Most fancy

On Discord (yes, we are that cool) MrDispatch wished for a way to collect a bunch of classes right after their declaration. I believe, with the power of the MOP, we can go a step further and register the type-object even before the definition is finished.

class MetamodelX::RegisteredHOW is Metamodel::ClassHOW {
    our @registered;

    method new_type(|) {
        my \type = callsame;
        type.HOW.remember_type(type);

        type
    }

    method remember_type(Mu \type) {
        @registered.push: type
    }

    method recall-types {
        @registered;
    }
}

sub recall-types is export(:recall-types) {
    MetamodelX::RegisteredHOW.recall-types;
}

my package EXPORTHOW {
    package DECLARE {
        constant registered = MetamodelX::RegisteredHOW;
    }
}

We introduce a new declarator registered that hooks up a new meta-class. It would be possible to overload class to register all classes in a compilation unit. For now, doing so would not be good conduct, because playing with EXPORTHOW is not quite lexical yet. The list of declared type objects will reside as a class-attribute inside the meta-class. We can access it through any type object or instance via .HOW or with the exported sub recall-types.

use v6.*;

use Registered :recall-types;

registered Foo {
    method answer { 42 }
    method common { self.^name }
}

registered Bar {
    method ohai { ‚Hello Universe!‘ }
    method common { self.^name }
}

Foo.HOW.recall-types.say;
say Foo.new.HOW.recall-types;
say recall-types; # requires the import adverb :recall-types;
say recall-types».common;

# OUTPUT: [(Foo) (Bar)]
#         [(Foo) (Bar)]
#         [(Foo) (Bar)]
#         [Foo Bar]

There are plenty of other ways to implement a registry of types. Using a custom meta-class is the most fancy and flexible way. We could hook into many other things here too. Rakudo is a dynamic compiler for a dynamic language. It uses introspection much more then you do. Sometimes it introspects a package called EXPORTHOW for a key called DECLARE to create a keyword that is more classy then class.