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

[Van Snyder]

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.

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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