1YAML(3)               User Contributed Perl Documentation              YAML(3)
2
3
4

NAME

6       YAML - YAML Ain't Markup Language (tm)
7

NOTE

9       This module has been released to CPAN as YAML::Old, and soon YAML.pm
10       will be changed to just be a frontend interface module for all the
11       various Perl YAML implementation modules, including YAML::Old.
12
13       If you want robust and fast YAML processing using the normal Dump/Load
14       API, please consider switching to YAML::XS. It is by far the best Perl
15       module for YAML at this time. It requires that you have a C compiler,
16       since it is written in C.
17
18       If you really need to use this version of YAML.pm it will always be
19       available as YAML::Old.
20
21       If you don't care which YAML module use, as long as it's the best one
22       installed on your system, use YAML::Any.
23
24       The rest of this documentation is left unchanged, until YAML.pm is
25       switched over to the new UI-only version.
26

SYNOPSIS

28           use YAML;
29
30           # Load a YAML stream of 3 YAML documents into Perl data structures.
31           my ($hashref, $arrayref, $string) = Load(<<'...');
32           ---
33           name: ingy
34           age: old
35           weight: heavy
36           # I should comment that I also like pink, but don't tell anybody.
37           favorite colors:
38               - red
39               - green
40               - blue
41           ---
42           - Clark Evans
43           - Oren Ben-Kiki
44           - Ingy doet Net
45           --- >
46           You probably think YAML stands for "Yet Another Markup Language". It
47           ain't! YAML is really a data serialization language. But if you want
48           to think of it as a markup, that's OK with me. A lot of people try
49           to use XML as a serialization format.
50
51           "YAML" is catchy and fun to say. Try it. "YAML, YAML, YAML!!!"
52           ...
53
54           # Dump the Perl data structures back into YAML.
55           print Dump($string, $arrayref, $hashref);
56
57           # YAML::Dump is used the same way you'd use Data::Dumper::Dumper
58           use Data::Dumper;
59           print Dumper($string, $arrayref, $hashref);
60

DESCRIPTION

62       The YAML.pm module implements a YAML Loader and Dumper based on the
63       YAML 1.0 specification. <http://www.yaml.org/spec/>
64
65       YAML is a generic data serialization language that is optimized for
66       human readability. It can be used to express the data structures of
67       most modern programming languages. (Including Perl!!!)
68
69       For information on the YAML syntax, please refer to the YAML
70       specification.
71

WHY YAML IS COOL

73       YAML is readable for people.
74           It makes clear sense out of complex data structures. You should
75           find that YAML is an exceptional data dumping tool. Structure is
76           shown through indentation, YAML supports recursive data, and hash
77           keys are sorted by default. In addition, YAML supports several
78           styles of scalar formatting for different types of data.
79
80       YAML is editable.
81           YAML was designed from the ground up to be an excellent syntax for
82           configuration files. Almost all programs need configuration files,
83           so why invent a new syntax for each one? And why subject users to
84           the complexities of XML or native Perl code?
85
86       YAML is multilingual.
87           Yes, YAML supports Unicode. But I'm actually referring to
88           programming languages. YAML was designed to meet the serialization
89           needs of Perl, Python, Ruby, Tcl, PHP, Javascript and Java. It was
90           also designed to be interoperable between those languages. That
91           means YAML serializations produced by Perl can be processed by
92           Python.
93
94       YAML is taint safe.
95           Using modules like Data::Dumper for serialization is fine as long
96           as you can be sure that nobody can tamper with your data files or
97           transmissions. That's because you need to use Perl's "eval()"
98           built-in to deserialize the data. Somebody could add a snippet of
99           Perl to erase your files.
100
101           YAML's parser does not need to eval anything.
102
103       YAML is full featured.
104           YAML can accurately serialize all of the common Perl data
105           structures and deserialize them again without losing data
106           relationships. Although it is not 100% perfect (no serializer is or
107           can be perfect), it fares as well as the popular current modules:
108           Data::Dumper, Storable, XML::Dumper and Data::Denter.
109
110           YAML.pm also has the ability to handle code (subroutine) references
111           and typeglobs. (Still experimental) These features are not found in
112           Perl's other serialization modules.
113
114       YAML is extensible.
115           The YAML language has been designed to be flexible enough to solve
116           it's own problems. The markup itself has 3 basic construct which
117           resemble Perl's hash, array and scalar. By default, these map to
118           their Perl equivalents. But each YAML node also supports a tagging
119           mechanism (type system) which can cause that node to be interpreted
120           in a completely different manner. That's how YAML can support
121           object serialization and oddball structures like Perl's typeglob.
122

