Defining an alias to a pointer type in ctypes

Learning
1

I’m experimenting with ctypes, following the instructions here. I have something basic working:

module Types (F : Ctypes.TYPE) = struct
  open F

  type error_stack
  let error_stack: error_stack structure typ = structure "ErrorStack"
end

module Functions (F : Ctypes.FOREIGN) = struct
  open F
  open Types

  let error_stack_create = foreign "error_stack_create" (void @-> returning (ptr error_stack))
end

and now I would like to define a type alias to the ErrorStruct* and use it in a record. This works:

  (* in Types *)
  type error_stack_p = error_stack ptr
  let error_stack_p = ptr error_stack

  (* in Functions *)
  let error_stack_create = foreign "error_stack_create" (void @-> returning error_stack_p)

but when I add this to my main code

type err = {
  error_stack: C.Types.error_stack_p;
}

let () =
  let error_stack = C.Functions.create_error_stack () in
  { error_stack }

it errors with

File "lib/piet.ml", line 10, characters 4-15:
10 |   { error_stack }
         ^^^^^^^^^^^
Error: This expression has type
         Piet__Function_description.Types.error_stack structure
         Ctypes_static.ptr =
           (Piet__Function_description.Types.error_stack structure, [ `C ])
           pointer
       but an expression was expected of type
         C.Types.error_stack_p =
           (Types_generated.error_stack, [ `C ]) pointer
       Type
         Piet__Function_description.Types.error_stack structure =
           (Piet__Function_description.Types.error_stack, [ `Struct ])
           structured
       is not compatible with type Types_generated.error_stack