J3/03-145r1

To: J3
From: UK Fortran panel
Subject: Edits for UK comment TC7 (Extending any non-SEQUENCE type)
Date: 1 April 2003
Author: Lawrie Schonfelder

1. Comment TC7

The UK comment was:

    Allow any non-SEQUENCE type to be extended.

    There is no technical reason for the requirement that only
    extensible types can be extended; therefore it should be possible
    to extend any non-SEQUENCE derived type.  However, SELECT TYPE
    should continue to require that its type guards specify extensible
    types (or intrinsic types).

2. Discussion

The perceived difficulty which underlay the introduction of the EXTENSIBLE
keyword was the interaction of previously compiled libraries (in particular,
sharable libraries) together with the need for a unique type "signature" for
identifying the type of a polymorphic variable.

On further reflection, this concern was somewhat of a shibboleth.  For an
ordinary polymorphic pointer, a "null" signature would suffice to identify
the base type.

Even in the case of an unlimited polymorphic pointer, there are reasonably
effective implementation techniques available.  A plausible technique for
identifying a type in the absence of a previously compiled type signature is
to store a string identifying the scope of the type declaration plus a string
identifying the type.  Although string comparison is relatively slow (vs.
comparing pointers to a type signature block), this would need to be done
relatively rarely and in a well-defined situation, viz:
   (a) the object is unlimited polymorphic
   (b) the type test is TYPE IS or CLASS IS of a base type that does not
       have a type signature block.

Alternatively, as mentioned in the UK comment, SELECT TYPE could continue
to require its type guards to specify extensible types (which always have
type signature blocks).  This restriction would simplify implementation
techniques slightly, at the cost of obstructing the user -- forcing him
to keep track of which type an unlimited polymorphic pointer is pointing
to (when it is not an extensible type) and to use unsafe pointer assignments.
In this case, the EXTENSIBLE keyword should still be kept as it would still
be useful.

This paper recommends that SELECT TYPE be extended to detect all nonsequence
types.

3. Interaction with UK comment TC6

Note that being able to extend any (non-SEQUENCE) derived type implies that
CLASS(*) can point to any (non-SEQUENCE) derived type.  UK comment TC6 also
addresses this area, and proposes that CLASS(*) be allowed to point to objects
of any type (i.e. not limited to non-SEQUENCE derived types).

4. Edits to 02-007r3 and 03-166

[42:1] Delete.
[42:8] Delete.
[42:10] Replace with
  "C418 (R423) If EXTENDS appears, SEQUENCE shall not appear."
{Remove EXTENSIBLE keyword (and attribute), but leave the concept of extensible
 type.  Allow any nonsequence nonBIND type to be extended.
 Note that for C418, the BIND(C) part was already redundant with C1503.}

[53:31+4] Delete ", EXTENSIBLE ::".
{Remove EXTENSIBLE keyword.}

[56:5] Change "Extensible types" to "Type extension".
{Better section title.}

[56:6] Replace with
  "A nonsequence derived type that does not have the BIND attribute is an
   <<extensible type>>."
{New definition of extensibility, i.e. most types.}

[56:7] Replace the first sentence with
  "An extensible type that does not have the EXTENDS attribute is a <<base
   type>>."
{Remove EXTENSIBLE attribute.}

[57:0+4] Delete ", EXTENSIBLE ::".
{Ditto.}

[51:4] Change "extensible type" to "extended type (4.5.6)".
{Note: This is a mere wording improvement, as only extended types have parent
 components...}

[163:29+3] Delete ", EXTENSIBLE ::".

[387:1] Delete "EXTENSIBLE or".

[416:2-3] Delete "A ... attribute.".
{Glossary entry for base type.  This sentence was unnecessary.}

[419:25] Replace with
  "A nonsequence type that does not have the BIND attribute."
{Glossary entry for extensible type.}

[432:37] Change "Extensible types (4.5.3)" to
  "Type extension and component accessibility (4.5.1.8, 4.5.3)".
{Better title.}

[433:1] Delete ", extensible ::".
{Type extension example.}

[437:22] Delete ", EXTENSIBLE ::".
{Final subroutine example.}

[441:30] Delete ", EXTENSIBLE ::".
{Component access example.}

[474:28] Delete ", EXTENSIBLE ::".

[477:29] Change "extensible types" to "polymorphic variables".
{This sentence was previously misleading.}

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