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

NAME

6       YAML - YAML Ain't Markup Language™
7

VERSION

9       This document describes YAML version 1.30.
10

NOTE

12       This module has been released to CPAN as YAML::Old, and soon YAML.pm
13       will be changed to just be a frontend interface module for all the
14       various Perl YAML implementation modules, including YAML::Old.
15
16       If you want robust and fast YAML processing using the normal Dump/Load
17       API, please consider switching to YAML::XS. It is by far the best Perl
18       module for YAML at this time. It requires that you have a C compiler,
19       since it is written in C.
20
21       If you really need to use this version of YAML.pm it will always be
22       available as YAML::Old.
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, $string) = Load(<<'...');
32           ---
33           name: ingy       # A Mapping
34           age: old
35           weight: heavy
36           # I should comment that I also like pink, but don't tell anybody.
37           favorite colors:
38             - red
39             - green
40             - blue
41           ---
42           - Clark Evans    # A Sequence
43           - Oren Ben-Kiki
44           - Ingy döt Net
45           --- >            # A Block Scalar
46           You probably think YAML stands for "Yet Another Markup Language". It
47           ain't! YAML is really a data serialization language. But if you want
48           to think of it as a markup, that's OK with me. A lot of people try
49           to use XML as a serialization format.
50
51           "YAML" is catchy and fun to say. Try it. "YAML, YAML, YAML!!!"
52           ...
53
54           # Dump the Perl data structures back into YAML.
55           print Dump($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
61           Since version 1.25 YAML.pm supports trailing comments.
62

DESCRIPTION

64       The YAML.pm module implements a YAML Loader and Dumper based on the
65       YAML 1.0 specification. <http://www.yaml.org/spec/>
66
67       YAML is a generic data serialization language that is optimized for
68       human readability. It can be used to express the data structures of
69       most modern programming languages. (Including Perl!!!)
70
71       For information on the YAML syntax, please refer to the YAML
72       specification.
73

WHY YAML IS COOL

75       YAML is readable for people.
76           It makes clear sense out of complex data structures. You should
77           find that YAML is an exceptional data dumping tool. Structure is
78           shown through indentation, YAML supports recursive data, and hash
79           keys are sorted by default. In addition, YAML supports several
80           styles of scalar formatting for different types of data.
81
82       YAML is editable.
83           YAML was designed from the ground up to be an excellent syntax for
84           configuration files. Almost all programs need configuration files,
85           so why invent a new syntax for each one? And why subject users to
86           the complexities of XML or native Perl code?
87
88       YAML is multilingual.
89           Yes, YAML supports Unicode. But I'm actually referring to
90           programming languages. YAML was designed to meet the serialization
91           needs of Perl, Python, Ruby, Tcl, PHP, Javascript and Java. It was
92           also designed to be interoperable between those languages. That
93           means YAML serializations produced by Perl can be processed by
94           Python.
95
96       YAML is taint safe.
97           Using modules like Data::Dumper for serialization is fine as long
98           as you can be sure that nobody can tamper with your data files or
99           transmissions. That's because you need to use Perl's "eval()"
100           built-in to deserialize the data.  Somebody could add a snippet of
101           Perl to erase your files.
102
103           YAML's parser does not need to eval anything.
104
105       YAML is full featured.
106           YAML can accurately serialize all of the common Perl data
107           structures and deserialize them again without losing data
108           relationships. Although it is not 100% perfect (no serializer is or
109           can be perfect), it fares as well as the popular current modules:
110           Data::Dumper, Storable, XML::Dumper and Data::Denter.
111
112           YAML.pm also has the ability to handle code (subroutine) references
113           and typeglobs. (Still experimental) These features are not found in
114           Perl's other serialization modules.
115
116       YAML is extensible.
117           The YAML language has been designed to be flexible enough to solve
118           it's own problems. The markup itself has 3 basic construct which
119           resemble Perl's hash, array and scalar. By default, these map to
120           their Perl equivalents. But each YAML node also supports a tagging
121           mechanism (type system) which can cause that node to be interpreted
122           in a completely different manner. That's how YAML can support
123           object serialization and oddball structures like Perl's typeglob.
124

