> From: Charles Bouillaguet <Charles.Bouillaguet@crans.org>
> > The problem appear when I want to define my values :
> > 
> > type 'sort array_state = 'sort * [`ArrayStateVar of string |  
> > `ArrayWrite of 'me  * 'sort base_value * [`Int] base_value * 'sort  
> > base_value] as 'me
> > and 'sort base_value = 'sort * [`Inert of unit | `FieldRead of  
> > [`Object] base_value * 'sort field | `ArrayRead of 'sort array_state  
> > * ('sort array_) base_value * [`Int] base_value]
> > and 'sort field = 'sort * [`FieldVar of string | `FieldWrite of 'me *  
> > [`Object] base_value * 'sort base_value] as 'me

> The trouble is that the array case takes 'sort as a parameter, meaning
> that we would need an infinity of such duplicates.
> Here is the second problem: by nature, polymorphic variants only allow
> regular types (that can be represented by a regular graph), and your
> definitions do not represent a regular type (you can get ever deeper
> arrays.)

Note that this can be solved by introducing a nominal type (record or
sum type) that breaks the irregular cycles. Here the following is
sufficient.

type 'sort array_state =
    'sort *
    [ `ArrayStateVar of string
    | `ArrayWrite of
        'me  * 'sort base_value * [`Int] base_value * 'sort base_value] as 'me
and 'sort base_value =
    {sort: 'sort; desc:
     [ `Inert of unit
     | `FieldRead of [`Object] base_value * 'sort field
     | `ArrayRead of
        'sort array_state * ('sort array_) base_value * [`Int] base_value]}
and 'sort field =
    'sort *
    [ `FieldVar of string
    | `FieldWrite of 'me *  [`Object] base_value * 'sort base_value] as 'me

Probably this is not you wanted, but just to make things clear.

Jacques Garrigue