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Showing posts from September, 2007

Visitor Pattern Deprecated: First-Class Messages Are All You Need!

In previous posts [1,2], I argued that the visitor pattern was a verbose, object-oriented equivalent of a pattern matching function. The verbosity stems from the need to add the dispatching code to each data class in the hierarchy, and because the encapsulation inherent to objects is awkward when dealing with pure data.

A single addition to an OOP language could completely do away with the need for the dispatching code and make OO pattern matching simple and concise: first-class messages (FCM). By this I mean, messages sent to an object are themselves 'objects' that can be passed around as parameters.

To recap, functional languages reify the structure of data (algebraic data types), and they abstract operations (functions). OOP languages reify operations (interfaces), but they abstract the structure of data (encapsulation). They are duals of one another.

All the Exp data classes created in [1,2] shouldn't be classes at all, they should be messages that are sent to the IVisito…