Currently, OCaml allows value expressions to "locally open" modules, but it would be helpful to extend this feature for use in type expressions.

For example, take the following code:

module Abc = struct
    type a
    type b
    type c
end
type abc_group = (Abc.a * Abc.b * Abc.c)

I would like to be able to define abc_group with syntax similar to this:

type abc_group = let open Abc in (a * b * c)

This would drastically reduce the verbosity of complex type signatures, especially those involving nested modules. Used in a reasonable way, this could lead to more readable code.

As for the specific syntax, I lack the experience to know what makes the most sense, so I leave that discussion to those who know more about OCaml syntax.

Edit:
@aantron proposed the following syntax:

type abc_group = Abc.(a * b * c)

This syntax is less verbose, and probably less confusing to most people.