How can a closure in Swift only be passed in a * for the reduce function?-CodePudding

extension Array {
func reduce<T>(_ initial: T, combine: (T, Element) -> T) -> T {
    var result = initial
    for x in self {
        result = combine(result, x)
    }
    return result
  }
 }

How does the below function work when it only passes a * to the combine closure?

func productUsingReduce(integers: [Int]) -> Int {
    return integers.reduce(1, combine: *)
}

CodePudding user response：

static func * (lhs: Self, rhs: Self) -> Self is an operator function defined on the Numeric protocol. So, you are really just passing in a function that takes two arguments.

CodePudding user response：

What does the declaration mean?

combine: (T, Element) -> T

It says: the combine parameter is a function that takes two parameters, a T and an Element, and returns a T.

Well, in Swift, operators are functions, and * is such a function. So it suffices to pass a reference to this function as the combine parameter. The bare name, *, is that reference.