YAML IMPLEMENTATIONS IN PERL

126       This module, YAML.pm, is really just the interface module for YAML
127       modules written in Perl. The basic interface for YAML consists of two
128       functions: "Dump" and "Load". The real work is done by the modules
129       YAML::Dumper and YAML::Loader.
130
131       Different YAML module distributions can be created by subclassing
132       YAML.pm and YAML::Loader and YAML::Dumper. For example, YAML-Simple
133       consists of YAML::Simple YAML::Dumper::Simple and YAML::Loader::Simple.
134
135       Why would there be more than one implementation of YAML? Well, despite
136       YAML's offering of being a simple data format, YAML is actually very
137       deep and complex. Implementing the entirety of the YAML specification
138       is a daunting task.
139
140       For this reason I am currently working on 3 different YAML
141       implementations.
142
143       YAML
144           The main YAML distribution will keeping evolving to support the
145           entire YAML specification in pure Perl. This may not be the fastest
146           or most stable module though. Currently, YAML.pm has lots of known
147           bugs. It is mostly a great tool for dumping Perl data structures to
148           a readable form.
149
150       YAML::Tiny
151           The point of YAML::Tiny is to strip YAML down to the 90% that
152           people use most and offer that in a small, fast, stable, pure Perl
153           form. YAML::Tiny will simply die when it is asked to do something
154           it can't.
155
156       YAML::Syck
157           "libsyck" is the C based YAML processing library used by the Ruby
158           programming language (and also Python, PHP and Pugs). YAML::Syck is
159           the Perl binding to "libsyck". It should be very fast, but may have
160           problems of its own. It will also require C compilation.
161
162           NOTE: Audrey Tang has actually completed this module and it works
163           great and is
164                 10 times faster than YAML.pm.
165
166       In the future, there will likely be even more YAML modules. Remember,
167       people other than Ingy are allowed to write YAML modules!
168

