In this chapter we take our first proper steps with Haskell. We start by introducing the Hugs system and the standard prelude, then explain the notation for function application, develop our first Haskell script, and conclude by discussing a number of syntactic conventions concerning scripts.

The Hugs system

As we saw in the previous chapter, small Haskell programs can be executed by hand. In practice, however, we usually require a system that can execute programs automatically. In this book we use an interactive system called Hugs, which is the most widely used implementation of Haskell.

The interactive nature of Hugs makes it well suited for teaching and prototyping, and its performance is sufficient for most applications. However, if greater performance or a stand-alone executable version of a program is required, a number of compilers for Haskell are also available, of which the most widely used is the Glasgow Haskell Compiler. This compiler also has an interactive version that operates in a similar manner to Hugs, and can readily be used in its place for the purposes of this book.

The standard prelude

When the Hugs system is started it first loads a library file called Prelude.hs, and then displays a > prompt to indicate that the system is waiting for the user to enter an expression to be evaluated.

This book is about an approach to programming in which simplicity, clarity, and elegance are the key goals. More specifically, it is an introduction to the functional style of programming, using the language Haskell.

The functional style is quite different to that promoted by most current languages, such as Java, C++, C, and Visual Basic. In particular, most current languages are closely linked to the underlying hardware, in the sense that programming is based upon the idea of changing stored values. In contrast, Haskell promotes a more abstract style of programming, based upon the idea of applying functions to arguments. As we shall see, moving to this higher-level leads to considerably simpler programs, and supports a number of powerful new ways to structure and reason about programs.

The book is primarily aimed at students studying computing science at university level, but is also appropriate for a broader spectrum of readers who would like to learn about programming in Haskell. No previous programming experience is required or assumed, and all the concepts are explained from first principles, with the aid of carefully chosen examples.

In this chapter we show how Haskell can be used to write interactive programs. We start by explaining what interactive programs are, show how such programs can naturally be viewed as functions, define a number of basic interactive programs and higher-order functions for combining interactive programs, and conclude by developing a calculator and the game of life.

Interaction

A batch program is one that does not interact with the user while it is running. In the early days of computing, most programs were batch programs, run in isolation from their users in order to maximise the amount of time that the computer was performing useful work. For example, a route-planning program may take start and finish points as its input, silently perform a large number of calculations, and then produce a recommended route as its output.

Up to this point in the book we have considered how Haskell can be used to write batch programs. In Haskell such programs, and more generally all programs, are modelled as pure functions that take all their input as explicit arguments, and produce all their output as explicit results. For example, a route planner may be modelled as a function of type (Point, Point) → Route that takes a pair of points and produces a route between them.

In contrast, an interactive program is one that may take additional input from the user, and produce additional output for the user, while the program is running.