is without branching by doing (x > 0) - (x < 0)
> but I couldn't get the types right so I'm open to suggestions.
>
>
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>
>
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