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XML::Simple::FAQ(3)   User Contributed Perl Documentation  XML::Simple::FAQ(3)

NAME
       XML::Simple::FAQ	- Frequently Asked Questions about XML::Simple

Basics
   What	should I use XML::Simple for?
       Nothing!

       It's as simple as that.

       Choose a	better module. See Perl	XML::LibXML by Example
       <http://grantm.github.io/perl-libxml-by-example/> for a gentle
       introduction to XML::LibXML with	lots of	examples.

   What	was XML::Simple	designed to be used for?
       XML::Simple is a	Perl module that was originally	developed as a tool
       for reading and writing configuration data in XML format.  You could
       use it for other	purposes that involve storing and retrieving
       structured data in XML but it's likely to be a frustrating experience.

   Why store configuration data	in XML anyway?
       It seemed like a	good idea at the time.	Now, I use and recommend
       Config::General which uses a format similar to that used	by the Apache
       web server.  This is easier to read than	XML while still	allowing
       advanced	concepts such as nested	sections.

       At the time XML::Simple was written, the	advantages of using XML	format
       for configuration data were thought to include:

       o   Using existing XML parsing tools requires less development time, is
	   easier and more robust than developing your own config file parsing
	   code

       o   XML can represent relationships between pieces of data, such	as
	   nesting of sections to arbitrary levels (not	easily done with .INI
	   files for example)

       o   XML is basically just text, so you can easily edit a	config file
	   (easier than	editing	a Win32	registry)

       o   XML provides	standard solutions for handling	character sets and
	   encoding beyond basic ASCII (important for internationalization)

       o   If it becomes necessary to change your configuration	file format,
	   there are many tools	available for performing transformations on
	   XML files

       o   XML is an open standard (the	world does not need more proprietary
	   binary file formats)

       o   Taking the extra step of developing a DTD allows the	format of
	   configuration files to be validated before your program reads them
	   (not	directly supported by XML::Simple)

       o   Combining a DTD with	a good XML editor can give you a GUI config
	   editor for minimal coding effort

   What	isn't XML::Simple good for?
       The main	limitation of XML::Simple is that it does not work with	'mixed
       content'	(see the next question).  If you consider your XML files
       contain marked up text rather than structured data, you should probably
       use another module.

       If your source XML documents change regularly, it's likely that you
       will experience intermittent failures.  In particular, failure to
       properly	use the	ForceArray and KeyAttr options will produce code that
       works when you get a list of elements with the same name, but fails
       when there's only one item in the list.	These types of problems	can be
       avoided by not using XML::Simple	in the first place.

       If you are working with very large XML files, XML::Simple's approach of
       representing the	whole file in memory as	a 'tree' data structure	may
       not be suitable.

   What	is mixed content?
       Consider	this example XML:

	 <document>
	   <para>This is <em>mixed</em>	content.</para>
	 </document>

       This is said to be mixed	content, because the <para> element contains
       both character data (text content) and nested elements.

       Here's some more	XML:

	 <person>
	   <first_name>Joe</first_name>
	   <last_name>Bloggs</last_name>
	   <dob>25-April-1969</dob>
	 </person>

       This second example is not generally considered to be mixed content.
       The <first_name>, <last_name> and <dob> elements	contain	only character
       data and	the  <person> element contains only nested elements.  (Note:
       Strictly	speaking, the whitespace between the nested elements is
       character data, but it is ignored by XML::Simple).

   Why doesn't XML::Simple handle mixed	content?
       Because if it did, it would no longer be	simple :-)

       Seriously though, there are plenty of excellent modules that allow you
       to work with mixed content in a variety of ways.	 Handling mixed
       content correctly is not	easy and by ignoring these issues, XML::Simple
       is able to present an API without a steep learning curve.

