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Data::Dumper(3)	       Perl Programmers	Reference Guide	       Data::Dumper(3)

       Data::Dumper - stringified perl data structures,	suitable for both
       printing	and "eval"

	   use Data::Dumper;

	   # simple procedural interface
	   print Dumper($foo, $bar);

	   # extended usage with names
	   print Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]);

	   # configuration variables
	     local $Data::Dumper::Purity = 1;
	     eval Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]);

	   # OO	usage
	   $d =	Data::Dumper->new([$foo, $bar],	[qw(foo	*ary)]);
	   print $d->Dump;
	   eval	$d->Dump;

       Given a list of scalars or reference variables, writes out their
       contents	in perl	syntax.	The references can also	be objects.  The
       content of each variable	is output in a single Perl statement.  Handles
       self-referential	structures correctly.

       The return value	can be "eval"ed	to get back an identical copy of the
       original	reference structure.  (Please do consider the security
       implications of eval'ing	code from untrusted sources!)

       Any references that are the same	as one of those	passed in will be
       named $VARn (where n is a numeric suffix), and other duplicate
       references to substructures within $VARn	will be	appropriately labeled
       using arrow notation.  You can specify names for	individual values to
       be dumped if you	use the	"Dump()" method, or you	can change the default
       $VAR prefix to something	else.  See $Data::Dumper::Varname and
       $Data::Dumper::Terse below.

       The default output of self-referential structures can be	"eval"ed, but
       the nested references to	$VARn will be undefined, since a recursive
       structure cannot	be constructed using one Perl statement.  You should
       set the "Purity"	flag to	1 to get additional statements that will
       correctly fill in these references.  Moreover, if "eval"ed when
       strictures are in effect, you need to ensure that any variables it
       accesses	are previously declared.

       In the extended usage form, the references to be	dumped can be given
       user-specified names.  If a name	begins with a "*", the output will
       describe	the dereferenced type of the supplied reference	for hashes and
       arrays, and coderefs.  Output of	names will be avoided where possible
       if the "Terse" flag is set.

       In many cases, methods that are used to set the internal	state of the
       object will return the object itself, so	method calls can be
       conveniently chained together.

       Several styles of output	are possible, all controlled by	setting	the
       "Indent"	flag.  See "Configuration Variables or Methods"	below for

	   Returns a newly created "Data::Dumper" object.  The first argument
	   is an anonymous array of values to be dumped.  The optional second
	   argument is an anonymous array of names for the values.  The	names
	   need	not have a leading "$" sign, and must be comprised of
	   alphanumeric	characters.  You can begin a name with a "*" to
	   specify that	the dereferenced type must be dumped instead of	the
	   reference itself, for ARRAY and HASH	references.

	   The prefix specified	by $Data::Dumper::Varname will be used with a
	   numeric suffix if the name for a value is undefined.

	   Data::Dumper	will catalog all references encountered	while dumping
	   the values. Cross-references	(in the	form of	names of substructures
	   in perl syntax) will	be inserted at all possible points, preserving
	   any structural interdependencies in the original set	of values.
	   Structure traversal is depth-first,	and proceeds in	order from the
	   first supplied value	to the last.

       $OBJ->Dump  or  PACKAGE->Dump(ARRAYREF [, ARRAYREF])
	   Returns the stringified form	of the values stored in	the object
	   (preserving the order in which they were supplied to	"new"),
	   subject to the configuration	options	below.	In a list context, it
	   returns a list of strings corresponding to the supplied values.

	   The second form, for	convenience, simply calls the "new" method on
	   its arguments before	dumping	the object immediately.

	   Queries or adds to the internal table of already encountered
	   references.	You must use "Reset" to	explicitly clear the table if
	   needed.  Such references are	not dumped; instead, their names are
	   inserted wherever they are encountered subsequently.	 This is
	   useful especially for properly dumping subroutine references.

	   Expects an anonymous	hash of	name =>	value pairs.  Same rules apply
	   for names as	in "new".  If no argument is supplied, will return the
	   "seen" list of name => value	pairs, in a list context.  Otherwise,
	   returns the object itself.

	   Queries or replaces the internal array of values that will be
	   dumped.  When called	without	arguments, returns the values as a
	   list.  When called with a reference to an array of replacement
	   values, returns the object itself.  When called with	any other type
	   of argument,	dies.

