Weirdness of SwiftUI's ForEach Constructor with Ranges

April 25, 2023

In SwiftUI, when using a Range of integers with ForEach, a constant range can be used as an argument without an id, but a non-constant range requires an id parameter. This is because the compiler considers the range non-constant at runtime even if it’s declared using let. Example:

// No warning:
ForEach(0..<10) { number in
    Text("\(number)")
}

// Warning: Non-constant range
let numbers = 0..<10
ForEach(numbers) { number in
    Text("\(number)")
}

// No warning with `id` parameter
let numbers = 0..<10
ForEach(numbers, id: \.self) { number in
    Text("\(number)")
}