快速循环中的 let 或 var

Question

Here is a sample of Swift UI code, a simple loop with a "variable" number value being changed.

    import SwiftUI

struct ContentView: View {
    let a = 1.0
    let a = 2.0 // invalid redeclaration

    var body: some View {
        VStack {
            ForEach (0..<3) {i in
                var number = i+1 // variable number was never mutated - use let instead
                Text("var \(number)")
                }
            ForEach (0..<3) {i in
                let number = i+1
                Text("let \(number)")
                }
//            ForEach (0..<3) {i in
                //number = i+1 // cannot find number in scope
//                Text("nothing \(number)")
//                }
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

The bizarre thing is that :

• "var number" : a recommandation to use "let" because "there are no change in the value of number"
• "let number" : is OK despite it is in a loop and therefore modified several times

Both work but I am surprised that let (designed for constants) is used and even preferred within a loop. Any explanation ?

Answer 1

Paradoxes rely on some misunderstanding and therefore make progressing.

Here, the question is : what is the scope with the for(ForEach). In most languages, it is all the instructions between the for and its end. Here, the scope is each instance of the iteration . Then a let, reserved for constants, is fully adequate since the variable is never modified within each instance of the iteration.

If one wants to cumulate the variable, say number, number must be declared in the environment of the for loop.

Thanks to all who commented.