General Functions with Typeclasses

Last week, we looked at the basics of Haskell’s data types. We saw that haskell is not an object oriented language, and we don’t have inheritance between data types. This would get very confusing with all the different constructors that a data type can have. Haskell gives a lot of the same functionality as inheritance by using Typeclasses. This week we’ll take a quick look at this concept.

What is a Typeclass?

A typeclass encapsulates functionality that is common to different types. In practice, a typeclass describes a series of functions that you expect to exist for a given type. When these functions exist, you can create what is called an “instance” of a typeclass.

Typeclasses are a lot like interfaces in Java. You specify a group of functions, but only with the type signatures. Then for each relevant type, you'll need to specify an implementation for each function. As an example, suppose we had two different types referring to different kinds of people.

data Student = Student String Int

data Teacher = Teacher
  { teacherName:: String
  , teacherAge:: Int
  , teacherDepartment :: String
  , teacherSalary :: Int
  }

We could then make a typeclass called IsPerson. We'll give it a couple functions that refer to the name and age of the person. Then we parameterize the class by the single type a. We'll use that type parameter in the type signatures of the functions:

class IsPerson a where
  personName :: a -> String
  personAge :: a -> Int

Creating Instances of Typeclasses

Now let's create an instance of the typeclass. All we have to do is implement each function under the instance keyword:

instance IsPerson Student where
  personName (Student name _) = name
  personAge (Student _ age) = age

instance IsPerson Teacher where
  personName = teacherName
  personAge = teacherAge

There are a lot of simple typeclasses in the base libraries that you’ll need to know for some basic tasks. For instance, to compare two items for equality, you’ll need the Eq typeclass:

class Eq a where
  (==) :: a -> a -> Bool
  (/=) :: a -> a -> Bool

We can define instances for these for all our types. But for simple, base library classes like these, GHC can define them for us! All we need is the deriving keyword:

data Student = Student String Int
  deriving (Eq)

data Teacher = Teacher
  { teacherName:: String
  , teacherAge:: Int
  , teacherDepartment :: String
  , teacherSalary :: Int
  }
  deriving (Eq)

Using Typeclass Constraints

But why are typeclasses important? Well, often times we want to write code that is as general as possible. We want to write functions that assume as little about their inputs as they can. For instance, suppose we have this function that will print a teacher’s name:

printName :: Teacher -> IO ()
printName teacher = putStrLn $ personName teacher

We can use this function for more types than Teacher though! Any type that implements IsPerson will do. So we can make the function polymorphic, and add the IsPerson constraint on our a type:

printName :: (IsPerson a) => a-> IO ()
printName person = putStrLn $ personName person

You can also use typeclasses to constrain a type parameter of a new data type.

data (IsPerson a) => EmployeeRecord a = EmployeeRecord
  { employee :: a
  , employeeTenure :: Int
  }

Typeclasses can even provide a form of inheritance. You can constrain a typeclass by another typeclass! A couple base classes show an example of this. The “Orderable” typeclass Ord depends on the type having an instance of Eq:

class (Eq a) => Ord a where
  compare :: a -> a -> Ordering
  (<=) :: a -> a -> Bool
  ...

Conclusion

Haskell programmers like code that is as general as possible. Object oriented languages try to accomplish this with inheritance. But Haskell gets most of the same functionality with typeclasses instead. They describe common features between types, and provide a lot of flexibility.

To continue learning more about the Haskell basics, take a look at our Getting Started Checklist and get going!

Do you already understand the basics and want more of a challenge? Check out our Recursion Workbook!