This chapter is devoted to an example of how to build a small library in Haskell. A library is an organised collection of types and functions made available to users for carrying out some task. The task we have chosen to discuss is pretty-printing, the idea of taking a piece of text and laying it out over a number of lines in such a way as to make the content easier to view and understand. We will ignore many of the devices for improving the readability of a piece of text, devices such as a change of colour or size of font. Instead we concentrate only on where to put the line breaks and how to indent the contents of a line. The library won't help you to lay out bits of mathematics, but it can help in presenting tree-shaped information, or in displaying lists of words as paragraphs.

Setting the scene

Let's begin with the problem of displaying conditional expressions. In this book we have used three ways of displaying such expressions:

These three layouts, which occupy one, two or three lines, respectively, are considered acceptable, but the following two are not:

The decision as to what is or is not acceptable is down to me, the author.