   Which Perl modules do handle	mixed content?
       Every one of them except	XML::Simple :-)

       If you're looking for a recommendation, I'd suggest you look at the
       Perl-XML	FAQ at:

	 http://perl-xml.sourceforge.net/faq/

Installation
   How do I install XML::Simple?
       If you're running ActiveState Perl, or Strawberry Perl
       <http://strawberryperl.com/> you've probably already got	XML::Simple
       and therefore do	not need to install it at all.	But you	probably also
       have XML::LibXML, which is a much better	module,	so just	use that.

       If you do need to install XML::Simple, you'll need to install an	XML
       parser module first.  Install either XML::Parser	(which you may have
       already)	or XML::SAX.  If you install both, XML::SAX will be used by
       default.

       Once you	have a parser installed	...

       On Unix systems,	try:

	 perl -MCPAN -e	'install XML::Simple'

       If that doesn't work, download the latest distribution from
       ftp://ftp.cpan.org/pub/CPAN/authors/id/G/GR/GRANTM , unpack it and run
       these commands:

	 perl Makefile.PL
	 make
	 make test
	 make install

       On Win32, if you	have a recent build of ActiveState Perl	(618 or
       better) try this	command:

	 ppm install XML::Simple

       If that doesn't work, you really	only need the Simple.pm	file, so
       extract it from the .tar.gz file	(eg: using WinZIP) and save it in the
       \site\lib\XML directory under your Perl installation (typically
       C:\Perl).

   I'm trying to install XML::Simple and 'make test' fails
       Is the directory	where you've unpacked XML::Simple mounted from a file
       server using NFS, SMB or	some other network file	sharing?  If so, that
       may cause errors	in the following test scripts:

	 3_Storable.t
	 4_MemShare.t
	 5_MemCopy.t

       The test	suite is designed to exercise the boundary conditions of all
       XML::Simple's functionality and these three scripts exercise the
       caching functions.  If XML::Simple is asked to parse a file for which
       it has a	cached copy of a previous parse, then it compares the
       timestamp on the	XML file with the timestamp on the cached copy.	 If
       the cached copy is *newer* then it will be used.	 If the	cached copy is
       older or	the same age then the file is re-parsed.  The test scripts
       will get	confused by networked filesystems if the workstation and
       server system clocks are	not synchronised (to the second).

       If you get an error in one of these three test scripts but you don't
       plan to use the caching options (they're	not enabled by default), then
       go right	ahead and run 'make install'.  If you do plan to use caching,
       then try	unpacking the distribution on local disk and doing the
       build/test there.

       It's probably not a good	idea to	use the	caching	options	with networked
       filesystems in production.  If the file server's	clock is ahead of the
       local clock, XML::Simple	will re-parse files when it could have used
       the cached copy.	 However if the	local clock is ahead of	the file
       server clock and	a file is changed immediately after it is cached, the
       old cached copy will be used.

       Is one of the three test	scripts	(above)	failing	but you're not running
       on a network filesystem?	 Are you running Win32?	 If so,	you may	be
       seeing a	bug in Win32 where writes to a file do not affect its
       modification timestamp.

       If none of these	scenarios match	your situation,	please confirm you're
       running the latest version of XML::Simple and then email	the output of
       'make test' to me at grantm@cpan.org

   Why is XML::Simple so slow?
       If you find that	XML::Simple is very slow reading XML, the most likely
       reason is that you have XML::SAX	installed but no additional SAX	parser
       module.	The XML::SAX distribution includes an XML parser written
       entirely	in Perl.  This is very portable	but not	very fast.  For	better
       performance install either XML::SAX::Expat or XML::LibXML.

Usage
   How do I use	XML::Simple?
       If you don't know how to	use XML::Simple	then the best approach is to
       learn to	use XML::LibXML	<http://grantm.github.io/perl-libxml-by-
       example/> instead.  Stop	reading	this document and use that one
       instead.

       If you are determined to	use XML::Simple, it come with copious
       documentation, so read that.

   There are so	many options, which ones do I really need to know about?
       Although	you can	get by without using any options, you shouldn't	even
       consider	using XML::Simple in production	until you know what these two
       options do:

       o   forcearray

       o   keyattr

       The reason you really need to read about	them is	because	the default
       values for these	options	will trip you up if you	don't.	Although
       everyone	agrees that these defaults are not ideal, there	is not wide
       agreement on what they should be	changed	to.  The answer	therefore is
       to read about them (see below) and select values	which are right	for
       you.

