If in your first attempt (with unless @!items) you instead use: say "Items: " ~ $hamper.items.perl; you will get

Name: Christmas Basket
Items: Array.new(:shape(3,), [Any, Any, Any])

[download]

The code has @.items[3] is rw; creates a positional attribute which contains a fixed-length array with 3 elements. Thus, the array is truthy and the unless is not triggered.

When you change to @!items := ('Mince Pie', 'White Wine', 'Stinky Cheese', 'Sardines', 'Dogfood'); you replace the fixed-kength array with a new List. Since @!items is rw that is allowed -- you aren't modifying the fixed length array, you are replacing it. Since the [3] describes the array, not the attribute, this is all fine -- to raku, though not perhaps to you :).

There may be a way to do this using has, but at some point it is reasonable to write your own accessor:

class Hamper {
    has $.name = 'Christmas Basket';
    has Str @!items;

    method TWEAK(){
        self.items = 'Mince Pie', 'White Wine', 'Stinky Cheese' unless
+ @!items;
    }

    method items() is rw {
        return-rw Proxy.new:
            FETCH => sub ($) { return @!items },
            STORE => sub ($, @items) {
                die "Wrong length" unless @items.elems == 3;
                @!items = @items;
            };
    }
}

my $hamper = Hamper.new;

# $hamper.items = 'Mince Pie', 'White Wine', 'Stinky Cheese', 'Dogfood
+';

say "Name: " ~ $hamper.name;
say "Items: " ~ $hamper.items;

[download]

Good Day,
    Dean

In reply to Re: [Raku] Assigning defaults to attributes that are fixed length lists (and other confusions) by duelafn
in thread [Raku] Assigning defaults to attributes that are fixed length lists (and other confusions) by tomgracey

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