	   Queries or replaces the internal array of user supplied names for
	   the values that will	be dumped.  When called	without	arguments,
	   returns the names.  When called with	an array of replacement	names,
	   returns the object itself.  If the number of	replacement names
	   exceeds the number of values	to be named, the excess	names will not
	   be used.  If	the number of replacement names	falls short of the
	   number of values to be named, the list of replacement names will be
	   exhausted and remaining values will not be renamed.	When called
	   with	any other type of argument, dies.

	   Clears the internal table of	"seen" references and returns the
	   object itself.

	   Returns the stringified form	of the values in the list, subject to
	   the configuration options below.  The values	will be	named $VARn in
	   the output, where n is a numeric suffix.  Will return a list	of
	   strings in a	list context.

   Configuration Variables or Methods
       Several configuration variables can be used to control the kind of
       output generated	when using the procedural interface.  These variables
       are usually "local"ized in a block so that other	parts of the code are
       not affected by the change.

       These variables determine the default state of the object created by
       calling the "new" method, but cannot be used to alter the state of the
       object thereafter.  The equivalent method names should be used instead
       to query	or set the internal state of the object.

       The method forms	return the object itself when called with arguments,
       so that they can	be chained together nicely.

       o   $Data::Dumper::Indent  or  $OBJ->Indent([NEWVAL])

	   Controls the	style of indentation.  It can be set to	0, 1, 2	or 3.
	   Style 0 spews output	without	any newlines, indentation, or spaces
	   between list	items.	It is the most compact format possible that
	   can still be	called valid perl.  Style 1 outputs a readable form
	   with	newlines but no	fancy indentation (each	level in the structure
	   is simply indented by a fixed amount	of whitespace).	 Style 2 (the
	   default) outputs a very readable form which takes into account the
	   length of hash keys (so the hash value lines	up).  Style 3 is like
	   style 2, but	also annotates the elements of arrays with their index
	   (but	the comment is on its own line,	so array output	consumes twice
	   the number of lines).  Style	2 is the default.

       o   $Data::Dumper::Trailingcomma	 or  $OBJ->Trailingcomma([NEWVAL])

	   Controls whether a comma is added after the last element of an
	   array or hash. Even when true, no comma is added between the	last
	   element of an array or hash and a closing bracket when they appear
	   on the same line. The default is false.

       o   $Data::Dumper::Purity  or  $OBJ->Purity([NEWVAL])

	   Controls the	degree to which	the output can be "eval"ed to recreate
	   the supplied	reference structures.  Setting it to 1 will output
	   additional perl statements that will	correctly recreate nested
	   references.	The default is 0.

       o   $Data::Dumper::Pad  or  $OBJ->Pad([NEWVAL])

	   Specifies the string	that will be prefixed to every line of the
	   output.  Empty string by default.

       o   $Data::Dumper::Varname  or  $OBJ->Varname([NEWVAL])

	   Contains the	prefix to use for tagging variable names in the
	   output. The default is "VAR".

       o   $Data::Dumper::Useqq	 or  $OBJ->Useqq([NEWVAL])

	   When	set, enables the use of	double quotes for representing string
	   values.  Whitespace other than space	will be	represented as
	   "[\n\t\r]", "unsafe"	characters will	be backslashed,	and
	   unprintable characters will be output as quoted octal integers.
	   The default is 0.

       o   $Data::Dumper::Terse	 or  $OBJ->Terse([NEWVAL])

	   When	set, Data::Dumper will emit single, non-self-referential
	   values as atoms/terms rather	than statements.  This means that the
	   $VARn names will be avoided where possible, but be advised that
	   such	output may not always be parseable by "eval".

       o   $Data::Dumper::Freezer  or  $OBJ->Freezer([NEWVAL])

	   Can be set to a method name,	or to an empty string to disable the
	   feature.  Data::Dumper will invoke that method via the object
	   before attempting to	stringify it.  This method can alter the
	   contents of the object (if, for instance, it	contains data
	   allocated from C), and even rebless it in a different package.  The
	   client is responsible for making sure the specified method can be
	   called via the object, and that the object ends up containing only
	   perl	data types after the method has	been called.  Defaults to an
	   empty string.

