Example of using the `Monad` instance from `Maybe`.

MonadMaybe.hs

data Person = Person { personId :: PersonId, personName :: Name }

parseInt :: String -> Maybe PersonId
getById :: PersonId -> Maybe Person

-- Accepts a raw string from the user
-- Attempts to parse it as an `Int`
-- Attempts to get a `Person` by that ID
-- Returns the `Name` of the Person
f :: String -> Maybe Name
f userInput = do
  id' <- parseInt userInput
  person <- getById id'
  return $ personName person