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Dear group,

  Consider this situation: a program can run either interactive
or automatic: In interactive mode, you can fix errors and try again.
So if something fails, an optional stat argument passes the error
up the call procedure(s), user gets notified, fights his frustration and
tries again. In automatic mode, as there is trouble, we print a 
confusing message and quit.
This codes as:

if(interactive) then
 call do_some_work(stat)
else
  call do_some_work()
endif

Not so nice- the same situation occurs in do_some_work and lotsa places.

Now, do_some_work cannot access interactive, as this causes circular
module dependencies. Besides, i find it quite nice and consistent that
my routines behave as allocate et al.

So I remembered that my NAG f95 seems to pass optional arguments as
null pointers:

program tst
  integer, pointer :: p
  integer, target :: q

  nullify(p)
  q=0
  print *, "call tstsub()" ; call tstsub()
  print *, "call tstsub(q)" ; call tstsub(q)
  print *, "call tstsub(p)" ; call tstsub(p)
  p => q
  print *, "call tstsub(p=>q)" ; call tstsub(p)
contains
 subroutine tstsub(arg)
   integer, intent(in), optional :: arg
   print *, "Arg present: ", present(arg)
 end subroutine tstsub
end program tst

and hey! - it worked:
[klaus@platon klaus]$ a.out
 call tstsub()
 Arg present:  F
 call tstsub(q)
 Arg present:  T
 call tstsub(p)
 Arg present:  F
 call tstsub(p=>q)
 Arg present:  T
[klaus@platon klaus]$             

Great- I can associate or nullify stat at least once for each
subroutine, or can I?

I doubt it is very portable, my f90 texts say nothing about how
optionals are to be implemented.

Besides- maybe this is a compiler bug? What if I wanted this
routine to know I passed a nullified pointer? OK- the dummy arg
is not a pointer, so cannot be "NULL". (If I make arg a pointer,
the actual arg must be a pointer to and gets detected as present
even if nullified). Hm.

Comments? 

Klaus
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