   What	is the forcearray option all about?
       Consider	this XML in a file called ./person.xml:

	 <person>
	   <first_name>Joe</first_name>
	   <last_name>Bloggs</last_name>
	   <hobbie>bungy jumping</hobbie>
	   <hobbie>sky diving</hobbie>
	   <hobbie>knitting</hobbie>
	 </person>

       You could read it in with this line:

	 my $person = XMLin('./person.xml');

       Which would give	you a data structure like this:

	 $person = {
	   'first_name'	=> 'Joe',
	   'last_name'	=> 'Bloggs',
	   'hobbie'	=> [ 'bungy jumping', 'sky diving', 'knitting' ]
	 };

       The <first_name>	and <last_name>	elements are represented as simple
       scalar values which you could refer to like this:

	 print "$person->{first_name} $person->{last_name}\n";

       The <hobbie> elements are represented as	an array - since there is more
       than one.  You could refer to the first one like	this:

	 print $person->{hobbie}->[0], "\n";

       Or the whole lot	like this:

	 print join(', ', @{$person->{hobbie}} ), "\n";

       The catch is, that these	last two lines of code will only work for
       people who have more than one hobbie.  If there is only one <hobbie>
       element,	it will	be represented as a simple scalar (just	like
       <first_name> and	<last_name>).  Which might lead	you to write code like
       this:

	 if(ref($person->{hobbie})) {
	   print join(', ', @{$person->{hobbie}} ), "\n";
	 }
	 else {
	   print $person->{hobbie}, "\n";
	 }

       Don't do	that.

       One alternative approach	is to set the forcearray option	to a true
       value:

	 my $person = XMLin('./person.xml', forcearray => 1);

       Which will give you a data structure like this:

	 $person = {
	   'first_name'	=> [ 'Joe' ],
	   'last_name'	=> [ 'Bloggs' ],
	   'hobbie'	=> [ 'bungy jumping', 'sky diving', 'knitting' ]
	 };

       Then you	can use	this line to refer to all the list of hobbies even if
       there was only one:

	 print join(', ', @{$person->{hobbie}} ), "\n";

       The downside of this approach is	that the <first_name> and <last_name>
       elements	will also always be represented	as arrays even though there
       will never be more than one:

	 print "$person->{first_name}->[0] $person->{last_name}->[0]\n";

       This might be OK	if you change the XML to use attributes	for things
       that will always	be singular and	nested elements	for things that	may be
       plural:

	 <person first_name="Jane" last_name="Bloggs">
	   <hobbie>motorcycle maintenance</hobbie>
	 </person>

       On the other hand, if you prefer	not to use attributes, then you	could
       specify that any	<hobbie> elements should always	be represented as
       arrays and all other nested elements should be simple scalar values
       unless there is more than one:

	 my $person = XMLin('./person.xml', forcearray => [ 'hobbie' ]);

       The forcearray option accepts a list of element names which should
       always be forced	to an array representation:

	 forcearray => [ qw(hobbie qualification childs_name) ]

       See the XML::Simple manual page for more	information.

   What	is the keyattr option all about?
       Consider	this sample XML:

	 <catalog>
	   <part partnum="1842334" desc="High pressure flange" price="24.50" />
	   <part partnum="9344675" desc="Threaded gasket"      price="9.25" />
	   <part partnum="5634896" desc="Low voltage washer"   price="12.00" />
	 </catalog>

       You could slurp it in with this code:

	 my $catalog = XMLin('./catalog.xml');

       Which would return a data structure like	this:

