From: Jacques Carette <carette@mcmaster.ca>
> Jacques Garrigue wrote:
> > The message is maybe not clear enough: for pattern-matching and
> > inheritance, you need an "exact" variant type, i.e. a type whose lower
> > and upper bounds are identical. 
> Indeed - I would suggest that the error include some clearer diagnostics 
> that indicate how the variant is 'inexact'.
> 
> > The standard workaround is to write:
> >
> > type c0 = [a | b]
> > type 'a c = 'a constraint 'a = [< c0]
> >   
> This got me further, thanks.  But not far enough... 
> Here is a simple example of the problems I am now running into:
> 
> type 'a a = [`A of 'a ]
> type 'a b = [`B of 'a ]
> type    d = [`D ]
> type ('a,'b) e = ['b a | 'a b | d]
> 
> type ('a,'b) c = ('a,'b) e
>     constraint 'a = [< 'b a ]
>     constraint 'b = [< 'a b ]
> 
> type 'd cc = 'd
>     constraint 'd = [< ('a,'b) c]
> 
> let is_cc = function
>     | #cc -> true
>     | _   -> false 
> ;;
> 
> (* complains that cc is not a variant type when trying to define is_cc *)

Sure, you must use #c as pattern, not #cc.
Or are you thinking of another problem?

Jacques Garrigue