From: "Lukasz Stafiniak" <lukstafi@gmail.com>

> Try the syntax:
> 
> # type r = {x : [> foo]};;
> Characters 14-21:
>   type r = {x : [> foo]};;
>                 ^^^^^^^
> Unbound type parameter [..]
> 
> The open variant type introduces a type parameter, which is not bound
> in definition of type r. I don't think these unnamed parameters can be
> bound, so that you could write:

The correct syntax in this case is

  type r = {x : 'a. [> foo] as 'a}

Unfortunately there seems to be a bug in 3.09, so that it doesn't
work. This will work again in 3.10.

(This was a stupid bug, as usual. Other cases, like
  type r = {x : 'a. <m : int; ..> as 'a}
do work in 3.09.)

Note that this seems not to be what the original poster wanted
actually, which was just a constrained type.

Jacques Garrigue