Person.hs

module Person (Person, null, newPerson, isNull, getAge, getName) where

-- constructor is not exported! only this module has access to the internal Maybe
newtype Person = Person (Maybe (Int, String)) 

null :: Person
null = Person Nothing

newPerson :: Int -> String -> Person
newPerson age name = Person (Just (age, name))

isNull :: Person -> Bool
isNull (Person maybeData) = isJust maybeData

getAge :: Person -> Int
getAge (Person (Just (age, _))) = age
getAge _ = error "NullPointerException"

getName :: Person -> String
getName (Person (Just (_, name))) = name
getName _ = error "NullPointerException"

Or heck it would probably be even cleaner as data Person = Person { age :: Int, name :: String } | Null, and then you don't even need to hide anything.