	   If an object	does not support the method specified (determined
	   using UNIVERSAL::can()) then	the call will be skipped.  If the
	   method dies a warning will be generated.

       o   $Data::Dumper::Toaster  or  $OBJ->Toaster([NEWVAL])

	   Can be set to a method name,	or to an empty string to disable the
	   feature.  Data::Dumper will emit a method call for any objects that
	   are to be dumped using the syntax "bless(DATA, CLASS)->METHOD()".
	   Note	that this means	that the method	specified will have to perform
	   any modifications required on the object (like creating new state
	   within it, and/or reblessing	it in a	different package) and then
	   return it.  The client is responsible for making sure the method
	   can be called via the object, and that it returns a valid object.
	   Defaults to an empty	string.

       o   $Data::Dumper::Deepcopy  or	$OBJ->Deepcopy([NEWVAL])

	   Can be set to a boolean value to enable deep	copies of structures.
	   Cross-referencing will then only be done when absolutely essential
	   (i.e., to break reference cycles).  Default is 0.

       o   $Data::Dumper::Quotekeys  or	 $OBJ->Quotekeys([NEWVAL])

	   Can be set to a boolean value to control whether hash keys are
	   quoted.  A defined false value will avoid quoting hash keys when it
	   looks like a	simple string.	Default	is 1, which will always
	   enclose hash	keys in	quotes.

       o   $Data::Dumper::Bless	 or  $OBJ->Bless([NEWVAL])

	   Can be set to a string that specifies an alternative	to the "bless"
	   builtin operator used to create objects.  A function	with the
	   specified name should exist,	and should accept the same arguments
	   as the builtin.  Default is "bless".

       o   $Data::Dumper::Pair	or  $OBJ->Pair([NEWVAL])

	   Can be set to a string that specifies the separator between hash
	   keys	and values. To dump nested hash, array and scalar values to
	   JavaScript, use: "$Data::Dumper::Pair = ' : ';". Implementing
	   "bless" in JavaScript is left as an exercise	for the	reader.	 A
	   function with the specified name exists, and	accepts	the same
	   arguments as	the builtin.

	   Default is: " => ".

       o   $Data::Dumper::Maxdepth  or	$OBJ->Maxdepth([NEWVAL])

	   Can be set to a positive integer that specifies the depth beyond
	   which we don't venture into a structure.  Has no effect when
	   "Data::Dumper::Purity" is set.  (Useful in debugger when we often
	   don't want to see more than enough).	 Default is 0, which means
	   there is no maximum depth.

       o   $Data::Dumper::Maxrecurse  or  $OBJ->Maxrecurse([NEWVAL])

	   Can be set to a positive integer that specifies the depth beyond
	   which recursion into	a structure will throw an exception.  This is
	   intended as a security measure to prevent perl running out of stack
	   space when dumping an excessively deep structure.  Can be set to 0
	   to remove the limit.	 Default is 1000.

       o   $Data::Dumper::Useperl  or  $OBJ->Useperl([NEWVAL])

	   Can be set to a boolean value which controls	whether	the pure Perl
	   implementation of "Data::Dumper" is used. The "Data::Dumper"	module
	   is a	dual implementation, with almost all functionality written in
	   both	pure Perl and also in XS ('C').	Since the XS version is	much
	   faster, it will always be used if possible. This option lets	you
	   override the	default	behavior, usually for testing purposes only.
	   Default is 0, which means the XS implementation will	be used	if

       o   $Data::Dumper::Sortkeys  or	$OBJ->Sortkeys([NEWVAL])

	   Can be set to a boolean value to control whether hash keys are
	   dumped in sorted order. A true value	will cause the keys of all
	   hashes to be	dumped in Perl's default sort order. Can also be set
	   to a	subroutine reference which will	be called for each hash	that
	   is dumped. In this case "Data::Dumper" will call the	subroutine
	   once	for each hash, passing it the reference	of the hash. The
	   purpose of the subroutine is	to return a reference to an array of
	   the keys that will be dumped, in the	order that they	should be
	   dumped. Using this feature, you can control both the	order of the
	   keys, and which keys	are actually used. In other words, this
	   subroutine acts as a	filter by which	you can	exclude	certain	keys
	   from	being dumped. Default is 0, which means	that hash keys are not

       o   $Data::Dumper::Deparse  or  $OBJ->Deparse([NEWVAL])

	   Can be set to a boolean value to control whether code references
	   are turned into perl	source code. If	set to a true value,
	   "B::Deparse"	will be	used to	get the	source of the code reference.
	   In older versions, using this option	imposed	a significant
	   performance penalty when dumping parts of a data structure other
	   than	code references, but that is no	longer the case.

