[J3] Thoughts on the semantics of generics

[Van Snyder]

On Sun, 2018-08-12 at 16:25 -0600, William Clodius via J3 wrote:

> The iterator proper usually provides a set sequential access, but it
> might be useful to allow a parallel access form, though
> the history of the FORALL construct is discouraging.... Iterators and
> maps would often be simplified if Fortran included co-routines.


I don't think a CONCURRENT ITERATE construct using an iterator is
problematic.  Its body would have the same rules as a DO CONCURRENT
construct.  The loop control would initiate the iterator, then resume it
repeatedly, launching a separate iteration of the body, until the
iterator terminates instead of suspending.  If the processor actually
parallelizes the iterations, each iteration would simply terminate
instead of resuming the iterator.  This could be useful for a set
container, but I don't see much use of it for iterating over
intrinsically ordered containers, such as trees, stacks, queues, ....
That's not to say it should be prohibited, just that I don't see users
using parallel iterators for intrinsically ordered containers.

Coroutines that are monitors could be resumed in different iterations of
a DO CONCURRENT construct.  I don't see how one could initiate a
coroutine in one iteration of a DO CONCURRENT construct and resume it in
others.  I think it would have to be active first.  The statement to
resume it would need a status, in case some iteration terminated it, or
it terminated itself by running out of work to do.  An alternative to a
monitor coroutine is to generalize critical sections to iterations of DO
CONCURRENT constructs, and require testing and resuming coroutines to be
done in critical sections.  Another alternative is simply a monitor
subroutine that tests the coroutine's active status and if it's active
resumes it.  This isn't as secure as a coroutine RESUME statement that
returns a status, or a generalized critical section.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.j3-fortran.org/pipermail/j3/attachments/20180813/69acc76f/attachment-0001.html>