In this chapter, we explore, in more detail, the software development methodology that is used with CDTs. It is a methodology based on transformation. Many of the transformations that are useful for list programming were already known informally in the Lisp community, and more formally in the APL and functional programming community. The chief contributions of the categorical data type perspective are:

• a guarantee that the set of transformation rules is complete (which becomes important for more complex types); and

• a style of developing programs that is terse but expressive.

This style has been extensively developed by Bird and Meertens, and by groups at Oxford, Amsterdam, and Eindhoven. A discussion of many of the stylistic and notational issues, and a comparison of the Bird–Meertens approach with Eindhoven quantifier notation, can be found in [17]. Developments in the Bird-Meertens style are an important interest of IFIP Working Group 2.1.

An Integrated Software Development Methodology

A software development methodology must handle specifications that are abstract, large, and complex. The categorical data type approach we have been advocating plays only a limited role in such a methodology because it is restricted (at the moment) to a single data type at a time. Although it is useful for handling the interface to parallel architectures, it is too limited, by itself, to provide the power and flexibility needed for large application development.