Subject: Interface Block / Host Association Proposal    J3/01-397
From: Kurt W. Hirchert                  (Meeting 159) 06 Dec 2001

==========
Background
==========

In his tutorial on the current state of the enhanced modules TR,
Van expressed dissatisfaction with the current host association
rules concerning interface blocks.  During the resulting
discussion, I expressed other concerns about the IMPORT statement
in the current draft.  As a result we were charged with proposing
something better.  After a couple of brainstorming sessions at
lunch, this is what we came up with.  It contains preliminary
specs and syntax.  (I.e., fine details of the specs probably
need more refining, and the things like the choice of syntax
keywords and punctuation are still up for grabs.)

========
Strategy
========

1. Eliminate the IMPORT statement:
   a. Remove it from R204.
   b. Remove R1209, C1209, and C1210.
   c. Remove the description of IMPORT following Note 12.4.
   d. Eliminate or modify Note 12.6 (the example of using IMPORT).

2. Add the CONTEXT statement:
   a. Add " or <context-stmt>" to R1202.
   b. R1209a <context-stmt> is CONTEXT <context>
      R1209b <context>      is EXTERNAL
                            or MODULE
   c. Description of CONTEXT can go where we took out the
      description of IMPORT (following Note 12.4).
   d. Fix the example in Note 12.6 by removing "IMPORT T"
      after the "SUBROUTINE MONITOR ..." statement and
      adding "CONTEXT MODULE" before that statement.  (Changes
      to the description of the example are left as an exercise
      for the reader.)

===============
Basic Semantics
===============

The CONTEXT statement is used to declare what host association
is used in the interpretation of the interface bodies that follow
it (i.e., until the next CONTEXT statement or the end of the
interface block).

CONTEXT EXTERNAL is the default and indicates that the affected
interface bodies access nothing through host association (i.e.,
they are like external procedures).  This is legal in interface
bodies everywhere, and may be applied to interface bodies for
external procedures, dummy procedures, and abstract interfaces.

CONTEXT MODULE indicates that the affected interface bodies use
host association to access entities from the _module_ that
ultimately contains it, _not_ the scoping unit which immediately
contains the interface block.  (In other words, the affected
interface bodies are interpreted like module procedures.)  It is
legal only within a module (including its module procedures and
their internal procedures).  It may be applied to interface
bodies for dummy procedures and abstract interfaces, but not to
module procedures (since they already have explicit interfaces).

=============================
IMPORT vs. CONTEXT Comparison
=============================

1. IMPORT requires a list of the specific entities to be
   accessed.  CONTEXT MODULE provides everything in the module
   automatically.

2. Declarations in the immediate host of an interface block
   can change the meaning of a name accessed via IMPORT from
   its meaning in the class of module procedures being described.
   CONTEXT MODULE operates directly out of the module, so no such
   distortion can take place.

3. It is possible for a single CONTEXT MODULE statement to apply
   to multiple interface bodies.  Each interface body would have
   required its own specification of an IMPORT list.

4. Because the IMPORT statement appears within the interface body,
   it may interfere with the ability to use techniques like
   INCLUDE files or cut and paste to use the identical text in an
   interface body as in the procedure being described.  The
   CONTEXT statement appears outside the interface body and
   should should not have this effect.

=================================================
Extending CONTEXT to the Enhanced Module Facility
=================================================

CONTEXT MODULE is now also allowed in a submodule and accesses
the module of which that submodule is logically a part.

" or SUBMODULE [<submodule-name>]" is added to R1209b.

CONTEXT SUBMODULE is similar to CONTEXT MODULE.  If an affected
interface body is for a dummy procedure or abstract interface,
CONTEXT SUBMODULE access the submodule in which it appears, and
CONTEXT SUBMODULE <submodule-name> accesses the named submodule,
subject to the requirement that this be a submodule of which the
current submodule is logically a part.  If an affected interface
body is not for a dummy procedure or abstract interface, it is
a declaration of the interface for a module procedure in a
submodule of the module or submodule in which the interface block
appears.  Host association in the interface body is from that
module or submodule.  If <submodule-name> is present, the actual
module procedure must be in that submodule (or one of its
submodules).

                               - end -