YAML IMPLEMENTATIONS IN PERL

124       This module, YAML.pm, is really just the interface module for YAML
125       modules written in Perl. The basic interface for YAML consists of two
126       functions: "Dump" and "Load". The real work is done by the modules
127       YAML::Dumper and YAML::Loader.
128
129       Different YAML module distributions can be created by subclassing
130       YAML.pm and YAML::Loader and YAML::Dumper. For example, YAML-Simple
131       consists of YAML::Simple YAML::Dumper::Simple and YAML::Loader::Simple.
132
133       Why would there be more than one implementation of YAML? Well, despite
134       YAML's offering of being a simple data format, YAML is actually very
135       deep and complex. Implementing the entirety of the YAML specification
136       is a daunting task.
137
138       For this reason I am currently working on 3 different YAML
139       implementations.
140
141       YAML
142           The main YAML distribution will keeping evolving to support the
143           entire YAML specification in pure Perl. This may not be the fastest
144           or most stable module though. Currently, YAML.pm has lots of known
145           bugs. It is mostly a great tool for dumping Perl data structures to
146           a readable form.
147
148       YAML::Lite
149           The point of YAML::Lite is to strip YAML down to the 90% that
150           people use most and offer that in a small, fast, stable, pure Perl
151           form.  YAML::Lite will simply die when it is asked to do something
152           it can't.
153
154       YAML::Syck
155           "libsyck" is the C based YAML processing library used by the Ruby
156           programming language (and also Python, PHP and Pugs). YAML::Syck is
157           the Perl binding to "libsyck". It should be very fast, but may have
158           problems of its own. It will also require C compilation.
159
160           NOTE: Audrey Tang has actually completed this module and it works
161           great
162                 and is 10 times faster than YAML.pm.
163
164       In the future, there will likely be even more YAML modules. Remember,
165       people other than Ingy are allowed to write YAML modules!
166

FUNCTIONAL USAGE

168       YAML is completely OO under the hood. Still it exports a few useful top
169       level functions so that it is dead simple to use. These functions just
170       do the OO stuff for you. If you want direct access to the OO API see
171       the documentation for YAML::Dumper and YAML::Loader.
172
173   Exported Functions
174       The following functions are exported by YAML.pm by default. The reason
175       they are exported is so that YAML works much like Data::Dumper. If you
176       don't want functions to be imported, just use YAML with an empty import
177       list:
178
179           use YAML ();
180
181       Dump(list-of-Perl-data-structures)
182           Turn Perl data into YAML. This function works very much like
183           Data::Dumper::Dumper(). It takes a list of Perl data strucures and
184           dumps them into a serialized form. It returns a string containing
185           the YAML stream. The structures can be references or plain scalars.
186
187       Load(string-containing-a-YAML-stream)
188           Turn YAML into Perl data. This is the opposite of Dump. Just like
189           Storable's thaw() function or the eval() function in relation to
190           Data::Dumper. It parses a string containing a valid YAML stream
191           into a list of Perl data structures.
192
193   Exportable Functions
194       These functions are not exported by default but you can request them in
195       an import list like this:
196
197           use YAML qw'freeze thaw Bless';
198
199       freeze() and thaw()
200           Aliases to Dump() and Load() for Storable fans. This will also
201           allow YAML.pm to be plugged directly into modules like POE.pm, that
202           use the freeze/thaw API for internal serialization.
203
204       DumpFile(filepath, list)
205           Writes the YAML stream to a file instead of just returning a
206           string.
207
208       LoadFile(filepath)
209           Reads the YAML stream from a file instead of a string.
210
211       Bless(perl-node, [yaml-node | class-name])
212           Associate a normal Perl node, with a yaml node. A yaml node is an
213           object tied to the YAML::Node class. The second argument is either
214           a yaml node that you've already created or a class (package) name
215           that supports a yaml_dump() function. A yaml_dump() function should
216           take a perl node and return a yaml node. If no second argument is
217           provided, Bless will create a yaml node. This node is not returned,
218           but can be retrieved with the Blessed() function.
219
220           Here's an example of how to use Bless. Say you have a hash
221           containing three keys, but you only want to dump two of them.
222           Furthermore the keys must be dumped in a certain order. Here's how
223           you do that:
224
225               use YAML qw(Dump Bless);
226               $hash = {apple => 'good', banana => 'bad', cauliflower => 'ugly'};
227               print Dump $hash;
228               Bless($hash)->keys(['banana', 'apple']);
229               print Dump $hash;
230
231           produces:
232
233               ---
234               apple: good
235               banana: bad
236               cauliflower: ugly
237               ---
238               banana: bad
239               apple: good
240
241           Bless returns the tied part of a yaml-node, so that you can call
242           the YAML::Node methods. This is the same thing that
243           YAML::Node::ynode() returns. So another way to do the above example
244           is:
245
246               use YAML qw(Dump Bless);
247               use YAML::Node;
248               $hash = {apple => 'good', banana => 'bad', cauliflower => 'ugly'};
249               print Dump $hash;
250               Bless($hash);
251               $ynode = ynode(Blessed($hash));
252               $ynode->keys(['banana', 'apple']);
253               print Dump $hash;
254
255           Note that Blessing a Perl data structure does not change it anyway.
256           The extra information is stored separately and looked up by the
257           Blessed node's memory address.
258
259       Blessed(perl-node)
260           Returns the yaml node that a particular perl node is associated
261           with (see above). Returns undef if the node is not (YAML) Blessed.
262

