Closures makes no sense! Please help me understand..

Question

Hi everybody
I am currently on the mathOperation challenge in the closures course. But I simply cannot understand it. I get that I need to define a function called mathOperations that takes in a function - for example "myMathOp". This function should return (Int, Int) and then the "second" return type should also be Int (but the function returns a function?). Aaaaand, that's as far as my brain will go.
I don't understand this whole crazy line: 
Swift
(Int, Int) -> Int, _ a: Int, _ b: Int)

(the example from https://developer.apple.com/library/ios/documentation/Swift/Conceptual/SwiftProgrammingLanguage/Functions.html#//apple_ref/doc/uid/TP40014097-CH10-ID160)
I need some really patient explaining here please? :-)
Also - when and why do we use void?

Brendan Whiting · Accepted Answer

Maybe it will help if I use the "machine" analogy.  So let's say I have this machine, a blender.  It takes some inputs: banana, juice, strawberries, it carries out a series of instructions and produces an output: smoothie.  Now this might be a weird analogy, but let's imagine I build another machine: let's call it RobotCook.  It takes ingredients, but it doesn't have a default task that it does with those ingredients.  It has a spot where it requires you to stick in another machine.  So if you stick a blender into that spot, it will make your ingredients into a smoothie.  If you stick a waffle maker into that spot, it will make your ingredients into waffles.  
So in this challenge, the function mathOperation is going to be like that RobotCook.  It will perform some math operation on two numbers, but we don't know yet what that operation will be.  It could be add, subtract, multiply, anything.  When you call the function, in addition to passing in two numbers as arguments, it requires you to pass in a function as an argument.  This will be the operation it performs on those two numbers.  But it can't just be any kind of function.  It has to be a function that takes in two integers as arguments, and returns one integer as its output.  Otherwise our machine would break - it has to fit these constraints.  Swift is designed so that we have to explicitly say what kind of function mathOperation will accept.
Here is my code:
Swift
func mathOperation(mathOp: (Int, Int) -> Int, _ a: Int, _ b:Int) -> (Int) {
  return mathOp(a, b)
}

The function mathOperation has three parameters: 
one called mathOp, which is a function.  This function takes two Ints and returns an Int.
one called a, which is an Int
one called b, which is an Int
I've put the _ before the a and b parameter names because then it doesn't get messy calling them by name later.  
The return value of mathOperation is an Int
Inside mathOperation, it will return the result of calling mathOp with the arguments a and b.
mathOp could end up being anything we pass in.  In the next step of the challenge, we pass in the function differenceBetweenNumbers, but it could have been any other operation as long as it fits the constraints that we spelled out.  
Let me know if you need more explanation.

Michael Reining · Answer

Hi Marina,
Yes, this can be confusing indeed!  
Here is how I thought about it.  
```swift
func differenceBetweenNumbers(a: Int, b:Int) -> (Int) {
    return a - b
}
// Think of a function as just another TYPE such as Int, String, etc.
// So in this case, you want a function called mathOperation() and the
// first parameter is of type (Int, Int) -> Int followed by to Ints.
// so you end up with this...
func mathOperation(mathOp: (Int, Int) -> Int, a: Int, b: Int) -> Int{
    return mathOp(a,b)
}
// Calling the function
let difference = mathOperation(differenceBetweenNumbers, a: 12, b: 6)
// NOTE: Part 2 of the challenge is a bit tricky because it does not accept
// the parameter lables which as of swift 2.1 are actually required.
// So enter the code below into the code challenge
let difference mathOperation(differenceBetweenNumbers, 12, 6)
```
I hope that helps.  In regards to your question about void... void just
means that nothing is being returned.  
```swift
func returnNothing(myString: String) {
    print(myString)
}
func returnWithVoid(myString: String) -> Void {
    print(myString)
}
returnNothing("Test String")
returnWithVoid("Test String")
// Void is required for functions with closures like this
// Another way to think about it is that if you include a
// function that returns nothing, then you have to use void
func exampleClosure(completion: (result: String) -> Void) {
        completion(result: "we finished!")
}
// Call closure
exampleClosure() {
    (result: String) in
    // Now execute any code you like...
    print("got back: (result)")
}
```
In the examples above, you have two functions that will print something
to the console.  Both return void but you don't have to write that in Swift.
Void, however, has to be used if you have a function that includes another
function that returns nothing.
I hope that helps a little,
Mike
PS: Thanks to the awesome resources on Team Treehouse, I just launched my first app. :-)
Code! Learn how to program with Swift (https://itunes.apple.com/app/code!-learn-how-to-program/id1032546737?mt=8)

Jhoan Arango · Answer

Hello Marina:
After reading the answers above, you can check my answer (https://teamtreehouse.com/community/im-stuck-at-this-one), from someone else's question. They were having difficulties understanding as well. 
The focus on this particular challenge is that you are not doing a "closure" per say, you are working with higher order functions. Which are functions that take "other" functions as arguments. Not only that, but you are also working with function type.
Function types are a topic of its own, that once you understand that, then closures will be an easy topic for you. 
I recommend reading this book (https://itun.es/us/k5SW7.l), and search in it "function type". 
Good luck and let me know if my answer helped you or if you need further explanation.

Ken Alger · Answer

Marina;
Take a look at the docs (https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html) on closures in Swift. 
Post back if you have additional questions.
Happy coding,  
Ken

Marina Alenskaja · Answer

Hi guys!
Thank you so much for your responses, they are awesome!! It definitly helps me understand. 
I love this community!! :-)

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Marina Alenskaja

Marina Alenskaja

Closures makes no sense! Please help me understand..

Marina Alenskaja

Marina Alenskaja

5 Answers

Brendan Whiting

Brendan Whiting

Marina Alenskaja

Marina Alenskaja

Brendan Whiting

Brendan Whiting

Andy Chen

Andy Chen

Michael Reining

Michael Reining

Ken Alger

Ken Alger

Jhoan Arango

Jhoan Arango

Marina Alenskaja

Marina Alenskaja