Page "Dylan (programming language)" Paragraph 47

The implication here is that a programmer can add functionality to existing classes by defining functions in a separate file.

For instance, you might wish to add spell checking to all s, which in most languages would require access to the source code of the string class — and such basic classes are rarely given out in source form.

In Dylan ( and other " extensible languages ") the spell checking method could be added in the module, defining all of the classes on which it can be applied via the construct.

In this case the actual functionality might be defined in a single generic function, which takes a string and returns the errors.

When the module is compiled into your program, all strings ( and other objects ) will get the added functionality.