John Carmack on mutable variables

> I wish it was the default, and mutable was a keyword.

I wish the IDE would simply provide a small clue, visible but graphically unobtrusive, that it was mutated.

In fact, I end up wishing this about almost every language feature that passes my mind. For example, I don't need to choose whether I can or can't append to a list; just make it unappendable if you can prove I don't append. I don't care if it's a map, list, set, listOf, array, vector, arrayOf, Array.of(), etc unless it's going to get in my way because I have ten CPU cores and I'll optimize the loop when I need to.

In my IntelliJ (a recent version), if I write a small Java function like this:

    private static void blah()
    {
        final int abc = 3;
        for (int def = 7; def < 20; ++def)
        {
            System.out.print(def);
        }
    }

The variable 'def' is underlined. Mouse-over hint shows: 'Reassigned local variable'. To be clear, 'abc' is not underlined. When I write Java, I try to use the smallest variable scopes possible with as much final (keyword) as possible. It helps me to write more maintainable code, that is easier to read.