8. Declaring types and classes

Question 1

What is the keyword for declaring a type? Give a simple example

Answer 1

type
i.e. type Pos = (Int, Int)
(must be capital P)

Question 2

Can the type keyword be used for declaring recursive types?

Answer 2

No, use the ‘data’ keyword

Question 3

What keyword can be used for declaring new types? Give an example

Answer 3

Data
i.e. data Bool = False | True

Question 4

How to declare a new type containing of a single argument? Give an example

Answer 4

newtype
i.e. newtype Nat = N Int

Question 5

Give an example of a recursive type

Answer 5

data Nat = Zero | Succ Nat

Question 6

How to declare a class? Give an example

Answer 6

class
i.e. class Eq a where
(==), (/=) :: a -> a -> Bool
x /= y = not (x == y)

Question 7

How to define a type as an instance of a class?

Answer 7

instance Eq Bool where
False == False = True
True == True = True
_ == _ = False

Question 8

How to make a type an instance of a class?

Answer 8

deriving
data Bool = False | True
deriving (Eq, Ord, Show, Read)