type definition by 'data' and 'newtype'

def_data.hs

data Hoge = Hoge { get :: Int }

foo :: Hoge -> String
foo (Hoge _) = "foo!"

foo undefined
-- "*** Exception: Prelude.undefined

def_newtype.hs

newtype Hoge = Hoge { get :: Int }

foo :: Hoge -> String
foo (Hoge _) = "foo!"

foo undefined
-- "foo!"