Function with throws doesn't exit the function with executes trow - swift

Question

I have the following function with throws:
func doSomething(_ myArray: [Int]) throws -> Int {
if myArray.count == 0 {
throw arrayErrors.empty
}
return myArray[0]
}
But when the array is equal to 0 it will throw the error but it will continue executing the function returning empty array.
How can I exit the function when goes to the if statement ?

Answer 1

The function does return when you throw an error. Pop this in a playground and look at the output.
//: Playground - noun: a place where people can play
enum MyError: Error {
case emptyArray
}
func doSomething<T>(_ array: [T]) throws -> T {
guard !array.isEmpty else {
throw MyError.emptyArray
}
return array[0]
}
do {
let emptyArray: [Int] = []
let x = try doSomething(emptyArray)
print("x from emptyArray is \(x)")
} catch {
print("error calling doSeomthing on emptyArray: \(error)")
}
do {
let populatedArray = [1]
let x = try doSomething(populatedArray)
print("x from populatedArray is \(x)")
} catch {
print("error calling doSeomthing on emptyArray: \(error)")
}
You'll see the output is
error calling doSeomthing on emptyArray: emptyArray
x from populatedArray is 1
Note that you don't see the output from print("x from emptyArray is \(x)") because it was never called, because throw ends the execution of that function. You can also confirm this because guard statements require the function to exit.
Also, just to note, if you want the first thing from an array you can just use myArray.first which will return T?, and you can handle the nil case instead of having to deal with an error. For example:
//: Playground - noun: a place where people can play
let myArray = [1, 2, 3]
if let firstItem = myArray.first {
print("the first item in myArray is \(firstItem)")
} else {
print("myArray is empty")
}
let myEmptyArray: [Int] = []
if let firstItem = myEmptyArray.first {
print("the first item in myEmptyArray is \(firstItem)")
} else {
print("myEmptyArray is empty")
}
which outputs:
the first item in myArray is 1
myEmptyArray is empty

Answer 2

You need to understand error handling. If you insert a throws keyword, anybody calling it will need to use a do-catch statement, try?, try! or continue to propagate them. So what will happen if the error is thrown is up to the caller.
Here's an example:
do {
try doSomething(foo)
} catch arrayErrors.empty {
fatalError("Array is empty!")
}
Apple's documentation for error handling
If you want to exit once you reach the if, simply don't use error handling and call fatalError inside the if.
if myArray.count = 0 {
fatalError("Error happened")
}