If your set of symbol is closed, you can use a variant type (sum type).

type symbols =
| A
| B


If you really need open symbols, you can use [polymorphic variants].

Let tag_a foo = (`A, foo)

 [polymorphic variants]
http://caml.inria.fr/pub/docs/manual-ocaml/manual006.html#toc36


However, you won't have convenience functions such as string_of_symbol; you
would have to define them yourself.