Beefy Boxes and Bandwidth Generously Provided by pair Networks
The stupid question is the question not asked

Re: Nervously....How do I access the Name of a Perl variable?

by wog (Curate)
on Jun 11, 2002 at 01:35 UTC ( #173363=note: print w/replies, xml ) Need Help??

in reply to Nervously....How do I access the Name of a Perl variable?

Getting the line number and filename is relatively easy:

sub trace { my($package, $filename, $line) = caller; # ... }

You can also get the subroutine its in. See the docs for caller.

Getting the names of the relevant variables is much harder, because perl does not such a convinent mechanism for compile-time expanded macros as C. However there are things called source filters, and the Filter::Simple module provides a nice interface to them. So, we can use that to make a module that adds a TRACE() macro:

package TraceMacro; use Filter::Simple; FILTER_ONLY code => sub { s/TRACE\(\s*([^)]+)\s*\)/__PACKAGE__."::_trace(q($1),$1)"/eg; }; # replace each TRACE(...) with <this package>::_trace(q(...),...) sub _trace { my($names,@values) = @_; my @names = split /\s*,\s*/, $names; # extract varnames my($package,$file,$line) = caller; foreach my $i(0..$#names) { print STDERR "$file ($line): $names[$i]:'$values[$i]'"; } }

(The module just needs to be saved appropriately to a file called by the name of where Package is whatever's specified by the package declaration. (You could change that.))

This has the side-effect of allowing arbitrary expressions instead of just variable names.

update: Usage would be like:

use TraceMacro; # ... TRACE( $foo, $bar );
If you want to change TRACE to trace (I prefer that compile-time-special things be uppercased), then it shouldn't be hard to fiquire that out.

Replies are listed 'Best First'.
Re: Re: Nervously....How do I access the Name of a Perl variable?
by Anonymous Monk on Jun 11, 2002 at 03:42 UTC

    Outstanding! Everything I wanted and an object lesson in creating my first package, which I didnt really think I was up for yet but this looks simple enough that even I can do it! Thankyou. Anyone with a spare vote handy, give it to this man.

    Of course, it will start out as cut&paste for now, but you gave me enough hints for possibilities that I can add (like sub name) to encourage me to investigate the references.

    To other responders:

    I am aware of the debugger and use it if I get something that needs it, but while im just cranking code out, sticking a TRACE in various places and running the prog is a nice quick way of checking things are doing what I expect or not.

Re: Re: Nervously....How do I access the Name of a Perl variable?
by Anonymous Monk on Jun 11, 2002 at 03:59 UTC

    Sorry for the second post here but reading the code I thought of one more feature that may be possible.and will try to work out for myself, but I am unsure of the side effects.

    If a SINGLE value is passed, how hard would it be to have it eval the passed expression and then return that from _trace()?

    That way, the TRACE macro could be used in-situ in the code, and by a suitable test of some flag (possibly the systems debug settings) either Trace or not trace and the program would still fuction (possibly inefficiently).

    Does this thought have any merit?

Log In?

What's my password?
Create A New User
Domain Nodelet?
Node Status?
node history
Node Type: note [id://173363]
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others cooling their heels in the Monastery: (6)
As of 2022-10-04 20:42 GMT
Find Nodes?
    Voting Booth?
    My preferred way to holiday/vacation is:

    Results (19 votes). Check out past polls.