Hi,

Note that you can replace your pattern-matching with something like:

let ud_type type =
   match type with
     Primitive(ud,_)
   | Pointer(ud,_) -> ud

But although it is more factorized (which is good) it doesn't solve your 
problem.

One solution which I like better is something like:

type userdata = ... whatever you want (code location...) ...
type 'a node = {
   user_data: userdata;
   node: 'a;
}
type type_tree =
   Primitive of string
| Pointer of type_tree node
type expr_tree =
   Unary of string * expr_tree node
| Binary of string * expr_tree node * expr_tree node

Now instead of matching on a type_tree x, you'll match on x.node. And if 
you want the user data you just use x.user_data.

Hope it helps.

-- 
Romain Bardou