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Gisle Aas


Data::Dump - Pretty printing of data structures


 use Data::Dump qw(dump ddx);

 $str = dump(@list);
 @copy_of_list = eval $str;

 # or use it for easy debug printout
 ddx localtime;


This module provide functions that takes a list of values as their argument and produces a string as its result. The string contains Perl code that, when evaled, produces a deep copy of the original arguments.

The main feature of the module is that it strives to produce output that is easy to read. Example:

    @a = (1, [2, 3], {4 => 5});


    (1, [2, 3], { 4 => 5 })

If you dump just a little data, it is output on a single line. If you dump data that is more complex or there is a lot of it, line breaks are automatically added to keep it easy to read.

The following functions are provided (only the dd* functions are exported by default):

dump( ... )
pp( ... )

Returns a string containing a Perl expression. If you pass this string to Perl's built-in eval() function it should return a copy of the arguments you passed to dump().

If you call the function with multiple arguments then the output will be wrapped in parenthesis "( ..., ... )". If you call the function with a single argument the output will not have the wrapping. If you call the function with a single scalar (non-reference) argument it will just return the scalar quoted if needed, but never break it into multiple lines. If you pass multiple arguments or references to arrays of hashes then the return value might contain line breaks to format it for easier reading. The returned string will never be "\n" terminated, even if contains multiple lines. This allows code like this to place the semicolon in the expected place:

   print '$obj = ', dump($obj), ";\n";

If dump() is called in void context, then the dump is printed on STDERR and then "\n" terminated. You might find this useful for quick debug printouts, but the dd*() functions might be a better alternative for this.

There is no difference between dump() and pp(), except that dump() shares its name with a not-so-useful perl builtin. Because of this some might want to avoid using that name.

quote( $string )

Returns a quoted version of the provided string.

It differs from dump($string) in that it will quote even numbers and not try to come up with clever expressions that might shorten the output.

dd( ... )
ddx( ... )

These functions will call dump() on on their argument and print the result to STDOUT (actually, it's the currently selected output handle, but STDOUT is the default for that).

The difference between them is only that ddx() will prefix the lines it prints with "# " and mark the first line with the file and line number where it was called. This is meant to be useful for debug printouts of state within programs.


Code references will be displayed as simply 'sub { "???" }' when dumped. Thus, evaling them will not reproduce the original routine.

If you forget to explicitly import the dump function, your code will core dump. That's because you just called the builtin dump function by accident, which intentionally dumps core. Because of this you can also import the same function as pp, mnemonic for "pretty-print".


The Data::Dump module grew out of frustration with Sarathy's in-most-cases-excellent Data::Dumper. Basic ideas and some code are shared with Sarathy's module.

The Data::Dump module provides a much simpler interface than Data::Dumper. No OO interface is available and there are no configuration options to worry about (yet :-). The other benefit is that the dump produced does not try to set any variables. It only returns what is needed to produce a copy of the arguments. This means that dump("foo") simply returns "foo", and dump(1..5) simply returns (1, 2, 3, 4, 5).


Data::Dumper, Storable


The Data::Dump module is written by Gisle Aas <gisle@aas.no>, based on Data::Dumper by Gurusamy Sarathy <gsar@umich.edu>.

 Copyright 1998-2000,2003-2004,2008 Gisle Aas.
 Copyright 1996-1998 Gurusamy Sarathy.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.