[J3] Default values for optional arguments

[Shterenlikht, Anton]

I see no problem here, that needs to be solved.

Perhaps this style is marginally more consise and clear:

 subroutine foo(a, b_optional)
   integer, intent(in), optional :: b_optional
   integer :: b = 42
...
   if (present(b_optional)) b = b_optional

On the other hand, being verbose and explicit
with optional arguments is probably a good thing,
as this is a frequent source of user errors.
So your original form might be preferred.

Anton

> On 20 Jan 2021, at 07:32, Thomas König via J3 <j3 at mailman.j3-fortran.org> wrote:
> 
> 
> A somewhat common thing to do for optional arguments is to set them
> to a default value if the caller did not specify anything.  One idiom
> to do this would be
> 
>  subroutine foo(a, b)
>    integer, intent(in), optional :: b
>    integer :: b_val
> ...
>    if (present(b)) then
>      b_val = b
>    else
>      b_val = 42
>    end if
> 
> and then use b_val and c_val. This idiom is wasteful of lines (as
> written, it takes five lines of code for a single default value). More
> importantly, it is also error-prone because it is necessary to remember
> to use b_val instead of b.
> 
> The first drawback could be mitigated by doing something like
> 
>  if (present(b)) b_val = b
>  if (.not.present(b)) b_val = 42
> 
> (not very clear, in my opinion) or by using yet-to-be-added conditional
> expressions, but people would still have to use a different variable
> name for essentially the same thing.
> 
> What could help is to add some way to express default initialization for
> optional arguments, something along the lines of
> 
>  integer, intent(in), optional :: b = 42
> 
> or
> 
>  integer, intent(in), optional, default (42) :: b
> 
> Opinions?
> 
> Best regards
> 
>    Thomas