Iterating over a collection in Swift: var vs. let

Question

I have a method that iterates over an array and call other method with every element as argument. If I declare this method as:

func didFinishedListFiles(files: [FileModel]) {
    for var fileData in files {
        self.downloadSingleFile(NSUUID(UUIDString: fileData.uuid!)!);
    }
}

Xcode shows a warning:

Variable 'fileData' was never mutated; consider changing to 'let' constant

But if I change var to let:

func didFinishedListFiles(files: [FileModel]) {
    for let fileData in files {
        self.downloadSingleFile(NSUUID(UUIDString: fileData.uuid!)!);
    }
}    

Xcode shows an error:

'let' pattern cannot appear nested in an already immutable context

How is a correct way to implement it without any warnings/errors?

Answer 1

The for-in pattern implicitly uses a constant binding (in the scope it creates. That is, your fileData binding is automatically a local let, and therefore constant for each pass through the loop.

So the following:

for fileData in files { /*...*/ }

...is equivalent to :

var index = 0
while index < files.count {
    let fileData = files[index]
    //...
    index += 1
}

You'd want to add var to the for-in binding only when you want to mutate that binding -- that is, if it's an object reference that you want to be able to point at something else during a single pass through the loop, or a value type that you want to be able to change. But it doesn't look like you're doing either of those things, so using var for this binding would be superfluous here.

(Swift 3 got rid of a lot of the places where you could make implicitly immutable bindings mutable, but left for var as an exception — it's still possible if you want to change something during a loop.)