Jacques Garrigue a écrit :
> [...] union of polymorphic variant is flat [...]
> The reason is that dispatch only looks at the head constructor:
> 
> let f = function
>   | #b as x -> fb x
>   | #c as x -> fc x
> [...]

Actually, I was faced to the problem in a similar pattern-matching, and 
I reduced the problem to the example in my post :-)

> [...] is actually
> 
> let f = function
>   | `A _ as x -> fb x
>   | `A _ as x -> fc x
> 
> which only types if both parameters have the same type.

However, one can do:

let f = function
   | `A (`B _) -> ...
   | `A (`C _) -> ..

Would it be difficult to extend the system so that #b and #c expand to 
the constructions above?

> Polymorphic variants are _not_ XML types.
> Use ocamlduce for that :-)

I am having a look at it...

-- 
Stéphane Glondu