Swift - Declaring a Set using named variable-CodePudding

I am trying to understand Sets in Swift and how to declare them correctly but I have found the following a little confusing.

I understand that this works:

let elements = [ 1, 2, 3, 4, 5, 1, 2, 6, 7]
let setFromElements = Set(elements)

But I don't understand why the following doesn't:

let setFromElements : Set = elements

Or even:

let setFromElements : Set<Int> = elements

When the following is valid:

let setFromArray : Set = [ 1, 2, 4, 5]

Can someone please help me understand why this is the case?

CodePudding user response：

let setFromArray: Set = [ 1, 2, 4, 5] works because Set conforms to ExpressibleByArrayLiteral and hence has an initializer that takes an ArrayLiteral. See Set.init(arrayLiteral:). This conformance gives syntactic sugar for not having to explicitly call the init.

On the other hand, once you save the array literal into a variable using let elements = [ 1, 2, 3, 4, 5, 1, 2, 6, 7], elements becomes an Array, not an ArrayLiteral and hence another initializer of Set has to be called that takes an Array. This init does not provide syntactic sugar like ExpressibleByArrayLiteral does, so you explicitly have to call the init by doing Set(array).

CodePudding user response：

Set has an initializer that takes an array, and that makes a set, by taking the unique items in the array. But a set is not an array, two different types, so you can't just use = to assign one to the other.