12. Monads and more

Question 1

What is a Functor used for?

Answer 1

For mapping a function over each element in a class

Question 2

What is the declaration of the Functor class?

Answer 2

class Functor f where
fmap :: (a -> b) -> f a -> f b

Question 3

Describe the functor definition of the Maybe class

Answer 3

instance Functor Maybe where
fmap _ Nothing = Nothing
fmap f Just x = Just (f x)

Question 4

What is an Applicative used for?

Answer 4

As an extension of the Functor class so that it can take multiple arguments.

Question 5

What is the decleration of the Applicative class?

Answer 5

class Functor => Applicative f where
pure :: a -> f a
(<*>) :: f (a -> b) -> f a - > f b

Question 6

What is a Monad used for?

Answer 6

To abstract away details of computations that involve side-effects or other complexities.

Question 7

What is the definition of the Monad class?

Answer 7

class Applicative m => Monad m where
return :: a -> m a
(»=) :: m a -> (a -> m b) -> m b

Question 8

Describe the bind function “»=”

Answer 8

Takes a value of the monad type and a function that takes a plain value and returns a monad value, and applies the function to the plain value.

Question 9

Describe the monad ‘return’ function

Answer 9

Also called pure, takes a plain value and wraps it in the monad type.

Question 10

Describe the Monad laws

Answer 10

1. return x&raquo_space;= f = f x
2. mx&raquo_space;= return = mx
3. (mx&raquo_space;= f)&raquo_space;= g = mx&raquo_space;= (\x -> (f x&raquo_space;= g))