Implementing the Iterator Design Pattern in Lisp: Tasks

Iterators

• Define the generic function mapover, and define a mapover method for lists.
• Define generic functions make-cursor-for, cursor-next and cursor-finished-p. Define methods for each of them for lists. Note: it is possible, and rather natural in Lisp, to define cursors as functions. This saves the proliferation of classes that are often seen in other languages - list-iterator, hashtable-iterator. See if you can work out how to do this. Remember that you can define methods for built-in types in Lisp, including functions.

Childed collections

• Define simple-childed-mixin. Implement its children as a list in its children slot.
• Define generic functions add-child, delete-child and delete-children-if. Define methods for these for simple-childed-mixin. Ensure when defining add-child that your method ensures that children within a given collection (instance of simple-childed-mixin) are unique when compared using eql.
• Implement the iterator protocol for this class, both internal and external. You may want to use your knowledge that the internal representation of the children is as a list, but be careful not to reveal this list to the users of the interface.
• Check that your implementation works! In particular make sure that the objects can only store one copy of each child, and make sure they adhere to the protocols.
• Discuss the following:
  • What are the shortcomings of this (secret) list-based implementation?
  • Can you think of a way of implementing the same protocol which might have better performance?
  • Would changing the protocol itself overcome these shortcomings?

Authors: Gail Anderson (ga@cley.com), Tim Bradshaw(tfb@cley.com), Cley Limited.
Copyright 1999–2003 Cley Ltd. & Franz Inc.