signature SYMBOL =
sig
  eqtype symbol
  val symbol : string -> symbol
  val name : symbol -> string
  type 'a table
  val empty : 'a table
  val enter : 'a table * symbol * 'a -> 'a table
  val look  : 'a table * symbol -> 'a option
end

abstraction Symbol : SYMBOL =
struct

  type symbol = string * int

  structure T = HashTable(struct type hash_key = string
				 val hashVal = HashString.hashString
				 fun sameKey(s1,s2) = s1=s2
			  end)

  exception Symbol
  val nextsym = ref 0
  val hashtable : int T.hash_table = T.mkTable(128,Symbol)
  
  fun symbol name =
      case T.peek hashtable name
       of SOME i => (name,i)
        | NONE => let val i = !nextsym
	           in nextsym := i+1;
		      T.insert hashtable (name,i);
		      (name,i)
		  end

  fun name(s,n) = s

  structure Table = IntMapTable(type key = symbol
				fun getInt(s,n) = n)

  type 'a table= 'a Table.table
  val empty = Table.empty
  val enter = Table.enter
  val look = Table.look
end