Why can't I use [1...10] ?

The 1...10 part creates what is called a sequence, and what you are doing is creating an array with one item only - that sequence. In this case it's just as easy to just type out the numbers, but if you do want to create an array based on values in a sequence, you can use the method map() on a sequence which, like filter(), is a built in higher order method that takes another method as an argument. In your case:

func returnIntUnchanged(i:Int) -> Int {
    return i
}
let array = (1...10).map(returnIntUnchanged)

You don't even have to actually type out the function, but instead you can pass it directly as a block like so:

let array = (1...10).map({(i:Int) -> Int in return i})

Of course inside that function or block you can do anything you like with the sequence, and you don't have to return an int...

Ps. as it turns out, there are even simpler ways... but it's not a bad thing to understand the mechanisms above.

let array = (1...10).map{$0}   //you can still "do stuff" to the values in the sequence here
let array = [Int](1...10)