[J3] [EXTERNAL] another PURE example

[Clune, Thomas L. (GSFC-6101)]

Hi Bob,

I know there would be terrible problems with backward compatibility, but would there otherwise be value in considering a PURE attribute for types?    Very roughly my idea is that a PURE type cannot do the things that allow the loopholes in your examples.    Arguments to PURE procedures would be required to be of PURE type, as would any local variables in the procedure.

The only way that I could see to squeeze a modicum of backwards compatibility with this approach would be to have some language that makes most types PURE by default, and thereby sidestepping the need for explicitly adding the PURE attribute to the types.   And certainly this is a bit of a big feature for an interp.

- Tom





On Oct 11, 2020, at 9:39 AM, Robert Corbett via J3 <j3 at mailman.j3-fortran.org<mailto:j3 at mailman.j3-fortran.org>> wrote:

I uploaded my interp request for PURE procedures.
It is paper 20-151.  I am sorry it took so long to
complete.

The paper does not provide a fix for the problem
it describes.  The paper describes the problem,
not the solution.  It mentions the fix that I favor
as of this date 2020-10-11, but it does not go into
detail.  I doubt the committee will accept my
proposed fix without amendment.

As I say in the paper, I think the committee needs
to rethink how pure procedures are to be used and
how they are to be specified.  Van raised good
points regarding additional constraints that might
be needed.

I want to thank everyone who provided feedback on
my examples and on earlier versions of my paper.
The paper is better (and longer) because of the
feedback.

Robert Corbett

On Tuesday, October 6, 2020, 10:37:21 PM PDT, Robert Corbett via J3 <j3 at mailman.j3-fortran.org<mailto:j3 at mailman.j3-fortran.org>> wrote:


I intend to file the interp paper
soon.

You have seen the original example.
You even commented on it.  I wrote
my new example, in part, because
of your comments.

Bob Corbett

On Oct 6, 2020, at 8:02 PM, Vipul Parekh via J3 <j3 at mailman.j3-fortran.org<mailto:j3 at mailman.j3-fortran.org>> wrote:



On Tue, Oct 6, 2020 at 3:10 PM Robert Corbett via J3 <j3 at mailman.j3-fortran.org<mailto:j3 at mailman.j3-fortran.org>> wrote:
My original program, which will be
included in my interp request, does
not use modules, uses only one
type definition, and does not use
a structure constructor.  Some
people proposed fixes to the
problem that work for that program
(which means they make that
program nonconforming), but
which do not solve the problem
in more complicated cases.  My
new program demonstrates a
more complicated case that is
not handled by some of the
proposed fixes.  Simplifying my
new program would make it
pointless, as it would no longer
exhibit the difficult case.

Hi Bob,

Can you share details on the  original problem, perhaps any link to the email chain if you have that handy?

Otherwise, can you please comment whether your original problem is. by any chance, similar to this reduced case?

--- begin example ---
module m
   integer, target, save :: x = 0
   type :: t
      integer, pointer :: p => x
   end type
end module
module n
   use m, only : x, t
contains
   pure subroutine sub(a, b)
      integer, intent(out) :: a, b
      type(t) :: y
      a = x
      y%p = 1
      b = x
   end subroutine
end module
   use n, only : sub
   integer :: a, b
   call sub(a, b)
   print *, "a = ", a, "; expected is 0"
   print *, "b = ", b, "; is it 1?"
end
--- end example ---

One processor I tried issued an error "Bad target in pointer assignment in PURE procedure" for module 'n'.  However, I am unable to come up with any clause in C1594 (or any constraints in 15.7 Pure Procedures) that clearly explains why this processor issues this diagnostic even if it is trying to do the right thing.

But now, no errors or warnings with another processor; the program built with it gave the following output:

 a =  0 ; expected is 0
 b =  1 ; is it 1?

The processor that issued the "Bad target in pointer assignment in PURE procedure" error in this reduced case also gave the same error with your new program.

Regards,
Vipul


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