Mathematical sign as function parameter

func doSomeCalculation(_ fun:((Int, Int) -> Int)) -> Int {
    return fun(12,13)
}

doSomeCalculation(+) // 25
doSomeCalculation(-) // -1

The same can be done for UInt, the IntXX, etc.

+ is basically simply a function that takes two arguments and returns the sum of it. Since functions are objects like any other you can pass them around as such.

The same way you can pass + into reduce.

In swift signs are functions name for a specified mathematic operation. To pass sign as parameter the parameter type must be function that takes two numbers and return a number. If you command + click on any sign you will see its definition as follow :

public func +(lhs: UInt8, rhs: UInt8) -> UInt8

public func +(lhs: Int8, rhs: Int8) -> Int8

public func +(lhs: UInt16, rhs: UInt16) -> UInt16

public func +(lhs: Int16, rhs: Int16) -> Int16

public func +(lhs: UInt32, rhs: UInt32) -> UInt32

public func +(lhs: Int32, rhs: Int32) -> Int32

public func +(lhs: UInt64, rhs: UInt64) -> UInt64

public func +(lhs: Int64, rhs: Int64) -> Int64

public func +(lhs: UInt, rhs: UInt) -> UInt

public func +(lhs: Int, rhs: Int) -> Int

In your case your reduce function should look as the following one

func reduce(sign: (Int,Int)->Int) -> Int{

    return sign(2,3)
}

reduce(*)
reduce(-)