module type Session_base = sig
  type t
  type a
  val mk_base : a -> t
end;;

module Protocol_base : Session_base = struct
  type t = protocol_base_formula
  type a = string
  let mk_base sender receiver formula = {...}
end;;

module Make_Session (Base: Session_base) = struct
  type t = Base.t

  type session =
    | SSeq  of ...
    | SBase of t

  let mk_base f d e = SBase (Base.mk_base f d e)
end;;

module Protocol = Make_Session(Protocol_base);;

Call site: Session.Protocol.mk_base first second c
where first is a string

Error:
This expression has type string but an expression was expected of type
         Session.Protocol_base.a