The Higher Order Functions assignment is not recognizing my answer as correct, when XCode Playground does

I solved it. But I'm not 100% why I solved it. I think it has to do with:

1) Your (and my) Xcode is now operating Swift 2.0. This course is not. There is a Swift 2.0 track, but it doesn't have a section on closures yet.

2) Something changed in Swift 2.0 with regards to whether you need to call parameters by name.

But anyway, I solved it by going back into the functions we defined in the previous steps and putting the underscore before all the parameters after the first one, so that I don't have to call them by name:

func differenceBetweenNumbers(a: Int, _ b:Int) -> (Int) {      // now an underscore before b: 
    return a - b
}

func mathOperation(mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) -> Int {     // now underscores before a and b
    return mathFunction(a, b)
}

let difference = mathOperation(differenceBetweenNumbers, 3, 4)

P.S. Looking again at your code, there's something wrong with the implementation of your mathOperation function. You're just calculating the difference between numbers explicitly just the same as in differenceBetweenNumbers. You need to run the two arguments of mathOperation through the mathFunction - which is the name of the function parameter we're defining. This placeholder function could theoretically could do anything to the two numbers. It will calculate the difference only later because we pass in the differenceBetweenNumbers function as an argument.