	   Caution : use this option only if you know that your	coderefs will
	   be properly reconstructed by	"B::Deparse".

       o   $Data::Dumper::Sparseseen or	 $OBJ->Sparseseen([NEWVAL])

	   By default, Data::Dumper builds up the "seen" hash of scalars that
	   it has encountered during serialization. This is very expensive.
	   This	seen hash is necessary to support and even just	detect
	   circular references.	It is exposed to the user via the "Seen()"
	   call	both for writing and reading.

	   If you, as a	user, do not need explicit access to the "seen"	hash,
	   then	you can	set the	"Sparseseen" option to allow Data::Dumper to
	   eschew building the "seen" hash for scalars that are	known not to
	   possess more	than one reference. This speeds	up serialization
	   considerably	if you use the XS implementation.

	   Note: If you	turn on	"Sparseseen", then you must not	rely on	the
	   content of the seen hash since its contents will be an
	   implementation detail!


       Run these code snippets to get a	quick feel for the behavior of this
       module.	When you are through with these	examples, you may want to add
       or change the various configuration variables described above, to see
       their behavior.	(See the testsuite in the Data::Dumper distribution
       for more	examples.)

	   use Data::Dumper;

	   package Foo;
	   sub new {bless {'a' => 1, 'b' => sub	{ return "foo" }}, $_[0]};

	   package Fuz;			      #	a weird	REF-REF-SCALAR object
	   sub new {bless \($_ = \ 'fu\'z'), $_[0]};

	   package main;
	   $foo	= Foo->new;
	   $fuz	= Fuz->new;
	   $boo	= [ 1, [], "abcd", \*foo,
		    {1 => 'a', 023 => 'b', 0x45	=> 'c'},
		    \\"p\q\'r",	$foo, $fuz];

	   # simple usage

	   $bar	= eval(Dumper($boo));
	   print($@) if	$@;
	   print Dumper($boo), Dumper($bar);  #	pretty print (no array indices)

	   $Data::Dumper::Terse	= 1;	    # don't output names where feasible
	   $Data::Dumper::Indent = 0;	    # turn off all pretty print
	   print Dumper($boo), "\n";

	   $Data::Dumper::Indent = 1;	    # mild pretty print
	   print Dumper($boo);

	   $Data::Dumper::Indent = 3;	    # pretty print with	array indices
	   print Dumper($boo);

	   $Data::Dumper::Useqq	= 1;	    # print strings in double quotes
	   print Dumper($boo);

	   $Data::Dumper::Pair = " : ";	    # specify hash key/value separator
	   print Dumper($boo);

	   # recursive structures

	   @c =	('c');
	   $c =	\@c;
	   $b =	{};
	   $a =	[1, $b,	$c];
	   $b->{a} = $a;
	   $b->{b} = $a->[1];
	   $b->{c} = $a->[2];
	   print Data::Dumper->Dump([$a,$b,$c],	[qw(a b	c)]);

	   $Data::Dumper::Purity = 1;	      #	fill in	the holes for eval
	   print Data::Dumper->Dump([$a, $b], [qw(*a b)]); # print as @a
	   print Data::Dumper->Dump([$b, $a], [qw(*b a)]); # print as %b

	   $Data::Dumper::Deepcopy = 1;	      #	avoid cross-refs
	   print Data::Dumper->Dump([$b, $a], [qw(*b a)]);

	   $Data::Dumper::Purity = 0;	      #	avoid cross-refs
	   print Data::Dumper->Dump([$b, $a], [qw(*b a)]);

	   # deep structures

	   $a =	"pearl";
	   $b =	[ $a ];
	   $c =	{ 'b' => $b };
	   $d =	[ $c ];
	   $e =	{ 'd' => $d };
	   $f =	{ 'e' => $e };
	   print Data::Dumper->Dump([$f], [qw(f)]);

	   $Data::Dumper::Maxdepth = 3;	      #	no deeper than 3 refs down
	   print Data::Dumper->Dump([$f], [qw(f)]);

