J3/06-373

Date: November 11, 2006
To: J3
From: Dr. Reinhold Bader
Subject: Extension of IMPORT statement

References: J3/06-007r1

This suggestion attempts to address a weakness of the submodule concept as
presently defined in the draft standard. Assume a module-defined interface

module x
  type :: tx
    :
  end type
  interface
    module procedure foo(yy, xx)
      use y        ! here be dragons
!     import :: tx - disallowed for separate module procedures
      type(tx), intent(out) :: xx
      type(ty), intent(in) :: yy
    end module procedure
  end interface
  :
end module x

which references a module y with a type ty defined there, and is subsequently
implemented in a submodule:

submodule(x) x_foo
  use y
  type(ty) :: g_foo_y
contains
  module procedure foo
    :
  end module
end submodule x_foo

One of the points of this exercise is to enable the module y to make use of
the type definitions in x:

module y
  use x
  type :: ty
    :
  end type
  :
end module y

Unfortunately this renders the "use y" line in the interface definition a
circular module reference and is hence disallowed, while the "use y" line
in the submodule is OK. For a number of software patterns it would be
desirable to be able to access other modules' entities from within an
interface block; the rationale is similar to that which led to the
introduction of the IMPORT statement. In fact an extension of IMPORT is
suggested to fix the above problem; the interface definition would then
be replaced by

  interface
    module procedure foo(yy, xx)
      import y :: ty
      type(tx), intent(out) :: xx
      type(ty), intent(in) :: yy
    end module procedure
  end interface

One use case for this is supplied by an article published in the August 2006
Fortran Forum ("Dynamic Polymorphism in Fortran 95: How Submodules can help").


Here is a suggestion for edits of J3/06-007r1:

[299:16]. In 12.4.3.2, change R1209 to read

import-stmt is    IMPORT [[ module-name ::] import-name-list [, rename-list]


[299:41-43]. Replace this by the following:

C1210 (R1209)  Using IMPORT to access entities from the local host scoping
               unit is allowed only in an interface body that is not a module
               procedure interface body; rename-list may not be specified in
               this case.


C1210a (R1209) Using IMPORT to access entities from a module different from
               the local host scoping unit is allowed only in an interface
               body that is a module procedure (or function) interface body.

C1211 (R1209)  Each import-name shall be the name of an entity in the
               referenced scoping unit. If no module-name is provided,
               this is the local host scoping unit. Any entities accessed by
               use association can be optionally renamed via rename-list.

[300:32].    Replace this by the following:

               The IMPORT statement within an interface body that is not a
               module procedure interface body specifies that the named
               entities from the host scoping unit are accessible in the
               interface body by host association.

[300:41].    Add the following section after this line:

               The IMPORT statement within an interface body that is a
               module procedure interface body specifies that the named
               entities from the referenced module host scoping unit are
               accessible in the interface body by use association. The
               rules listed in 11.2.2 for use association apply, with the
               exception that it is allowed to indirectly reference the
               local host scoping unit. Furthermore, omitting import-name-
               list has an effect analogous to omitting the ONLY part of
               the USE statement.

[488:6].     Replace this by the following:

               Use association is the association of names in different
               scoping units specified either by a USE statement, or by an
               IMPORT statement within an interface body that is a module
               procedure interface body.

Adding the above example as a note might be useful.
Furthermore, it might be appropriate to add a note to 11.2.3 indicating the
necessity to explicitly provide a USE statement referencing all modules which
are needed due to an IMPORT statement in the parent module for a locally
implemented separate module procedure, possibly with a consistent renaming
scheme. Correspondingly, the syntax rule annex requires part of the above
updates.
--------------------------------------------------------------------------
Dr. Reinhold Bader
Email: Reinh.Bader@t-online.de

Leibniz-Rechenzentrum, Abt. Hochleistungssysteme | Tel. +49 89 35831 8825
Boltzmannstr. 1, 85748 Garching                  | Fax  +49 89 35831 9700