Show description

A collection (implements a Factory Method [35] to return a separate subobject that) implements the Iterator interface to accept an Action, apply it to each element in turn, and replace the original elements with the possibly new ones returned. Internal Iterators are less flexible than external — for example, it is more difficult to have two linked iterations over the same collection, and to terminate an iteration early — but they are correspondingly simpler to use. Visitor. In the normal object-oriented paradigm, the definition of each traversal operation is spread across the whole class hierarchy of the structure being traversed — typically but not necessarily a Composite.

When the producer is an unfold, the composition of producer and consumer is (under certain mild strictness conditions) a hylomorphism. hylo :: Bifunctor s ⇒ (b → s a b) → (s a c → c) → b → c hylo f g = g ◦ bimap id (hylo f g) ◦ f More generally, but harder to reason with, the producer is a build, and the composition replaces the constructors in the builder by the body of the fold. 36 J. Gibbons foldBuild :: Bifunctor s ⇒ (∀b. ) Once again, both of these definitions are datatype-generic; both take as arguments a producer f and a consumer g, both with types parametrized by the shape s of the product to be built.

Finally, the Builder pattern ‘separates the construction of a complex object from its representation, so that the same construction process can create different representations’. As Figure 9 shows, this is done by delegating responsibility for the construction to a separate Builder object — in fact, an instance of the Strategy pattern [35], encapsulating a strategy for performing the construction. Director Builder 1 +addPart():void Product ConcreteBuilder +addPart():void +getProduct():Product Fig.