Real World Haskell

Chapter 3. Defining Types, Streamlining Functions

Defining a New Data Type

Although lists and tuples are useful, we’ll often want to construct new data types of our own. This allows us to add structure to the values in our programs. Instead of using an anonymous tuple, we can give a collection of related values a name and a distinct type. Defining our own types also improves the type safety of our code: Haskell will not allow us to accidentally mix values of two types that are structurally similar but have different names.

For motivation, we’ll consider a few kinds of data that a small online bookstore might need to manage. We won’t make any attempt at complete or realistic data definitions, but at least we’re tying them to the real world.

We define a new data type using the data keyword:

-- file: ch03/BookStore.hs
data BookInfo = Book Int String [String]
                deriving (Show)

BookInfo after the data keyword is the name of our new type. We call BookInfo a type constructor. Once we define a type, we will use its type constructor to refer to it. As we’ve already mentioned, a type name, and hence a type constructor, must start with a capital letter.

The Book that follows is the name of the value constructor (sometimes called a data constructor). We use this to create a value of the BookInfo type. A value constructor’s name must also start with a capital letter.

After Book, the Int, String, and [String] that follow are the components of the type. A component serves the same purpose in Haskell as ...