[Haskell-cafe] Test on identity?

Dear Café, I'm trying to build a DAG from a binary tree. I don't think there's a big trouble. Nevertheless, I do even some transformations. Thus, I would like to know it is still a DAG, not adding, accidentally, a node. Is there any way, if I have data like data Ex = Val Int | Add Ex Ex so that I can test that some value Val i === Val i ? I mean, the pointers go to the same data box? I could do that via some IORefs, AFAIK, but I don't think it is feasible. Maybe to tune the algorithm... Best regards, Dusan