	   # object-oriented usage

	   $d =	Data::Dumper->new([$a,$b], [qw(a b)]);
	   $d->Seen({'*c' => $c});	      #	stash a	ref without printing it
	   print $d->Dump;
	   $d->Reset->Purity(0);	      #	empty the seen cache
	   print join "----\n",	$d->Dump;

	   # persistence

	   package Foo;
	   sub new { bless { state => 'awake' }, shift }
	   sub Freeze {
	       my $s = shift;
	       print STDERR "preparing to sleep\n";
	       $s->{state} = 'asleep';
	       return bless $s,	'Foo::ZZZ';

	   package Foo::ZZZ;
	   sub Thaw {
	       my $s = shift;
	       print STDERR "waking up\n";
	       $s->{state} = 'awake';
	       return bless $s,	'Foo';

	   package main;
	   use Data::Dumper;
	   $a =	Foo->new;
	   $b =	Data::Dumper->new([$a],	['c']);
	   $c =	$b->Dump;
	   print $c;
	   $d =	eval $c;
	   print Data::Dumper->Dump([$d], ['d']);

	   # symbol substitution (useful for recreating	CODE refs)

	   sub foo { print "foo	speaking\n" }
	   *other = \&foo;
	   $bar	= [ \&other ];
	   $d =	Data::Dumper->new([\&other,$bar],['*other','bar']);
	   $d->Seen({ '*foo' =>	\&foo });
	   print $d->Dump;

	   # sorting and filtering hash	keys

	   $Data::Dumper::Sortkeys = \&my_filter;
	   my $foo = { map { (ord, "$_$_$_") } 'I'..'Q'	};
	   my $bar = { %$foo };
	   my $baz = { reverse %$foo };
	   print Dumper	[ $foo,	$bar, $baz ];

	   sub my_filter {
	       my ($hash) = @_;
	       # return	an array ref containing	the hash keys to dump
	       # in the	order that you want them to be dumped
	       return [
		 # Sort	the keys of %$foo in reverse numeric order
		   $hash eq $foo ? (sort {$b <=> $a} keys %$hash) :
		 # Only	dump the odd number keys of %$bar
		   $hash eq $bar ? (grep {$_ % 2} keys %$hash) :
		 # Sort	keys in	default	order for all other hashes
		   (sort keys %$hash)

       Due to limitations of Perl subroutine call semantics, you cannot	pass
       an array	or hash.  Prepend it with a "\"	to pass	its reference instead.
       This will be remedied in	time, now that Perl has	subroutine prototypes.
       For now,	you need to use	the extended usage form, and prepend the name
       with a "*" to output it as a hash or array.

       "Data::Dumper" cheats with CODE references.  If a code reference	is
       encountered in the structure being processed (and if you	haven't	set
       the "Deparse" flag), an anonymous subroutine that contains the string
       '"DUMMY"' will be inserted in its place,	and a warning will be printed
       if "Purity" is set.  You	can "eval" the result, but bear	in mind	that
       the anonymous sub that gets created is just a placeholder.  Even	using
       the "Deparse" flag will in some cases produce results that behave
       differently after being passed to "eval"; see the documentation for

       SCALAR objects have the weirdest	looking	"bless"	workaround.

       Pure Perl version of "Data::Dumper" escapes UTF-8 strings correctly
       only in Perl 5.8.0 and later.

       Starting	from Perl 5.8.1	different runs of Perl will have different
       ordering	of hash	keys.  The change was done for greater security, see
       "Algorithmic Complexity Attacks"	in perlsec.  This means	that different
       runs of Perl will have different	Data::Dumper outputs if	the data
       contains	hashes.	 If you	need to	have identical Data::Dumper outputs
       from different runs of Perl, use	the environment	variable
       PERL_HASH_SEED, see "PERL_HASH_SEED" in perlrun.	 Using this restores
       the old (platform-specific) ordering: an	even prettier solution might
       be to use the "Sortkeys"	filter of Data::Dumper.

       Gurusamy	Sarathy

       Copyright (c) 1996-2019 Gurusamy	Sarathy. All rights reserved.  This
       program is free software; you can redistribute it and/or	modify it
       under the same terms as Perl itself.

       Version 2.174


perl v5.32.0			  2020-06-14		       Data::Dumper(3)


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