[J3] Remove requirement that passed-object dummy argument is required to be scalar?

[Brad Richardson]

How would the procedure be called with a scalar then? I.e.

type(list_t) :: my_list

...

if (my_list%check_list()) then
...

I think to remove the constraint would require new features to deal with all these kinds of cases.

On Thu, 2021-12-09 at 15:06 -0800, Van Snyder via J3 wrote:

> Is there a technical reason that the passed-object dummy argument is required to be a scalar?
>
> In an old code, have a module with a "list" type that manages double-linked lists, where the links are subscripts in an array of "list" type. The current version of the "list" type has a single additional "item" component of type integer that is a subscript into another array. I'd like to extend the list type to incorporate a component of that other type (or its its components), rather than needing two types, and needing to connect the objects using subscripts. This would be trivial using a parameterized module instead of type extension.
>
> One of the module procedures checks that the list is consistent, by verifying that the "next" component of the previous list item is in fact the current item, and the same for the next one.
>
> If something goes wrong, it calls another procedure to dump the list item -- along with its predecessor and successor.
>
> I can't make those procedures type-bound procedures because the passed-object dummy argument is required to be a scalar.
>
> Therefore, I can't inherit those procedures into extension types, and override them.
>
> A work-around kludge is to make "list" have a single array component that is the real list. This adds another level of component selection in every reference.
>
> Another kludge is to use pointers to scalar "list" objects, instead of subscripts, but that's an invitation to memory leaks because we don't have reference-counted targets or intrinsic garbage collection.
>
> If there is no compelling reason that the passed-object dummy argument is required to be a scalar, can that clause of C760 be removed?
>
> logical function Check_List ( List, Message ) result ( Wrong )
>
> ! Verify that each list item's predecessor's "next" component indexes
>
> ! that item, and the same for its successor mutatis mutandis.
>
> class(list_t), intent(in) :: List(:)
>
> character(*), intent(in), optional :: Message
>
> integer :: I, N, P
>
> do i = 1, size(list)
>
> p = list(i)%prev
>
> n = list(i)%next
>
> wrong = .false.
>
> if ( p /= 0 ) wrong = list(p)%next /= i
>
> if ( n /= 0 ) wrong = wrong .or. list(n)%prev /= i
>
> if ( wrong ) then
>
> if ( present(message) ) write ( unit, '(a)' ) trim(message)
>
> call dump_list_item ( list, i, prevNext=1 )
>
> end if
>
> end do
>
> end function Check_List
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