PFL_KeywordEnforcer.F90

   ! This module implements a mechanism that can be used to enforce
   ! keyword association for dummy arguments in an interface.  The
   ! concept is to have a derived type for which no actual argument can
   ! ever be provided.
   
   ! The original idea comes (AFAIK) from ESMF which uses a PUBLIC
   ! derived type that is simply not exported in the main ESMF
   ! package.  That approach has one weakness, which is that a clever
   ! user can still access the module that defines the type.  Various
   ! workarounds for that are possible such as using a truly PRIVATE
   ! type, but these encounter further issues for type-bound
   ! procedures which are then overridden in a subclass.

   ! The approach here, suggested by Dan Nagle, is to use an ABSTRACT
   ! type which prevents variables from being declared with that type.
   ! Tom Clune improved upon this by introducing a DEFERRED type-bound
   ! procedure that prevents extending the type to a non-abstract
   ! class.  A DEFERRED, PRIVATE type-bound procedure is attached to
   ! the type and cannot be overridden outside of this module.  Any
   ! non-abstract extension must implement the method.  (Note that
   ! ABSTRACT extensions can be created, but do not circumvent the
   ! keyword enforcement.

module PFL_KeywordEnforcerMod
   implicit none
   private

   public :: KeywordEnforcer

   type, abstract :: KeywordEnforcer
   contains
      procedure (nonimplementable), deferred, nopass, private :: nonimplementable
   end type KeywordEnforcer

   abstract interface
      subroutine nonimplementable()
      end subroutine nonimplementable
   end interface

end module PFL_KeywordEnforcerMod