From: "Till Varoquaux" <till.varoquaux@gmail.com>

> This would work but it isn't an option since I cannot get it to work
> with functors etc...
> AFAIK there's no way to specify a type must be a polymorphic variant
> without specfying any of it's members:
> 
> if we are building a functor against:
> 
> module type Evaluator=
> sig
>  ...
>  type 'a exp (*Is an exact polymorphic variant type *)
>  ...
> end
> 
> we won't be able to write something like:
> 
> type 'a t=[ `Some_label of 'a | 'a exp ]
> 
> since the compiler won't be able to detect 'a exp was a polymorphic
> variant (note: it looks like there could also be an issue on the
> freshness of the new label)

There is indeed an issue of freshness :-)
There is code doing exactly what you ask for inside a branch in the
CVS for ocaml. The branch is still experimental, and is called
varunion.
You would have to write
  type 'a exp = private [> ] ~ [`Some_label of 'a]
    (* `Some_label may only have type 'a (if present) *)
or
  type 'a exp = private [> ] ~ [`Some_label of 'a]
    (* `Some_label is absent *)
in order to be able to use it later in combination with `Some_label.

You can look at the code in testlabl/varunion.ml, which includes
examples corresponding exactly to the code you are trying to write.

Jacques Garrigue