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.

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'

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>

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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That's great - you've put a lot of work into it. Personally, I do like the explicit join('::', @foo) in the output.

You guys are amazing! Thanks for such a thoughtful response. I'll have a play around with Devel::Declare.

2match comments /^ *warn __PACKAGE__.*/
highlight comments ctermfg=darkgrey
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Very nifty! I love Vim, so I'll give this a go too. Thanks.

Does caller() appeal to you in this case?

ps. I don't know much about C, so this is just a guess.

Update: caller() won't print out the var & contents inherently, but it does of course produce a stack trace.

Hi stevieb - caller() is very impressive, but, as you say, it seems to relate more to doing a stack trace about function calls etc. I'm after a simple function (or "macro" equivalent) to just plonk out trace values of the form "<line_no>: <var_name> = <var_value>", using a command like "DEBUG(<var_name>)". I guess I could use the debugger, but this would be simpler for little tasks. The trick is I can't see how to find the <var_name> itself.

I regretted posting originally, because I spoke before I fully comprehended what you were after.

I've written such tools before, but they aren't worthy of public consumption. Hopefully someone will be able to finger a CPAN module that will get you on your way.

Cheers,

Steve