I see. I know nothing about Nim but I do see in their example they used var when declaring the function. This way seems like it's hidden to the consumer of the function but hopefully editor tooling helps with this.

You have to declare the variable as `var` as well, eg.

    var arr1 = [4, 2, 3, 6]
    sort arr1

    # Doesn't work
    let arr2 = [4, 2, 3, 6]
    sort arr2

You'll get a warning about the latter from the compiler (and nimsuggest for editors).