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Re: main or procedure [message #54624 is a reply to message #54623]

Wed, 20 June 2007 07:39

Marshall Perrin
Messages: 44
Registered: December 2005

Member

David Fanning <news@dfanning.com> wrote:
> sujian@gmail.com writes:
>
>> When I .compile a procedure, it says,
>> You compiled a main program while inside a procedure. Returning.
>>
>> And all the information before the compilation was lost.
>>
>> How to fix it?
>
> You need to do two things: (1) Add more error handling
> to your code, so that when a program crashes it goes
> back to the main program level (ON_ERROR, 1), and (2)
> learn to type RETALL *immediately* when your program
> crashes, and certainly before you do any more compiling.

It feels odd to disagree with the great Dr. Fanning, but in this case
I want to offer a counter-suggestion to the above advice. I much
prefer for my programs to stop at the point of failure when they
crash, because that way I have a chance to examine the variables and
program state and try to figure out what went wrong. I *hate* it when
programs return back to the main level, since then it's usually much
harder to understand the situation that caused the crash.

Besides, often when the error is in a subroutine, it can be fixed and
the program execution resumed from that point. This can be a big timesaver
when running some computationally intensive code that you don't want to
have to restart from the very beginning:

dxfinish ; set breakpoint after broken subroutine
return ; or return,0 if you're in a function
.comp fixed_subroutine.pro ; after you've fixed the error
fixed_subroutine, /arguments ; paste in the relevant line to re-run the
; routine, which will hopefully now work

I think the difference between David's approach and mine probably
comes down to how we structure our main programs. His are probably
mostly Widget programs, so if you retall to the main level, hopefully
all your state is retained and you can resume your work without losing
all your information. I on the other hand usually just run long
computations or data analysis routines in regular procedures, so a
retall in the middle of that would indeed lose all my state. Hence my
deep aversion to "on_error, 1". I'd argue that these are equally
good, if different, approaches, but maybe other folks here will care
to chime in with their opinions.

For the original poster: The reason it returned automatically to the
top level in your case is that you compiles a main routine (i.e.
a file with IDL code but no "PRO whatever" or "FUNCTION whatever")
while inside a procedure or function. That, you can't do. You can
compile a PRO or FUNCTION while inside another function (but not while
inside itself), but main-level routines can only be compiles from the
main level. Which means if you want to have a batch of commands that
you can execute interactively from the command line while at a
breakpoint, you either need to package it up inside a procedure, or
you need to run it with the @FILENAME syntax (with all the hassles
about loops and punctuation that entails.)

- Marshall

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[Message index]

		Re: main or procedure By: Carsten Lechte on Thu, 21 June 2007 12:06
		Re: main or procedure By: JD Smith on Wed, 20 June 2007 17:02
		Re: main or procedure By: JD Smith on Wed, 20 June 2007 16:36
		Re: main or procedure By: Carsten Lechte on Wed, 20 June 2007 13:13
		Re: main or procedure By: David Fanning on Wed, 20 June 2007 12:14
		Re: main or procedure By: Carsten Lechte on Wed, 20 June 2007 11:50
		Re: main or procedure By: Conor on Wed, 20 June 2007 11:54
		Re: main or procedure By: Paul Van Delst[1] on Wed, 20 June 2007 10:48
		Re: main or procedure By: Vince Hradil on Wed, 20 June 2007 09:47
		Re: main or procedure By: Carsten Lechte on Wed, 20 June 2007 09:12
		Re: main or procedure By: Carsten Lechte on Wed, 20 June 2007 09:03
		Re: main or procedure By: Conor on Wed, 20 June 2007 08:42
		Re: main or procedure By: David Fanning on Wed, 20 June 2007 07:56
		Re: main or procedure By: David Fanning on Wed, 20 June 2007 07:40
		Re: main or procedure By: Marshall Perrin on Wed, 20 June 2007 07:39
		Re: main or procedure By: Carsten Lechte on Wed, 20 June 2007 07:24
		Re: main or procedure By: David Fanning on Wed, 20 June 2007 07:00
		Re: main or procedure By: Carsten Lechte on Wed, 20 June 2007 06:07
		Re: main or procedure By: Conor on Wed, 20 June 2007 05:39
		Re: main or procedure By: David Fanning on Wed, 20 June 2007 05:20

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