[swift-evolution] [Pitch] Replace the ternary operator with an in-language function

[Jay Abbott]

David, you can use two binary operators (or overload the same one twice if
you want) to create syntax that behaves like a ternary operator.

Here’s an example of using a pair of operators to interpolate between two
CGPoint values:

let interpolatedPoint = p1 <~~ 0.3 ~~> p2

See here
<https://github.com/j-h-a/Animation/blob/develop/Animation/Interpolation.swift>
for the code that defines them.

I went for two different operators in the end, but when experimenting I
also tried using the same one, and it works fine because of overloading,
for example:

infix operator ~~> : InterpolationPrecedence
public func ~~> <T: Interpolatable>(from: T, alpha: Double) -> (T, Double) {
    return (from, alpha)
}
public func ~~> <T: Interpolatable>(lhs: (T, Double), rhs: T) -> T {
    return lerp(from: lhs.0, to: rhs, alpha: lhs.1)
}let interpolatedPoint = p1 ~~> 0.3 ~~> p2

And as Anton demonstrated earlier, ?: can be emulated the same way. The
errors you would get if you omitted the second operator and third part are
not as useful as they can be with ?:. The compiler can probably do a much
better job of optimising with ?: as a special case, and it’s a common
pattern regardless of syntax, so people would just write their own if it
wasn’t there. So I think it makes sense to have it in the language. And if
it wasn’t already there, I do think that it would be something we should
add (same for ??).
​

On Thu, 27 Oct 2016 at 01:44 David Sweeris via swift-evolution <
swift-evolution at swift.org> wrote:

>
> On Oct 25, 2016, at 23:51, Charlotte Angela Tortorella via swift-evolution
> <swift-evolution at swift.org> wrote:
>
> Disadvantages of The Ternary Operator
> <https://gist.github.com/Qata/25a11c21200f1cf8f43ed78e9ffd727c#disadvantages-of-the-ternary-operator>
>
> [...]
>
>
> 6. This operator is only applicable to a single type, `Bool`.
>
> [...]
>
>
> Proposed Approach
> <https://gist.github.com/Qata/25a11c21200f1cf8f43ed78e9ffd727c#proposed-approach>
>
> We should drop the ternary operator in favor of a new extension to `Bool`.
>
>
> I'm not sure proposals should do exactly what they claim is a downside of
> the current approach. Especially when the downside in question is
> inherent to the problem being solved.
>
> FWIW, the only thing I find confusing about the ternary operator is that I
> can't overload it. Being able to define my own ternary operators would be
> great, but I don't have an answer to obvious potential ambiguities.
>
> - Dave Sweeris
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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