Course of Raku / Objects, I/O, and exceptions / Classes and objects / The metaobject protocol 🆕

Adding methods at runtime

The MOP is not only for inspection — it can also change a type while the program runs. The meta-method .^add_method adds a new method to a class:

class Empty {
}

Empty.^add_method('greet', method { 'hi' });

say Empty.new.greet; # hi

The class Empty is defined with no methods at all. Empty.^add_method('greet', …) then attaches a method named greet, given as an anonymous method { … }. From that point on, every Empty object responds to .greet, just as if it had been written into the class.

This is genuine meta-programming: the metaobject for Empty is being told to grow a new method. The same protocol underlies features you have already used — conceptually, when you write class, role, has, or method, the compiler builds your type through MOP methods like .^add_method and .^add_attribute on your behalf.

“Conceptually”, because this is a model of the language, not a promise about any one compiler. The MOP is what the object system is specified in terms of; whether a given implementation literally routes every declaration through those meta-methods — and whether you can observe it doing so — is up to that implementation. For the same reason, calling these meta-methods yourself is implementation-dependent: which ones a compiler exposes, and exactly how they behave, can differ, so code that reaches for .^add_method and friends may not run the same way on every Raku.

Adding methods at runtime is a powerful and rarely needed tool. Most programs never reach for it, but it is what makes Raku’s object system open: the rules of classes are themselves written in terms of the MOP, so you can extend them. For everyday code, the inspection meta-methods of the previous topic are the part of the MOP you will actually use.

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