ios - How to customize ternary operators in Swift

Question

I know how to customize binary operators, like this

infix operator ** { associativity left precedence 170 }
func ** (left: Double, right: Double) -> Double {
    return pow(left, right)
}

But, how to customize ternary operators in Swift? Can anyone give me some idea? Thanks so much!

Answer 1

You can actually do this by declaring two separate operators that work together and using a curried function for one of the operators.

Let's declare a ternary operator x +- y +|- z that will check the sign of the initial value x, and then return the second value y if the sign is zero or positive and the final value z if the sign is negative. That is, we should be able to write:

let sign = -5 +- "non-negative" +|- "negative"
// sign is now "negative"

We'll start by declaring the two operators. The important part is to have higher precedence on the second operator - we'll evaluate that part first and return a function:

infix operator +- { precedence 60 }
infix operator +|- { precedence 70 }

Then define the functions - we'll define the second first:

func +|-<T>(lhs: @autoclosure () -> T, rhs: @autoclosure () -> T)(left: Bool) -> T {
    return left ? lhs() : rhs()
}

The important part here is that this function is curried -- if you only call it with the first two parameters, instead of returning a T value, it returns a (left: Bool) -> T function. That becomes the second parameter of the function for our first operator:

func +-<I: SignedIntegerType, T>(lhs: I, rhs: (left: Bool) -> T) -> T {
    return rhs(left: lhs >= 0)
}

And now we can use our "ternary" operator, like this:

for i in -1...1 {
    let sign = i +- "" +|- "-"
    println("(i): '(sign)'")
}
// -1: '-'
// 0: ''
// 1: ''

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Note: I wrote a blog post on this subject with another example.