Monads

Question 1

write small program to ask user’s name and say you rock.

Answer 1

main = do
putStrLn “Hello, what’s your name?”
name

Question 2

how would you read: name

Answer 2

perform the I/O action getLine and then bind its result value to name. getLine

Question 3

what is type signature for main?

Answer 3

main :: IO something, where something is some concrete type

Question 4

what is the import statement for ‘when’?

Answer 4

import Control.Monad

Question 5

What does mapM do? How does it differ from mapM_ ?

Answer 5

mapM takes a function and a list, maps the function over the list and then sequences it. mapM_ does the same, only it throws away the result later. We usually use mapM_ when we don’t care what result our sequenced I/O actions have.

Question 6

what do monads do?

Answer 6

Monads apply a function that returns a wrapped value to a wrapped value. Monads have a function&raquo_space;= (pronounced “bind”) to do this.

Question 7

explain each section of the following type signature:

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

Answer 7

(»=) m a = takes a monad
(a-> m b) = and a function which returns a monad
-> m b = and returns a monad.

Question 8

A functor is a data type that implements the Functor typeclass.
An applicative is a data type that implements the Applicative typeclass.
A monad is a data type that implements the Monad typeclass.
A Maybe implements all three, so it is a functor, an applicative, and a monad.
What is the difference between the three?

Answer 8

functors: you apply a function to a wrapped value using fmap or
applicatives: you apply a wrapped function to a wrapped value using or liftA
monads: you apply a function that returns a wrapped value, to a wrapped value using&raquo_space;= or liftM

Question 9

What does the function liftM do?

Answer 9

Function liftM turns a function which takes input and produces output to a function which takes input in some monad and produces output in the same monad. Lets look at its definition: