Why can readonly array attributes on a Raku class be modified?

The baz attribute is read only.

The thing is that the baz attribute is an Array which has mutable elements.

---

When you call $boo.baz you basically get a reference to the array.

Once you have a reference to the array you can do anything that you could normally do to the array.

say $boo.baz.VAR.name; # @!baz

When you assign to an array, what you are really doing is telling the array that it will have new contents.

my @a = ('a','b','c');
say @a.WHICH; # Array|94070988080608

@a = (1, 2, 3);
say @a.WHICH; # Array|94070988080608

Notice that the .WHICH doesn't change. That is because it is still the same array. It just has new contents.

The exact same thing happens when you assign to a public array attribute.
You are not assigning a new array, you are altering the existing one.

All of the behaviours of a variable are not intrinsic to the variable, they are instead handled by an object.
In the case of arrays the object that handles the assignment is Array.

---

The simplest fix is to just overload the autogenerated accessor method.

class Boo {
    has $.bar;
    has @.baz;

    method baz () { @!baz.List }
}

If you never plan on changing the values in @!baz you could make it a List itself

class Boo {
    has $.bar;
    has @.baz is List;
}