A dictionary object is a data structure used to index
information in a user-defined way. In standard Scheme, the main
aggregate data types are lists and vectors. Lists are not really
indexed at all, and vectors are indexed only by number
(e.g. (vector-ref foo 5)). Often you will find it useful
to index your data on some other type; for example, in a library
catalog you might want to look up a book by the name of its
author. Dictionaries are used to help you organize information in
such a way.
An association list (or alist for short) is a list of
key-value pairs. Each pair represents a single quantity or
object; the car of the pair is a key which is used to
identify the object, and the cdr is the object’s value.
A hash table also permits you to index objects with arbitrary keys, but in a way that makes looking up any one object extremely fast. A well-designed hash system makes hash table lookups almost as fast as conventional array or vector references.
Alists are popular among Lisp programmers because they use only the language’s primitive operations (lists, car, cdr and the equality primitives). No changes to the language core are necessary. Therefore, with Scheme’s built-in list manipulation facilities, it is very convenient to handle data stored in an association list. Also, alists are highly portable and can be easily implemented on even the most minimal Lisp systems.
However, alists are inefficient, especially for storing large quantities of data. Because we want Guile to be useful for large software systems as well as small ones, Guile provides a rich set of tools for using either association lists or hash tables.