$fish.scales = ('green','blue','yellow');
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Interestingly, it also works if I declare the attribute as readonly.

See also StackOverflow.

https://docs.raku.org/type/Attribute#method_get_value
https://docs.raku.org/type/Attribute#method_set_value

Please learn how to format your posts properly.

1. the sigil - @ (or $, %, &)- provides guidance on the content of your attribute container - in the case of @ it does role Positional
2. the twigil - . or ! - provides guidance on the encapsulation, a "public attribute" with '.' has a getter accessor method automatically generated - with 'is rw' both getter and setter methods are provided
3. the actual contents - in this case an Array - and each item in an Array is a scalar container that may be assigned a value

• the twigil and is rw trait apply at the attribute container level - so you can assign an entirely different Array to your attribute if it is marked 'is rw', regardless of this, when you have the Array (even if the container is read only) individual items are still read/write and may be assigned values, sliced, pushed/popped (unless a where constraint or type subset is provided) - you can assign a List if you want immutability
• the generated accessor method therefore applies at the top level - and, as with $ scalars, the setter method provides an lvalue that can be assigned to with the '=' operator so $fish.scales = ('green','blue','yellow'); is fine
• raku public attributes are (deliberately) simple - so that is all you get from the '.' twigil ... I think that the idea is to make raku OO be very lightweight in the role of random data structure, but to have encapsulation also be very light and to kick in for all use cases that have even just the slightest type management demands - the colon formulation of the method call is awesome for this $fish.scales: ('green','blue','yellow');

  1 class Fish {
  2     has @!scales;
  3     
  4     multi method scales { @!scales }
  5     multi method scales( @new ) { @!scales = @new }
  6     method raku { "[ scales => {@!scales} ]" }
  7 }   
  8 
  9 my $fish = Fish.new;
 10 $fish.scales: ('green','blue','yellow');
 11 
 12 dd $fish;
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When you declare a public attribute (with the . twigil) you are telling it (among other things) to create a method with the same name.

class Fish {
    has @.scales;
}
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class Fish {
  has @!scales;
  method scales () { return @!scales }

  # other stuff it does (not exhaustive)
  submethod BUILD ( :@!scales ){}
  method gist () { … }
  method Capture () { \( :@!scales ) }
}
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When you declare it as is rw, you are telling it that the generated method should also be marked as is rw.

class Fish {
    has @.scales is rw;
}
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class Fish {
  has @!scales;
  #                v---v
  method scales () is rw { return-rw @!scales }

  # other stuff it does (not exhaustive)
  submethod BUILD ( :@!scales ){}
  method gist () { … }
  method Capture () { \( :@!scales ) }
}
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So then when you assign to the method, you replace the contents of the array.

my $fish = Fish.new;

$fish.scales = ('green','blue','yellow')
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If you instead wanted to handle it with the parameter list, you would replace the generated one with a method you created yourself.

(You still want to make it public with . so that the other stuff happens)

class Fish {
    has @.scales;
    multi method scales () { @!scales }
    multi method scales ( +@new ) { @!scales = @new }
}

my $fish = Fish.new;

$fish.scales('green','blue','yellow');
say $fish.scales(); # does not clear it
$fish.scales(()); # clears it
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You can do it with an only method if you prefer.

class Fish {
    has @.scales;
    method scales ( |c (+@new) ) {
        @!scales = @new if c;
        return @!scales;
    }
}

my $fish = Fish.new;

$fish.scales('green','blue','yellow');
say $fish.scales(); # does not clear it
$fish.scales(()); # clears it
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The |c is necessary so that an empty list given as the first and only argument will clear @!scales, but a truly empty argument list doesn't.

There is one thing that you should know, Array and Hash attributes currently don't need is rw to be mutable. They are mutable already.

class Fish {
    has @.scales;
}

my $fish = Fish.new;

$fish.scales = ('green','blue','yellow');
say $fish.scales();
$fish.scales = (); # clears it
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