in reply to How to get variable name in trace message

Perl doesn't make this sort of thing especially easy. Devel::Assert kinda pulls off this trick...

use Devel::Assert -all;

my $x = 2;
assert($x > 3);

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The above will die with the message: Assertion ' $x > 3 ' failed at assert.pl line 5.

How does Devel::Assert do it? With a bit of Devel::Declare magic. Devel::Declare hooks into the Perl compiler, rewriting code before it gets compiled. Using Devel::Declare you could define your own Dumper function which rewrites this:

Dumper($foo)

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to this:

Data::Dumper->Dump([$foo], [qw[$foo]])

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Not impossible, but probably more effort than it's worth.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

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Re^2: How to get variable name in trace message
by tobyink (Canon) on Jun 15, 2012 at 09:08 UTC

    While I still stand by the "more effort than it's worth" comment, I've been meaning to get more stuck into Devel::Declare lately, so I had a go, and this is what I came up with:

    Data::Dumper::Declare

    It gives you a Dumper function just like Data::Dumper (in fact, it uses Data::Dumper internally, so respects things like $Data::Dumper::Sortkeys), but figures out the variable names being dumped. So you can do this:

    use Data::Dumper::Declare;

my $foo = 1;
my $bar = 'Hello World';
my @baz = qw(1 2 3);

print Dumper($foo, substr($bar, 0, 5), @baz);

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    ... and you get:

    $foo = 1;
$EXPR = 'Hello';
\@baz = [
  1,
  2,
  3
];

    (Note that unlike Data::Dumper, you can't just eval the output. An assignment like \@bar = [...] is illegal.)

    Data::Dumper::Declare isn't on CPAN yet. Do people think it useful enough to upload?

    Update: just pushed a change which allows arrays and hashes to be dumped more nicely. e.g.

    @baz = (
  1,
  2,
  3
);

    Update 2: Data::Dumper::Declare is now on CPAN.

    Update 3: Given this:

    use Data::Dumper::Declare;

my @foo = qw(Hello World);

print Dumper(
    @foo,
    $foo[0],
    $foo[1],
    join('::', @foo),
);

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    The output is:

    @foo = (
  'Hello',
  'World'
);
$foo[0] = 'Hello';
$foo[1] = 'World';
$EXPR = 'Hello::World';

    Can anyone suggest something better than $EXPR in the output? It's certainly possible to output join('::', @foo) = 'Hello::World'; but I'm not especially fond of that. What do other people think?

    perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'
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[d/l]
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      Data::Dumper::Declare isn't on CPAN yet. Do people think it useful enough to upload?
      Definitely! This is something I always wanted to have!!!

      -- 
      Ronald Fischer <ynnor@mm.st>
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[d/l]

      There is no variable name associated with the expression, so undef could work, and it is ok to assign to undef.

      Quoting the expression might also be a nice option if possible.

      $foo = 1;
undef = 'StringResultOfExpression';
$str . result('expression') = 'StringResultOfExpression';
@bar = [
  1,
  2,
  3,
];

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[d/l]
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      That's great - you've put a lot of work into it. Personally, I do like the explicit join('::', @foo) in the output.
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Re^2: How to get variable name in trace message
by ChrisBeall (Novice) on Jun 15, 2012 at 06:15 UTC
    You guys are amazing! Thanks for such a thoughtful response. I'll have a play around with Devel::Declare.
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