GLOBAL OPTIONS

264       YAML options are set using a group of global variables in the YAML
265       namespace. This is similar to how Data::Dumper works.
266
267       For example, to change the indentation width, do something like:
268
269           local $YAML::Indent = 3;
270
271       The current options are:
272
273       DumperClass
274           You can override which module/class YAML uses for Dumping data.
275
276       LoaderClass
277           You can override which module/class YAML uses for Loading data.
278
279       Indent
280           This is the number of space characters to use for each indentation
281           level when doing a Dump(). The default is 2.
282
283           By the way, YAML can use any number of characters for indentation
284           at any level. So if you are editing YAML by hand feel free to do it
285           anyway that looks pleasing to you; just be consistent for a given
286           level.
287
288       SortKeys
289           Default is 1. (true)
290
291           Tells YAML.pm whether or not to sort hash keys when storing a
292           document.
293
294           YAML::Node objects can have their own sort order, which is usually
295           what you want. To override the YAML::Node order and sort the keys
296           anyway, set SortKeys to 2.
297
298       Stringify
299           Default is 0. (false)
300
301           Objects with string overloading should honor the overloading and
302           dump the stringification of themselves, rather than the actual
303           object's guts.
304
305       UseHeader
306           Default is 1. (true)
307
308           This tells YAML.pm whether to use a separator string for a Dump
309           operation. This only applies to the first document in a stream.
310           Subsequent documents must have a YAML header by definition.
311
312       UseVersion
313           Default is 0. (false)
314
315           Tells YAML.pm whether to include the YAML version on the
316           separator/header.
317
318               --- %YAML:1.0
319
320       AnchorPrefix
321           Default is ''.
322
323           Anchor names are normally numeric. YAML.pm simply starts with '1'
324           and increases by one for each new anchor. This option allows you to
325           specify a string to be prepended to each anchor number.
326
327       UseCode
328           Setting the UseCode option is a shortcut to set both the DumpCode
329           and LoadCode options at once. Setting UseCode to '1' tells YAML.pm
330           to dump Perl code references as Perl (using B::Deparse) and to load
331           them back into memory using eval(). The reason this has to be an
332           option is that using eval() to parse untrusted code is, well,
333           untrustworthy.
334
335       DumpCode
336           Determines if and how YAML.pm should serialize Perl code
337           references. By default YAML.pm will dump code references as dummy
338           placeholders (much like Data::Dumper). If DumpCode is set to '1' or
339           'deparse', code references will be dumped as actual Perl code.
340
341           DumpCode can also be set to a subroutine reference so that you can
342           write your own serializing routine. YAML.pm passes you the code
343           ref. You pass back the serialization (as a string) and a format
344           indicator. The format indicator is a simple string like: 'deparse'
345           or 'bytecode'.
346
347       LoadCode
348           LoadCode is the opposite of DumpCode. It tells YAML if and how to
349           deserialize code references. When set to '1' or 'deparse' it will
350           use "eval()". Since this is potentially risky, only use this option
351           if you know where your YAML has been.
352
353           LoadCode can also be set to a subroutine reference so that you can
354           write your own deserializing routine. YAML.pm passes the
355           serialization (as a string) and a format indicator. You pass back
356           the code reference.
357
358       UseBlock
359           YAML.pm uses heuristics to guess which scalar style is best for a
360           given node. Sometimes you'll want all multiline scalars to use the
361           'block' style. If so, set this option to 1.
362
363           NOTE: YAML's block style is akin to Perl's here-document.
364
365       UseFold
366           If you want to force YAML to use the 'folded' style for all
367           multiline scalars, then set $UseFold to 1.
368
369           NOTE: YAML's folded style is akin to the way HTML folds text,
370                 except smarter.
371
372       UseAliases
373           YAML has an alias mechanism such that any given structure in memory
374           gets serialized once. Any other references to that structure are
375           serialized only as alias markers. This is how YAML can serialize
376           duplicate and recursive structures.
377
378           Sometimes, when you KNOW that your data is nonrecursive in nature,
379           you may want to serialize such that every node is expressed in
380           full. (ie as a copy of the original). Setting $YAML::UseAliases to
381           0 will allow you to do this. This also may result in faster
382           processing because the lookup overhead is by bypassed.
383
384           THIS OPTION CAN BE DANGEROUS. *If* your data is recursive, this
385           option *will* cause Dump() to run in an endless loop, chewing up
386           your computers memory. You have been warned.
387
388       CompressSeries
389           Default is 1.
390
391           Compresses the formatting of arrays of hashes:
392
393               -
394                 foo: bar
395               -
396                 bar: foo
397
398           becomes:
399
400               - foo: bar
401               - bar: foo
402
403           Since this output is usually more desirable, this option is turned
404           on by default.
405

