Re: Top 10 IDL Requests

[kashyap(at)head-cfa.harvard.edu (Vinay L. Kashyap)]

Ah, the specific reason I came up with that request was that I was trying
to have CURVEFIT pass strange keywords down to a curve-defining function
(which is called via CALL_PROCEDURE), and failing.  In that particular
case, the fix is simple of course (edit CURVEFIT).

Also, I was not aware that the function definition does not *have* to
have the "_extra" -- not quite what the help file says:

	You can pass keyword parameters to called routines
	by value by adding the formal keyword parameter
	"_EXTRA" (note the underscore character) to the
	definition of your routine.  ...

now that I am primed to it, I notice that it goes on to say:

	When the keyword _EXTRA appears in a procedure or
	function call, its argument is either a structure
	containing additional keyword/value pairs which
	are inserted into the argument list ...

It's not tremendously clear that putting the _extra in a function
definition is optional.  I suppose it helps only if the function
in question calls other functions or subroutines to which it must
pass keywords.

Thanks for the clarifications,
Vinay

In article <on1z0hrdn4.fsf@cow.physics.wisc.edu>,
Craig Markwardt  <craigmnet@cow.physics.wisc.edu> wrote:
>davidf@dfanning.com (David Fanning) writes:
>
>> Craig Markwardt (craigmnet@cow.physics.wisc.edu) writes:
>> 
>> > davidf@dfanning.com (David Fanning) writes:
>> > > Vinay L. Kashyap (kashyap@head-cfa.harvard.edu) writes:
>> > > > 2. _EXTRA
>> > > > 
>> > > > Please consider having all built-in commands accept _EXTRA as a keyword.
>> > > 
>> > > Uh, this is the way it works. :-)
>> > 
>> > Uh, not quite.  There are some built in commands that don't accept any
>> > keywords at all.  The _EXTRA keyword doesn't work for them, *even* if
>> > the value passed is empty!
>> > 
>> > Why is this important?  Makes it a pain to write a wrapper procedure
>> > or function.
>> 
>> Alright, I must be obtuse today, but I can't figure out why
>> it would be hard to write wrapper routines for commands that
>> don't take keywords. Surely in writing the wrapper you give
>> at least *some* thought to what keywords you might expect
>> to be passed. Adding an _Extra to such a command seems
>> excessively anal at the very least, and certainly unnecessary. :-)
>> 
>> And what commands did you have in mind? I've never encountered
>> a built-in command that didn't accept this keyword mechanism.
>
>Hrmm.  The moment I need to find an example, and I can't find it.
>Arghh.  An example of a built-in command that doesn't take keywords is
>EMPTY, but I agree that it's a pretty lame example.  I actually would
>hope that *all* procedures and functions could be called with _EXTRA,
>whether or not they actually accept keywords.  For, example, this
>statement
>
>  call_procedure, 'EMPTY', _EXTRA=null
>
>will fail no matter what, even if null is an undefined variable.
>Shouldn't IDL be smart enough to test whether the _EXTRA value is
>undefined before it crashes?
>
>I'm always looking for ways to avoid special cases in wrapper
>routines.  Real world examples of such unavoidable abominations are
>given below.
>
>Craig
>
>
>(from XFWINDOW in XFWINDOW_CALL_PROCEDURE)
>  sz = size(key)   
>  if sz(sz(0)+1) EQ 8 then begin ;; Keywords are present
>      xfwindow_rekey, key
>      case n_args of 
>          0: call_procedure, cmd, _extra=key
>          1: call_procedure, cmd, x0, _extra=key
>          2: call_procedure, cmd, x0, x1, _extra=key
>          3: call_procedure, cmd, x0, x1, x2, _extra=key
>          4: call_procedure, cmd, x0, x1, x2, x3, _extra=key
>          5: call_procedure, cmd, x0, x1, x2, x3, x4, _extra=key
>      endcase
>  endif else begin               ;; No keywords are present
>      case n_args of 
>          0: call_procedure, cmd
>          1: call_procedure, cmd, x0
>          2: call_procedure, cmd, x0, x1
>          3: call_procedure, cmd, x0, x1, x2
>          4: call_procedure, cmd, x0, x1, x2, x3
>          5: call_procedure, cmd, x0, x1, x2, x3, x4
>      endcase
>  endelse
>
>(from MPFIT in MPFIT_CALL)
>  if proc then begin
>      if n_params() EQ 3 then begin
>          if n_elements(extra) GT 0 then $
>            call_procedure, fcn, x, f, fjac, _EXTRA=extra $
>          else $
>            call_procedure, fcn, x, f, fjac
>      endif else begin
>          if n_elements(extra) GT 0 then $
>            call_procedure, fcn, x, f, _EXTRA=extra $
>          else $
>            call_procedure, fcn, x, f
>      endelse
>  endif else begin
>      if n_params() EQ 3 then begin
>          if n_elements(extra) GT 0 then $
>            f = call_function(fcn, x, fjac, _EXTRA=extra) $
>          else $
>            f = call_function(fcn, x, fjac)
>      endif else begin
>          if n_elements(extra) GT 0 then $
>            f = call_function(fcn, x, _EXTRA=extra) $
>          else $
>            f = call_function(fcn, x)
>      endelse
>  endelse  
>
>-- 
>--------------------------------------------------------------------------
>Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
>Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
>--------------------------------------------------------------------------


-- 
______________________________________________________________________________
kashyap@head-cfa.harvard.edu         617 495 7173 [CfA/P-146] 617 496 7173 [F]