The power of packages becomes evident when packages have parameters. Usually these parameters are domains and the exported operations have types involving these parameters.
In Chapter ugTypes , you learned that categories denote classes of domains. Although we cover this notion in detail in the next chapter, we now give you a sneak preview of its usefulness.
In ugUserBlocks , we defined functions and to sort a list of integers. If you look at the code for these functions, you see that they may be used to sort any structure with the right properties. Also, the functions can be used to sort lists of any elements---not just integers. Let us now recall the code for .
What properties of ``lists of integers'' are assumed by the sorting algorithm? In the first line, the operation # computes the maximum index of the list. The first obvious property is that must have a finite number of elements. In Axiom, this is done by your telling Axiom that has the ``attribute'' finiteAggregate. An attribute is a property that a domain either has or does not have. As we show later in ugCategoriesAttributes , programs can query domains as to the presence or absence of an attribute.
The operation swap swaps elements of . Using Browse, you find that swap requires its elements to come from a domain of category IndexedAggregate with attribute shallowlyMutable. This attribute means that you can change the internal components of without changing its external structure. Shallowly-mutable data structures include lists, streams, one- and two-dimensional arrays, vectors, and matrices.
The category IndexedAggregate designates the class of aggregates whose elements can be accessed by the notation for suitable selectors . The category IndexedAggregate takes two arguments: , a domain of selectors for the aggregate, and , a domain of entries for the aggregate. Since the sort functions access elements by integers, we must choose Integer. The most general class of domains for which and are defined are those of category IndexedAggregate(Integer,Entry) with the two attributes shallowlyMutable and finiteAggregate.
Using Browse, you can also discover that Axiom has many kinds of domains with attribute shallowlyMutable. Those of class IndexedAggregate(Integer,Entry) include Bits, FlexibleArray, OneDimensionalArray, List, String, and Vector, and also HashTable and EqTable with integer keys. Although you may never want to sort all such structures, we nonetheless demonstrate Axiom's ability to do so.
Another requirement is that Entry has an operation <. One way to get this operation is to assume that Entry has category OrderedSet. By definition, will then export a < operation. A more general approach is to allow any comparison function to be used for sorting. This function will be passed as an argument to the sorting functions.
Our sorting package then takes two arguments: a domain of objects of any type, and a domain , an aggregate of type IndexedAggregate(Integer, S) with the above two attributes. Here is its definition using what are close to the original definitions of and for sorting lists of integers. The symbol ! is added to the ends of the operation names. This uniform naming convention is used for Axiom operation names that destructively change one or more of their arguments.