optional parameters - Fortran 2003 / 2008: Elegant default arguments?

Question

In fortran, we can define default arguments. However, if an optional argument is not present, it can also not be set. When using arguments as keyword arguments with default values, this leads to awkward constructs like

PROGRAM PDEFAULT 

  CALL SUB
  CALL SUB(3)

CONTAINS 
  SUBROUTINE SUB(VAL)
    INTEGER, OPTIONAL :: VAL
    INTEGER :: AVAL ! short for "actual val"

    IF(PRESENT(VAL)) THEN
       AVAL = VAL
    ELSE 
       AVAL = -1   ! default value 
    END IF

    WRITE(*,'("AVAL is ", I0)') AVAL
  END SUBROUTINE SUB

END PROGRAM PDEFAULT

Personally, I often ran into the problem of accidentially typing VAL instead of AVAL, i.e. the disconnect between the variable name in the interface, and the initialized value used in the code can introduce runtime bugs – let alone that this manner of initialization is rather verbose.

Is there some more elegant way of using optional arguments with a default value?

Example It would feel more natural to write something like

IF(NOT(PRESENT(VAL))) VAL = -1 

because it avoids the VAL vs AVAL confusion. But it isn't valid, presumably because Fortran passes arguments by reference and thus if VAL is not present in the CALL statement, no memory is associated with VAL and VAL = -1 would cause a segfault.

Answer 1

You described the situation rather well. There is no other way I am aware off and that is standard conforming. The pattern with a local variable named similarly is what people often use. The other option is to just put if (present()) else everywhere, but that is awkward.

The point is that they are optional arguments, not default arguments. Fortran doesn't have default arguments. The may have been better, but that is not what the committee members have chosen in the 80s when preparing Fortran 90.