	 $catalog = {
	     'part' => [
		 {
		   'partnum' =>	'1842334',
		   'desc'    =>	'High pressure flange',
		   'price'   =>	'24.50'
		 },
		 {
		   'partnum' =>	'9344675',
		   'desc'    =>	'Threaded gasket',
		   'price'   =>	'9.25'
		 },
		 {
		   'partnum' =>	'5634896',
		   'desc'    =>	'Low voltage washer',
		   'price'   =>	'12.00'
		 }
	     ]
	 };

       Then you	could access the description of	the first part in the catalog
       with this code:

	 print $catalog->{part}->[0]->{desc}, "\n";

       However,	if you wanted to access	the description	of the part with the
       part number of "9344675"	then you'd have	to code	a loop like this:

	 foreach my $part (@{$catalog->{part}})	{
	   if($part->{partnum} eq '9344675') {
	     print $part->{desc}, "\n";
	     last;
	   }
	 }

       The knowledge that each <part> element has a unique partnum attribute
       allows you to eliminate this search.  You can pass this knowledge on to
       XML::Simple like	this:

	 my $catalog = XMLin($xml, keyattr => ['partnum']);

       Which will return a data	structure like this:

	 $catalog = {
	   'part' => {
	     '5634896' => { 'desc' => 'Low voltage washer',   'price' => '12.00' },
	     '1842334' => { 'desc' => 'High pressure flange', 'price' => '24.50' },
	     '9344675' => { 'desc' => 'Threaded	gasket',      'price' => '9.25'	 }
	   }
	 };

       XML::Simple has been able to transform $catalog->{part} from an
       arrayref	to a hashref (keyed on partnum).  This transformation is
       called 'array folding'.

       Through the use of array	folding, you can now index directly to the
       description of the part you want:

	 print $catalog->{part}->{9344675}->{desc}, "\n";

       The 'keyattr' option also enables array folding when the	unique key is
       in a nested element rather than an attribute.  eg:

	 <catalog>
	   <part>
	     <partnum>1842334</partnum>
	     <desc>High	pressure flange</desc>
	     <price>24.50</price>
	   </part>
	   <part>
	     <partnum>9344675</partnum>
	     <desc>Threaded gasket</desc>
	     <price>9.25</price>
	   </part>
	   <part>
	     <partnum>5634896</partnum>
	     <desc>Low voltage washer</desc>
	     <price>12.00</price>
	   </part>
	 </catalog>

       See the XML::Simple manual page for more	information.

   So what's the catch with 'keyattr'?
       One thing to watch out for is that you might get	array folding even if
       you don't supply	the keyattr option.  The default value for this	option
       is:

	 [ 'name', 'key', 'id']

       Which means if your XML elements	have a 'name', 'key' or	'id' attribute
       (or nested element) then	they may get folded on those values.  This
       means that you can take advantage of array folding simply through
       careful choice of attribute names.  On the hand,	if you really don't
       want array folding at all, you'll need to set 'key attr to an empty
       list:

	 my $ref = XMLin($xml, keyattr => []);

       A second	'gotcha' is that array folding only works on arrays.  That
       might seem obvious, but if there's only one record in your XML and you
       didn't set the 'forcearray' option then it won't	be represented as an
       array and consequently won't get	folded into a hash.  The moral is that
       if you're using array folding, you should always	turn on	the forcearray
       option.

       You probably want to be as specific as you can be too.  For instance,
       the safest way to parse the <catalog> example above would be:

	 my $catalog = XMLin($xml, keyattr => {	part =>	'partnum'},
				   forcearray => ['part']);

       By using	the hashref for	keyattr, you can specify that only <part>
       elements	should be folded on the	'partnum' attribute (and that the
       <part> elements should not be folded on any other attribute).

       By supplying a list of element names for	forcearray, you're ensuring
       that folding will work even if there's only one <part>.	You're also
       ensuring	that if	the 'partnum' unique key is supplied in	a nested
       element then that element won't get forced to an	array too.

