Re: Passing optional parameters through a wrapper routine

["J.D. Smith" <jdsmith(at)astro.cornell.edu>]

William Thompson wrote:
> 
> edward.s.meinel@aero.org writes:
> 
> >In article <950129121.690143@clam-55>,
> >  "Mark Hadfield" <m.hadfield@niwa.cri.nz> wrote:
> 
> >> That's an interesting point David. The first few lines
> >> of my routines tend to look something like this:
> >>
> >>  if n_elements(arg1) then message, 'You haven't defined arg1'
> 
> >...
> 
> >> 2. The principle that in scientific programming
> >> (as opposed, say, to Web page programming)
> >> it is much better for programs to crash than to continue
> >> and return bad data.
> 
> >Ugh, I *hate* MESSAGE. Why cause a crash when it is easy to exit nicely?
> >How about:
> 
> >  IF N_ELEMENTS(arg1) EQ 0 THEN BEGIN
> >       print, 'You haven't defined arg1'
> >       RETURN
> >  ENDIF
> 
> >or even:
> 
> >  IF N_ELEMENTS(arg1) EQ 0 THEN BEGIN
> >       dummy = DIALOG_MESSAGE('You haven't defined arg1')
> >       RETURN
> >  ENDIF
> 
> >This way the user gets the message, but the program doesn't crash. This
> >is especially helpful when the procedure is used in a widget -- I don't
> >have to manually clean up everything before trying again.
> 
> >>  if size(arg2, /TNAME) ne 'STRING' then message, 'Arg2 must be string
> >> specifying the file name'
> 
> >...
> 
> >> if in doubt, stop and call for help.
> 
> >Right, but you can stop and ask for help without forcing a crash. How
> >about:
> 
> >IF SIZE(arg2, /TNAME) NE 'STRING' THEN BEGIN
> 
> >; Oooops! forgot the file name.
> 
> >  arg2 = DIALOG_PICKFILE(set_the_appropriate_keywords)
> >  IF arg2 EQ '' THEN BEGIN
> >    dummy = DIALOG_MESSAGE( $
> >            'You must provide a file name as the second argument')
> >    RETURN
> >  ENDIF
> >ENDIF
> 
> I tend to agree with Mark Hadfield that it's better to crash than to not catch
> the error and let the program continue on.  If one is operating in a
> user-driven environment, then bringing it to the user's attention, such as
> popping up an error widget as described above, is a good way to handle it.
> However, one must also think about the case where data analysis software is
> allowed to run in batch mode.
> 
> One trick I've adopted in many of my programs is to use an error message
> keyword, called ERRMSG.  Then, instead of using something like
> 
>         MESSAGE, 'You haven't defined arg1'
> 
> I substitute
> 
>         MESSAGE = 'You haven't defined arg1'
>         GOTO, HANDLE_ERROR
> 
> At the end of the program, I have lines like
> 
>                 GOTO, FINISH
>         ;
>         ;  Error handling point.
>         ;
>         HANDLE_ERROR:
>                 IF N_ELEMENTS(ERRMSG) NE 0 THEN                 $
>                         ERRMSG = 'My_Routine: ' + MESSAGE ELSE  $
>                         MESSAGE, MESSAGE
>         ;
>         ;  Exit point.
>         ;
>         FINISH:
>                 RETURN
>                 END
> 
> That way, if the calling routine calls "My_Routine" without passing the ERRMSG
> keyword, then messages are handled with the MESSAGE facility.  However, if
> ERRMSG is passed, then the error message is passed back to the calling routine
> and it's then responsible for deciding what to do about it.  The only drawback
> to this scheme is that one has to define ERRMSG first, so that "My_Routine"
> knows that it was passed, e.g
> 
>         ERRMSG = ''
>         My_Routine, ERRMSG=ERRMSG, ...
>         IF ERRMSG NE '' THEN ...


That's why arg_present() was invented!  It can detect undefined but nevertheless
passed-in parameters, available by reference from the calling level.  No need to
define them beforehand.  It also saves you from the silly user who does:

IDL> my_routine,ERROR_MESSAGE='This is a fine message'

You would change your code to:

IF arg_present(ERRMSG) THEN $
ERRMSG = 'My_Routine: ' + MESSAGE ELSE  $
MESSAGE, MESSAGE

Another side-benefit of arg_present() is that you can use it to annoy David
Fanning by forcing him to contradict bold statements such as "it is NOT possible
to reliably determine if a keyword was used in a call to your program", when in
fact the test:

n_elements(k) ne 0 OR arg_present(k)

will tell you precisely this ;). This might be useful if k is a flag, which
you'd like to set if anything, even an undefined variable, is passed it. 


JD

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