Fortran 2003 / 2008: Elegant default arguments?

问题:

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.

回答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.

回答2:

While also looking into this, I found out that you can in fact do something like the proposed example using the OPTIONAL and VALUE attributes (at least with gfortran, not sure how different compilers might handle it). E.g.:

PROGRAM PDEFAULT 

  CALL SUB
  CALL SUB(3)

CONTAINS 
  SUBROUTINE SUB(VAL)
    INTEGER, OPTIONAL,VALUE :: VAL

    IF(.NOT. PRESENT(VAL)) VAL = -1 ! default value

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

END PROGRAM PDEFAULT

This was implemented in version 4.9 of gfortran. And here's the relevant explanation in the documentation for argument passing conventions:

For OPTIONAL dummy arguments, an absent argument is denoted by a NULL pointer, except for scalar dummy arguments of type INTEGER, LOGICAL, REAL and COMPLEX which have the VALUE attribute. For those, a hidden Boolean argument (logical(kind=C_bool),value) is used to indicate whether the argument is present.

I also found this discussion interesting as historical context.

Maybe somebody more knowledgeable might have comments on whether doing this is a bad idea (aside from being compiler dependent), but at least at face value it seems like a nice workaround.

Note that this behavior is not part of the Fortran standard, and depends on the implementation of a given compiler. For example, the example code segfaults when using ifort (version 16.0.2).

回答3:

Whilst I certainly wouldn't advocate doing so in most situations (and indeed you can't in some situations), one may sometimes use an interface to provide a single entry point for multiple routines with different required arguments rather than using an optional argument. For example your code could be written like

MODULE subs
  implicit none
  public :: sub

  interface sub
    module procedure sub_default
    module procedure sub_arg
  end interface
 contains
  SUBROUTINE SUB_arg(VAL)
    INTEGER :: VAL
    WRITE(*,'("VAL is ", I0)') VAL
  END SUBROUTINE SUB_arg

  SUBROUTINE SUB_default
     integer, parameter :: default = 3
     CALL SUB_arg(default)
  END SUBROUTINE SUB_default
END MODULE SUBS

PROGRAM test
   use subs, only: sub
   call sub
   call sub(5)
END PROGRAM TEST

Again, I don't recommend this approach, but I thought I should include it anyway as an alternative way of providing something that looks like a default.

回答4:

I hope Fortran to support a popular syntax like

subroutine mysub( x, val = -1 )
integer, optional :: val

or in a more Fortran style

subroutine mysub( x, val )
integer, optional :: val = -1     !! not SAVE attribute intended

but this seems not supported (as of 2016). So some workaround needs to be done by the users' side...

In my case, after trial-and-errors, I settled down to attaching one underscore to the optional dummy argument, so doing something like (*)

subroutine mysub( x, val_)
integer, optional :: val_
integer val

Other people seem to like the opposite pattern (i.e., dummy variable => sep, local variable => sep_, see split() in StringiFor, for example). As seen in this line, the shortest way to set the default value is

val = -1 ; if (present(val_)) val = val_

But because even this line is somewhat verbose, I usually define a macro like

#define optval(x,opt,val) x = val; if (present(opt)) x = opt

in a common header file and use it as

subroutine mysub( x, val_, eps_ )
    integer :: x
    integer, optional :: val_
    real, optional :: eps_
    integer  val
    real     eps
    optval( val, val_, -1 )
    optval( eps, eps_, 1.0e-5 )    

    print *, "x=", x, "val=", val, "eps=", eps
endsubroutine

...
call mysub( 100 )
call mysub( 100, val_= 3 )
call mysub( 100, val_= 3, eps_= 1.0e-8 )

However, I believe this is still far from elegant and no more than an effort to make it slightly less error-prone (by using the desired variable name in the body of the subroutine).

---

Another workaround for a very "big" subroutine might be to pass a derived type that contains all the remaining keyword arguments. For example,

#define getkey(T) type(T), optional :: key_; type(T) key; if (present(key_)) key = key_

module mymod
    implicit none

    type mysub_k
        integer  :: val = -1
        real     :: eps = 1.0e-3
    endtype
contains

subroutine mysub( x, seed_, key_ )
    integer :: x
    integer, optional :: seed_
    integer :: seed
    getkey(mysub_k)   !! for all the remaining keyword arguments
    optval( seed, seed_, 100 )    

    print *, x, seed, key% val, key% eps
endsubroutine

endmodule

program main
    use mymod, key => mysub_k

    call mysub( 10 )
    call mysub( 20, key_= key( val = 3 ) )
    call mysub( 30, seed_=200, key_= key( eps = 1.0e-8 ) )  ! ugly...
endprogram

This might be a bit close to what is done by some dynamic languages under the hood, but this is again far from elegant in the above form...

---

(*) I know it is often considered ugly to use CPP macros, but IMO it depends on how they are used; if they are restricted to limited extensions of Fortran syntax, I feel it is reasonable to use (because there is no metaprogramming facility in Fortran); on the other hand, defining program-dependent constants or branches should probably be avoided. Also, I guess it would be more powerful to use Python etc to make more flexible preprocessors (e.g., PreForM.py and fypp and so on), e.g., to allow a syntax like subroutine sub( val = -1 )