Изучай Haskell ради Добра! Создание своих собственных типов и классов типов

Learn You a Haskell for Great Good! Making Our Own Types and Typeclasses

Making Our Own Types and Typeclasses

In the previous chapters, we covered some existing Haskell types and typeclasses. In this chapter, we'll learn how to make our own and how to put them to work!

Algebraic data types intro

So far, we've run into a lot of data types. Bool, Int, Char, Maybe, etc. But how do we make our own? Well, one way is to use the data keyword to define a type. Let's see how the Bool type is defined in the standard library.

data Bool = False | True

data means that we're defining a new data type. The part before the = denotes the type, which is Bool. The parts after the = are value constructors. They specify the different values that this type can have. The | is read as or. So we can read this as: the Bool type can have a value of True or False. Both the type name and the value constructors have to be capital cased.

In a similar fashion, we can think of the Int type as being defined like this:

data Int = -2147483648 | -2147483647 | ... | -1 | 0 | 1 | 2 | ... | 2147483647

The first and last value constructors are the minimum and maximum possible values of Int. It's not actually defined like this, the ellipses are here because we omitted a heapload of numbers, so this is just for illustrative purposes.

Now, let's think about how we would represent a shape in Haskell. One way would be to use tuples. A circle could be denoted as (43.1, 55.0, 10.4) where the first and second fields are the coordinates of the circle's center and the third field is the radius. Sounds OK, but those could also represent a 3D vector or anything else. A better solution would be to make our own type to represent a shape. Let's say that a shape can be a circle or a rectangle. Here it is:

data Shape = Circle Float Float Float | Rectangle Float Float Float Float

Now what's this? Think of it like this. The Circle value constructor has three fields, which take floats. So when we write a value constructor, we can optionally add some types after it and those types define the values it will contain. Here, the first two fields are the coordinates of its center, the third one its radius. The Rectangle value constructor has four fields which accept floats. The first two are the coordinates to its upper left corner and the second two are coordinates to its lower right one.