Functions are small modules or chunk of code. Swift’s function syntax is so flexible that it allows to write a function inside a function.

Here is a syntax:

func firstFunction(arguments) -> returnType{

	func secondFunction(arguments) -> returnType{


		func thirdFunction(arguments) -> returnType{

			and so on.
		}
	}
}

Here is an example(code) from above syntax:

func addRandomNumAndCheckIfOdd(inout passedNumber: Int) -> Bool {
        
	passedNumber +=  Int(arc4random_uniform(UInt32(10))); // adding random number between 0-9 to passed number
        
        // function will check if the obtained number is Odd or not
	func isOdd(numToCheck:Int)->Bool{

		if numToCheck%2 == 0 {
                
			return false
		}
		return true
	}
	return isOdd(passedNumber)
}

In the above example we have created a function “addRandomNumAndCheckIfOdd” which will add a random number and check if it’s odd or not.

We have passed an argument ‘passedNumber’ with reference so that it can be modified. A random number ranging from 0-9 will be added to passed number.
while returning we are checking if the modified number is odd or not using function inside a function flexibility provided by swift.

Function inside a function will not be executed until and unless it is called, compiler will skip the function otherwise.