YAML TERMINOLOGY

407       YAML is a full featured data serialization language, and thus has its
408       own terminology.
409
410       It is important to remember that although YAML is heavily influenced by
411       Perl and Python, it is a language in its own right, not merely just a
412       representation of Perl structures.
413
414       YAML has three constructs that are conspicuously similar to Perl's
415       hash, array, and scalar. They are called mapping, sequence, and string
416       respectively. By default, they do what you would expect. But each
417       instance may have an explicit or implicit tag (type) that makes it
418       behave differently. In this manner, YAML can be extended to represent
419       Perl's Glob or Python's tuple, or Ruby's Bigint.
420
421       stream
422           A YAML stream is the full sequence of unicode characters that a
423           YAML parser would read or a YAML emitter would write. A stream may
424           contain one or more YAML documents separated by YAML headers.
425
426               ---
427               a: mapping
428               foo: bar
429               ---
430               - a
431               - sequence
432
433       document
434           A YAML document is an independent data structure representation
435           within a stream. It is a top level node. Each document in a YAML
436           stream must begin with a YAML header line. Actually the header is
437           optional on the first document.
438
439               ---
440               This: top level mapping
441               is:
442                   - a
443                   - YAML
444                   - document
445
446       header
447           A YAML header is a line that begins a YAML document. It consists of
448           three dashes, possibly followed by more info. Another purpose of
449           the header line is that it serves as a place to put top level tag
450           and anchor information.
451
452               --- !recursive-sequence &001
453               - * 001
454               - * 001
455
456       node
457           A YAML node is the representation of a particular data stucture.
458           Nodes may contain other nodes. (In Perl terms, nodes are like
459           scalars.  Strings, arrayrefs and hashrefs. But this refers to the
460           serialized format, not the in-memory structure.)
461
462       tag This is similar to a type. It indicates how a particular YAML node
463           serialization should be transferred into or out of memory. For
464           instance a Foo::Bar object would use the tag 'perl/Foo::Bar':
465
466               - !perl/Foo::Bar
467                   foo: 42
468                   bar: stool
469
470       collection
471           A collection is the generic term for a YAML data grouping. YAML has
472           two types of collections: mappings and sequences. (Similar to
473           hashes and arrays)
474
475       mapping
476           A mapping is a YAML collection defined by unordered key/value pairs
477           with unique keys. By default YAML mappings are loaded into Perl
478           hashes.
479
480               a mapping:
481                   foo: bar
482                   two: times two is 4
483
484       sequence
485           A sequence is a YAML collection defined by an ordered list of
486           elements. By default YAML sequences are loaded into Perl arrays.
487
488               a sequence:
489                   - one bourbon
490                   - one scotch
491                   - one beer
492
493       scalar
494           A scalar is a YAML node that is a single value. By default YAML
495           scalars are loaded into Perl scalars.
496
497               a scalar key: a scalar value
498
499           YAML has many styles for representing scalars. This is important
500           because varying data will have varying formatting requirements to
501           retain the optimum human readability.
502
503       plain scalar
504           A plain sclar is unquoted. All plain scalars are automatic
505           candidates for "implicit tagging". This means that their tag may be
506           determined automatically by examination. The typical uses for this
507           are plain alpha strings, integers, real numbers, dates, times and
508           currency.
509
510               - a plain string
511               - -42
512               - 3.1415
513               - 12:34
514               - 123 this is an error
515
516       single quoted scalar
517           This is similar to Perl's use of single quotes. It means no
518           escaping except for single quotes which are escaped by using two
519           adjacent single quotes.
520
521               - 'When I say ''\n'' I mean "backslash en"'
522
523       double quoted scalar
524           This is similar to Perl's use of double quotes. Character escaping
525           can be used.
526
527               - "This scalar\nhas two lines, and a bell -->\a"
528
529       folded scalar
530           This is a multiline scalar which begins on the next line. It is
531           indicated by a single right angle bracket. It is unescaped like the
532           single quoted scalar. Line folding is also performed.
533
534               - >
535                This is a multiline scalar which begins on
536                the next line. It is indicated by a single
537                carat. It is unescaped like the single
538                quoted scalar. Line folding is also
539                performed.
540
541       block scalar
542           This final multiline form is akin to Perl's here-document except
543           that (as in all YAML data) scope is indicated by indentation.
544           Therefore, no ending marker is required. The data is verbatim. No
545           line folding.
546
547               - |
548                   QTY  DESC          PRICE  TOTAL
549                   ---  ----          -----  -----
550                     1  Foo Fighters  $19.95 $19.95
551                     2  Bar Belles    $29.95 $59.90
552
553       parser
554           A YAML processor has four stages: parse, load, dump, emit.
555
556           A parser parses a YAML stream. YAML.pm's Load() function contains a
557           parser.
558
559       loader
560           The other half of the Load() function is a loader. This takes the
561           information from the parser and loads it into a Perl data
562           structure.
563
564       dumper
565           The Dump() function consists of a dumper and an emitter. The dumper
566           walks through each Perl data structure and gives info to the
567           emitter.
568
569       emitter
570           The emitter takes info from the dumper and turns it into a YAML
571           stream.
572
573           NOTE: In YAML.pm the parser/loader and the dumper/emitter code are
574           currently very closely tied together. In the future they may be
575           broken into separate stages.
576
577       For more information please refer to the immensely helpful YAML
578       specification available at <http://www.yaml.org/spec/>.
579

