Effective Haskell

Exercises

Writing Type Classes Representing Emptiness

Imagine that we wanted to create a type class that represents things that can be “empty” for some definition of empty that will depend on the particular type. In this exercise, we’ll call the type class Nullable and give it two functions:

• isNull should return True if a value is “empty”.
• null should return an “empty” value.

module Nullable where
import Prelude hiding (null)

class Nullable a where
  isNull :: a -> Bool
  null :: a

Create instances of this type class for:

1. Any Maybe a where a is Nullable
2. Any tuple, (a,b) where a and b are Nullable
3. Any list type

Adding a Default Null Test

Add a new Eq constraint to the definition of Nullable:

class Eq a => Nullable