Unique identifiers - including a null Id.

\begin{code}
module Identify where

nullId :: Ident
nullId = IdentS ""

data Ident = IdentS String
 deriving (Eq,Ord,Read,Show)

class Identifiable a where
   identify :: a -> Ident


instance Identifiable String where
 identify s = IdentS s

instance Identifiable Int where
 identify n = IdentS $ show n
\end{code}