The Haskell School of Expression Learning Functional Programming through Multimedia

In the previous chapter you learned quite a few techniques for problem solving via calculation in Haskell It's time now to apply these ideas to a larger example, which will require learning even more problem-solving skills and Haskell language features.

Our job will be to design a simple module of geometric shapes, that is, a collection of functions and data types for computing with geometric shapes such as circles, squares, triangles, and others. Users of this module will be able to create new instances of geometric shapes and compute their areas. You will learn lots of new things in building this module, including how to design your own data types. Then in Chapter 4 we will extend this functionality with the ability to draw geometric shapes, and in Chapter 6 compute their perimeters.

In the description above I refer to the end product as a module, through which a user has access to certain well-defined functionalities. A module can be seen as a way to conveniently wrap up an application in such a way that only the functionality intended for the end-user is visible; everything else needed to implement the system is effectively hidden.

In Haskell we can create a module named Shape in the following way:

The “(…)” after the name Shape will ultimately be a list of names of the functions and data types that the end-user is intended to use, and is sometimes called the interface to a module. At the end of this chapter we will fill in the details of the interface once we know what they should be. The …body-of-module… is of course where we will place all the code developed in this chapter.

DETAILS

Module names must always be capitalized (just like type names).

A user of our shape module can later import it into a module that he or she is constructing, by writing:

Indeed, we will do exactly this in later chapters where new modules will be created in which users will be able to draw shapes, compute their perimeters, combine them into larger “regions,” color them, scale them, and place them on a “virtual desktop.”