possible to define a type where = is forbidden ?

[yoann padioleau <padator@wanadoo.fr>]

I am making a program analysis tool  and in my AST I had previously tokens represented as strings,
but now I want also to associate the line numbers of those strings.

For instance I had

type expr = 
 Plus of expr * expr
 | ...
 | Constant of string
 | Int of int
and I want to pass to 

type expr = 
 Plus of expr * expr
 | ...
 | Constant of  string extended
 | Int of int extended

where type 'a extended = ('a * int)

The problem is that I have in many places some code such as   x = y   that works well
when the AST contain only the string of the token, but now that there is also the line,  the x = y  is not the good one.
I would like a  '= modulo I dont care about line'   such as for example  (Constant "toto",45)  =modulo_line= (Constant "toto", 61)  be true.

There is even some places where I use List.mem  that dont work too.

Is it possible to define a type  such as '=' is forbidden,  so that the compiler and the type system will
help me to spot all the occurences of = (and List.mem, ...)  that I have to change.

For the moment my partial solution  is to use functions to represented the extended type, 
because '=' fail at the execution on functionnal value.
I want to do: 

type line_number = int
type 'a extended = Abstract of 'a | Concrete of (unit -> ('a * line_number))

where Constant "toto"   becomes now  (Constant (Concrete (fun () -> "toto", 45))

The problem is that it dectects bad use of = only at run-time. I would prefer a solution that detects the "incrorrect" use
of = at compile time.



PS: if only caml had typeclasses :)