Where was I called?

Ovid has asked for the wisdom of the Perl Monks concerning the following question:

The problem is a long story, but the minimal test case is pretty clear. I have a subroutine which needs to know where is was called. For example:

while ( my $value = some_sub() ) {
    # do something
    my $other_value = some_sub();
}
[download]

In the above case, it's pretty easy to disambiguate:

sub some_sub {
    my @caller = caller();
    # now I know where I was called
}
[download]

That assigns the package, filename, and line number to @caller. However, there's a very weird edge case here:

for ( 1 .. 10 ) {
    my ( $foo, $bar ) = ( some_sub(), some_sub() );
    # do stuff
}
[download]

@caller will hold the same value for each of those. I'm overriding a pre-existing interface, so I'm not at liberty to change said interface. Without more information available, is there any way in &some_sub to know which invocation on a line was called? I can't hardcode any knowledge about the structure of calling lines because this may be called in many different places.

Cheers,
Ovid

New address of my CGI Course.

Comment on Where was I called? Select or Download Code

Replies are listed 'Best First'.
Re: Where was I called? by BrowserUk (Patriarch) on Sep 28, 2006 at 12:54 UTC
Presumably this is for some kind of error reporting? How about simply reformatting the source code as: `for ( 1 .. 10 ) { my ( $foo, $bar ) = ( some_sub(), some_sub() ); # do stuff }` [download] Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal? "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l]
Re^2: Where was I called? by Ovid (Cardinal) on Sep 28, 2006 at 13:26 UTC
That's one thought, but it turns out there's a bug in the scheme. `while ( my $value = some_sub() ) { # line 50 my $next_value = some_sub(); # line 51 }` [download] This is something I forgot about, but it's very, very irritating. The first time some_sub() is called, caller will correctly report that line 50 is the calling line and the second time some_sub() is called, it reports line 51. However, the second time through the while loop, the first some_sub() is reported as being on line 51! That totally blows my initial scheme. Cheers, Ovid New address of my CGI Course.	[reply] [d/l]
Re^3: Where was I called? by diotalevi (Canon) on Sep 28, 2006 at 15:35 UTC
Wow, thanks Ovid! I never realized that problem existed. Just to explain, whenever a `nextstate` op is seen the global variable `PL_curcop` is set and that's some of what `caller()` uses. You know line 50 as you enter the loop, then the first line in the loop is 51 but you never reach that outer `nextstate` op to say you're testing a condition on line 50 again. I bet if you were really interested in solving this problem in a JFDI way that you could change that "`goto 9`" to be "`goto 2`" or promote the conditional expression into a do{} block. But that way lies madness so you won't do that. `### caller() will report line 50. 2 <;> nextstate(main 2 x:50) v 3 <{> enterloop(next->8 last->d redo->4) v 9 <0> pushmark s a <#> gv[X] s/EARLYCV b <1> entersub[t2] sKS/TARG,1 c <\|> and(other->4) vK/1 ### caller() will report line 51 4 <;> nextstate(main 1 x:51) v 5 <0> pushmark s 6 <#> gv[Y] s/EARLYCV 7 <1> entersub[t4] vKS/TARG,1 8 <0> unstack v goto 9 d <2> leaveloop vK/2 e <@> leave[1 ref] vKP/REFC` [download] ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊	[reply] [d/l] [select]
Re^4: Where was I called? by Ovid (Cardinal) on Sep 28, 2006 at 15:54 UTC
Re^5: Where was I called? by ysth (Canon) on Sep 29, 2006 at 02:39 UTC
Re: Where was I called? by cephas (Pilgrim) on Sep 28, 2006 at 13:20 UTC
This is less than ideal, but it gives you a little better visibility than just line number. `{ my ($line,$occurrence) = (0,0); sub some_sub { $, = ' - '; my @caller = caller(); if($caller[2] == $line) { $occurrence++; } else { $line = $caller[2]; $occurrence = 0; } print @caller, $occurrence, $/; } }` [download] You example shows the biggest limitation to this approach. `$occurrence` will just keep growing since it is the same line number. But if you know you are dealing with a specific number of calls on the line, you can mod whatever and get the specific occurence on the line.	[reply] [d/l] [select]
Re: Where was I called? by Anonymous Monk on Sep 28, 2006 at 11:08 UTC
The simplest way is to give this problem to Simon Cozens and claim it can't be done.	[reply]
Re^2: Where was I called? by larsen (Parson) on Sep 28, 2006 at 14:15 UTC
Which is the same way one of the head developers here at $work endorsed as "the best way to obtain things from system administrators". Never ask "Could you...?", but "Is it possible to...?"	[reply]
Re: Where was I called? by monkey_boy (Priest) on Sep 28, 2006 at 10:42 UTC
Would the use of caller combined with a Time-stamp help? The only way i can differentiate between the two is that one is executed before the other. This is not a Signature...	[reply] [d/l]
Re: Where was I called? by Anonymous Monk on Sep 28, 2006 at 10:41 UTC
use B;	[reply]


Don't ask to ask, just ask
	PerlMonks