(j3.2006) (SC22WG5.5369) Question on PURE subroutines

[Bill Long]

On Dec 4, 2014, at 2:03 PM, Van Snyder <Van.Snyder at jpl.nasa.gov> wrote:

> On Thu, 2014-12-04 at 16:55 +0000, Bader, Reinhold wrote:
>> Hello all, 
>> 
>> 
>> 
>> consider the program
>> 
>> 
>> 
>> module mod_proc_cont_01
>> 
>>  implicit none
>> 
>>  type :: cont
>> 
>>     real, pointer :: p(:) => null()
>> 
>>  end type cont
>> 
>>  real, target :: p(2)
>> 
>> contains
>> 
>>  pure subroutine pf(x) 
>> 
>>    type(cont), intent(inout) :: x
>> 
>>    integer :: i
>> 
>> 
>> 
>>    if (associated(x%p)) then
>> 
>>       x%p = [ (real(2*i), i=1, size(x%p)) ]
>> 
>>    end if
>> 
>>  end subroutine pf
>> 
>> end module mod_proc_cont_01
>> 
>> program proc_cont_01
>> 
>>  use mod_proc_cont_01
>> 
>>  implicit none
>> 
>>  type(cont) :: xx
>> 
>> 
>> 
>>  p = [ 1.0, 2.0 ]
>> 
>>  xx%p => p
>> 
>>  call pf(xx)
>> 
>>  write(*,*) p
>> 
>> end program proc_cont_01
>> 
>> 
>> 
>> 
>> 
>> It appears to conform to the Fortran 2008 standard; it is accepted by
>> the compilers available to me. Yet it 
>> 
>> appears to implement a side effect in a PURE procedure. Was this
>> intended? If not, would adding the following
>> 
>> text in 007 fix the problem?
>> 
>> 
>> 
>> [312:30] After "any designator", add " with the POINTER attribute or "
> 
> Assuming this is in the first line of C1283 in F08, it goes a bit too
> far.  It effectively prevents meaningful use of pointers within pure
> procedures.
> 
> For example, if you have several local variables upon which the code
> needs to act in several places, instead of repeating the test throughout
> the code, do the test once and associate a pointer with the necessary
> variable.  The proposed change to C1283 would make it impossible to
> assign a value using that pointer.


The issue that Reinhold points out is the case of a pointer appearing in a variable definition context when its target is not a local variable, but one that is declared in the specification part of a module.  We prohibit direct definition of such a variable in a pure procedure, so to seems reasonable to also prohibit definition of a pointer associated with such a variable. 

Cheers,
Bill



> 
> I'm not convinced this is an undesirable side effect, but if it is, a
> much more elaborate prohibition would be needed, to prevent a pointer
> component of a dummy argument from appearing in a variable definition
> context or as the data target in a pointer assignment statement.
> 
>> Cheers
>> 
>> Reinhold
>> 
>> 
>> 
>> 
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> 
> 
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Bill Long                                                                       longb at cray.com
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