   How do I know what my data structure	should look like?
       The rules are fairly straightforward:

       o   each	element	gets represented as a hash

       o   unless it contains only text, in which case it'll be	a simple
	   scalar value

       o   or unless there's more than one element with	the same name, in
	   which case they'll be represented as	an array

       o   unless you've got array folding enabled, in which case they'll be
	   folded into a hash

       o   empty elements (no text contents and	no attributes) will either be
	   represented as an empty hash, an empty string or undef - depending
	   on the value	of the 'suppressempty' option.

       If you're in any	doubt, use Data::Dumper, eg:

	 use XML::Simple;
	 use Data::Dumper;

	 my $ref = XMLin($xml);

	 print Dumper($ref);

   I'm getting 'Use of uninitialized value' warnings
       You're probably trying to index into a non-existant hash	key - try
       Data::Dumper.

   I'm getting a 'Not an ARRAY reference' error
       Something that you expect to be an array	is not.	 The two most likely
       causes are that you forgot to use 'forcearray' or that the array	got
       folded into a hash - try	Data::Dumper.

   I'm getting a 'No such array	field' error
       Something that you expect to be a hash is actually an array.  Perhaps
       array folding failed because one	element	was missing the	key attribute
       - try Data::Dumper.

   I'm getting an 'Out of memory' error
       Something in the	data structure is not as you expect and	Perl may be
       trying unsuccessfully to	autovivify things - try	Data::Dumper.

       If you're already using Data::Dumper, try calling Dumper() immediately
       after XMLin() - ie: before you attempt to access	anything in the	data
       structure.

   My element order is getting jumbled up
       If you read an XML file with XMLin() and	then write it back out with
       XMLout(), the order of the elements will	likely be different.
       (However, if you	read the file back in with XMLin() you'll get the same
       Perl data structure).

       The reordering happens because XML::Simple uses hashrefs	to store your
       data and	Perl hashes do not really have any order.

       It is possible that a future version of XML::Simple will	use
       Tie::IxHash to store the	data in	hashrefs which do retain the order.
       However this will not fix all cases of element order being lost.

       If your application really is sensitive to element order, don't use
       XML::Simple (and	don't put order-sensitive values in attributes).

   XML::Simple turns nested elements into attributes
       If you read an XML file with XMLin() and	then write it back out with
       XMLout(), some data which was originally	stored in nested elements may
       end up in attributes.  (However,	if you read the	file back in with
       XMLin() you'll get the same Perl	data structure).

       There are a number of ways you might handle this:

       o   use the 'forcearray'	option with XMLin()

       o   use the 'noattr' option with	XMLout()

       o   live	with it

       o   don't use XML::Simple

   Why does XMLout() insert <name> elements (or	attributes)?
       Try setting keyattr => [].

       When you	call XMLin() to	read XML, the 'keyattr'	option controls
       whether arrays get 'folded' into	hashes.	 Similarly, when you call
       XMLout(), the 'keyattr' option controls whether hashes get 'unfolded'
       into arrays.  As	described above, 'keyattr' is enabled by default.

   Why are empty elements represented as empty hashes?
       An element is always represented	as a hash unless it contains only
       text, in	which case it is represented as	a scalar string.

       If you would prefer empty elements to be	represented as empty strings
       or the undefined	value, set the 'suppressempty' option to '' or undef
       respectively.

   Why is ParserOpts deprecated?
       The "ParserOpts"	option is a remnant of the time	when XML::Simple only
       worked with the XML::Parser API.	 Its value is completely ignored if
       you're using a SAX parser, so writing code which	relied on it would bar
       you from	taking advantage of SAX.

       Even if you are using XML::Parser, it is	seldom necessary to pass
       options to the parser object.  A	number of people have written to say
       they use	this option to set XML::Parser's "ProtocolEncoding" option.
       Don't do	that, it's wrong, Wrong, WRONG!	 Fix the XML document so that
       it's well-formed	and you	won't have a problem.

       Having said all of that,	as long	as XML::Simple continues to support
       the XML::Parser API, this option	will not be removed.  There are
       currently no plans to remove support for	the XML::Parser	API.

perl v5.32.1			  2018-03-18		   XML::Simple::FAQ(3)

NAME | Basics | Installation | Usage

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