Functional Programming Patterns in Scala and Clojure

Pattern 10

Replacing Visitor

Intent

To encapsulate an action to be performed on a data structure in a way that allows the addition of new operations to the data structure without having to modify it.

Overview

A common sticking point in large, long-lived programs is how to extend a data type. We want to extend along two dimensions. First, we may want to add new operations to existing implementations of the data type. Second, we may want to add new implementations of the data type.

We’d like to be able to do this without recompiling the original source, indeed, possibly without even having access to it. This is a problem that’s as old as programming itself, and it’s now known as the expression problem.

For example, consider Java’s Collection ...