Example for Phantom Types in Haskell

phantom.hs

data PList a b = Nil
               | Cons a (PList a b)

test :: PList Int String
test = Cons 3 $ Cons 2 $ Cons 1 Nil

data Foo = Foo

test2 :: PList Int Foo
test2 = Cons 3 $ Cons 2 $ Cons 1 Nil