Recipe 3.6: Adding or Changing an Attribute or Method of a Class at Runtime

Last Updated: Sep 8, 2003 Status: Draft

Problem:  

You want to add an attribute or method to a class, or change the value or implementation of an existing attribute/method, but only under certain (runtime) conditions.

Solution:  

Use code like the following:

class MyClass { ... };     # the original class

MyClass.foo = 5;               # add an attribute
MyClass.bar = method { ... }   # add a method

Discussion:

Perl6 allows you to modify the structure of a class even at runtime, if that class does not specifically forbid it.

There are a number of circumstances in which you may not add an attribute or method to a class, or modify an existing attribute or method. Among them:

• The class has been declared is final
• The attribute or method has been declared is final
• The attribute or method is marked as is interface, and your new value or method does not match the previously defined type or signature.

If you are merely defining a new implementation of a method, but don't want to change the signature of the old method, don't put signature information in the anonymous method constructor:

MyClass.bar = method is prop1, prop2 (... args ...) { ... };  # with signature

MyClass.bar = method { ... };    # uses the existing signature

Issue: Or will we be using MyClass.bar := method { ... }?

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