J3/03-132

To: J3
From: UK Fortran panel
Subject: Edits for UK comment TC5 (Change to type extension)
Date: 10 March 2003

Comment TC5 of the UK vote was:

    Remove the facility to add type parameters during type extension.

    A derived type statement with an EXTENDS clause shall not declare
    any type parameters.

    The current description is incorrect, as SELECT TYPE provides no
    way to discover the values of any kind type parameters that were
    added during type extension.  Furthermore, this feature complicates
    the language and adds little benefit.

When constructing the edits for this, we realized that this change
would make CLASS(*) irregular since an object of CLASS(*) has no type
parameters but is type compatible with objects that do. Also, it would
not obviate the need to correct SELECT TYPE, since this is needed for
CLASS(*) objects.  Therefore, we wish to change our comment to a
request for correcting SELECT TYPE.  Edits follow.

{Note: These edits are all superseded if TC6 is adopted.}

162:16,17. Change "<extensible-type-name>" to "<type-guard-spec>".
{We need new BNF for type specifiers that exclude nonkind type params.}

162:19-20. Delete.
{We'll handle this in our new rule.}

162:25+. Insert new rules and constraints:

  R823a <type-guard-spec> <<is>> <type-name> [ ( <type-param-spec-list> ) ]

  C815a (R823a) <type-name> shall be the name of an extensible type.

  C815b (R823a) If <selector> is not unlimited polymorphic, <type-name>
       shall be the name of an extension of the declared type of
       <selector>.

  C815c (R823a) There shall be exactly one <type-param-spec>
       corresponding to each kind type parameter of the type and none
       corresponding to a nonkind type parameter.

If UK comment TC3 is accepted, the third new constraint will need to be

  C815c (R823a) There shall be exactly one <type-param-spec>
       corresponding to each kind type parameter of the type that does
       not have a default value, at most one corresponding to each kind
       type parameter of the type that has a default value, and none
       corresponding to a nonkind type parameter.

{New rule.}

162:21. Change '<extensible-type-name>' to 'type and kind type
   parameter values'.

163:6-14. Replace by EITHER

   The block to be executed is selected as follows:

   (1) If the dynamic type and kind type parameter values of the selector
       are the same as those specified by a TYPE IS type guard
       statement, the block following that statement is executed.

   (2) Otherwise, if exactly one CLASS IS type guard statement
       specifies a type of which the dynamic type of the selector is an
       extension and specifies kind type parameter values that match
       the corresponding type parameter values of the dynamic type of
       the selector, the block following that statement is executed.

   (3) Otherwise, if several CLASS IS type guard statements match as in
       (2), one of these statements must specify a type that is an
       extension of all the types specified in the others; the block
       following that statement is executed.

OR

   A TYPE IS type guard statement matches the selector if the dynamic
   type and type parameter values of the selector are the same as those
   specified by the statement.  A CLASS IS type guard statement matches
   the selector if the dynamic type of the selector is an extension of
   the type specified by the statement and the kind type parameter
   values specified by the statement are the same as the corresponding
   type parameter values of the dynamic type of the selector.

   The block to be executed is selected as follows:

   (1) If a TYPE IS type guard statement matches the selector, the block
       following that statement is executed.
   (2) Otherwise, if exactly one CLASS IS type guard statement matches
       the selector, the block following that statement is executed.
   (3) Otherwise, if several CLASS IS type guard statements match the
       selector, one of these statements must specify a type that is an
       extension of all the types specified in the others; the block
       following that statement is executed.

163:18,21,24: Change 'type parameters' to 'type parameter values'
on all three lines.
{In many places, the standard uses 'type parameter' when the context
makes it clear that 'type parameter value' is what is really meant.
Here, we think it needs to be written in full to make the meaning
clear.}

163:22 Replace line by

   are the corresponding type parameter values of the selector.