ysh - The YAML Shell

581       The YAML distribution ships with a script called 'ysh', the YAML shell.
582       ysh provides a simple, interactive way to play with YAML. If you type
583       in Perl code, it displays the result in YAML. If you type in YAML it
584       turns it into Perl code.
585
586       To run ysh, (assuming you installed it along with YAML.pm) simply type:
587
588           ysh [options]
589
590       Please read the "ysh" documentation for the full details. There are
591       lots of options.
592

BUGS & DEFICIENCIES

594       If you find a bug in YAML, please try to recreate it in the YAML Shell
595       with logging turned on ('ysh -L'). When you have successfully
596       reproduced the bug, please mail the LOG file to the author
597       (ingy@cpan.org).
598
599       WARNING: This is still *ALPHA* code. Well, most of this code has been
600       around for years...
601
602       BIGGER WARNING: YAML.pm has been slow in the making, but I am committed
603       to having top notch YAML tools in the Perl world. The YAML team is
604       close to finalizing the YAML 1.1 spec. This version of YAML.pm is based
605       off of a very old pre 1.0 spec. In actuality there isn't a ton of
606       difference, and this YAML.pm is still fairly useful. Things will get
607       much better in the future.
608

RESOURCES

610       http://lists.sourceforge.net/lists/listinfo/yaml-core
611       <http://lists.sourceforge.net/lists/listinfo/yaml-core> is the mailing
612       list. This is where the language is discussed and designed.
613
614       <http://www.yaml.org> is the official YAML website.
615
616       <http://www.yaml.org/spec/> is the YAML 1.0 specification.
617
618       <http://yaml.kwiki.org> is the official YAML wiki.
619

SEE ALSO

621       See YAML::Syck. Fast!
622

AUTHOR

624       Ingy doet Net <ingy@cpan.org>
625
626       is resonsible for YAML.pm.
627
628       The YAML serialization language is the result of years of collaboration
629       between Oren Ben-Kiki, Clark Evans and Ingy doet Net. Several others
630       have added help along the way.
631
633       Copyright (c) 2005, 2006, 2008. Ingy doet Net.
634
635       Copyright (c) 2001, 2002, 2005. Brian Ingerson.
636
637       Some parts copyright (c) 2009 Adam Kennedy
638
639       This program is free software; you can redistribute it and/or modify it
640       under the same terms as Perl itself.
641
642       See <http://www.perl.com/perl/misc/Artistic.html>
643
644
645
646perl v5.12.0                      2009-08-10                           YAML(3)
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