FUNCTIONAL USAGE

170       YAML is completely OO under the hood. Still it exports a few useful top
171       level functions so that it is dead simple to use. These functions just
172       do the OO stuff for you. If you want direct access to the OO API see
173       the documentation for YAML::Dumper and YAML::Loader.
174
175   Exported Functions
176       The following functions are exported by YAML.pm by default. The reason
177       they are exported is so that YAML works much like Data::Dumper. If you
178       don't want functions to be imported, just use YAML with an empty import
179       list:
180
181           use YAML ();
182
183       Dump(list-of-Perl-data-structures)
184           Turn Perl data into YAML. This function works very much like
185           Data::Dumper::Dumper(). It takes a list of Perl data structures and
186           dumps them into a serialized form. It returns a string containing
187           the YAML stream. The structures can be references or plain scalars.
188
189       Load(string-containing-a-YAML-stream)
190           Turn YAML into Perl data. This is the opposite of Dump. Just like
191           Storable's thaw() function or the eval() function in relation to
192           Data::Dumper. It parses a string containing a valid YAML stream
193           into a list of Perl data structures.
194
195   Exportable Functions
196       These functions are not exported by default but you can request them in
197       an import list like this:
198
199           use YAML qw'freeze thaw Bless';
200
201       freeze() and thaw()
202           Aliases to Dump() and Load() for Storable fans. This will also
203           allow YAML.pm to be plugged directly into modules like POE.pm, that
204           use the freeze/thaw API for internal serialization.
205
206       DumpFile(filepath, list)
207           Writes the YAML stream to a file instead of just returning a
208           string.
209
210       LoadFile(filepath)
211           Reads the YAML stream from a file instead of a string.
212
213       Bless(perl-node, [yaml-node | class-name])
214           Associate a normal Perl node, with a yaml node. A yaml node is an
215           object tied to the YAML::Node class. The second argument is either
216           a yaml node that you've already created or a class (package) name
217           that supports a "yaml_dump()" function. A "yaml_dump()" function
218           should take a perl node and return a yaml node. If no second
219           argument is provided, Bless will create a yaml node. This node is
220           not returned, but can be retrieved with the Blessed() function.
221
222           Here's an example of how to use Bless. Say you have a hash
223           containing three keys, but you only want to dump two of them.
224           Furthermore the keys must be dumped in a certain order. Here's how
225           you do that:
226
227               use YAML qw(Dump Bless);
228               $hash = {apple => 'good', banana => 'bad', cauliflower => 'ugly'};
229               print Dump $hash;
230               Bless($hash)->keys(['banana', 'apple']);
231               print Dump $hash;
232
233           produces:
234
235               ---
236               apple: good
237               banana: bad
238               cauliflower: ugly
239               ---
240               banana: bad
241               apple: good
242
243           Bless returns the tied part of a yaml-node, so that you can call
244           the YAML::Node methods. This is the same thing that
245           YAML::Node::ynode() returns.  So another way to do the above
246           example is:
247
248               use YAML qw(Dump Bless);
249               use YAML::Node;
250               $hash = {apple => 'good', banana => 'bad', cauliflower => 'ugly'};
251               print Dump $hash;
252               Bless($hash);
253               $ynode = ynode(Blessed($hash));
254               $ynode->keys(['banana', 'apple']);
255               print Dump $hash;
256
257           Note that Blessing a Perl data structure does not change it anyway.
258           The extra information is stored separately and looked up by the
259           Blessed node's memory address.
260
261       Blessed(perl-node)
262           Returns the yaml node that a particular perl node is associated
263           with (see above). Returns undef if the node is not (YAML) Blessed.
264

GLOBAL OPTIONS

266       YAML options are set using a group of global variables in the YAML
267       namespace.  This is similar to how Data::Dumper works.
268
269       For example, to change the indentation width, do something like:
270
271           local $YAML::Indent = 3;
272
273       The current options are:
274
275       DumperClass
276           You can override which module/class YAML uses for Dumping data.
277
278       LoadBlessed (since 1.25)
279           Default is undef (false)
280
281           The default was changed in version 1.30.
282
283           When set to true, YAML nodes with special tags will be
284           automatocally blessed into objects:
285
286               - !perl/hash:Foo::Bar
287                   foo: 42
288
289           When loading untrusted YAML, you should disable this option by
290           setting it to 0. This will also disable setting typeglobs when
291           loading them.
292
293           You can create any kind of object with YAML. The creation itself is
294           not the critical part. If the class has a "DESTROY" method, it will
295           be called once the object is deleted. An example with File::Temp
296           removing files can be found at
297           <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=862373>
298
299       LoaderClass
300           You can override which module/class YAML uses for Loading data.
301
302       Indent
303           This is the number of space characters to use for each indentation
304           level when doing a Dump(). The default is 2.
305
306           By the way, YAML can use any number of characters for indentation
307           at any level. So if you are editing YAML by hand feel free to do it
308           anyway that looks pleasing to you; just be consistent for a given
309           level.
310
311       SortKeys
312           Default is 1. (true)
313
314           Tells YAML.pm whether or not to sort hash keys when storing a
315           document.
316
317           YAML::Node objects can have their own sort order, which is usually
318           what you want. To override the YAML::Node order and sort the keys
319           anyway, set SortKeys to 2.
320
321       Stringify
322           Default is 0. (false)
323
324           Objects with string overloading should honor the overloading and
325           dump the stringification of themselves, rather than the actual
326           object's guts.
327
328       Numify
329           Default is 0. (false)
330
331           Values that look like numbers (integers, floats) will be numified
332           when loaded.
333
334       UseHeader
335           Default is 1. (true)
336
337           This tells YAML.pm whether to use a separator string for a Dump
338           operation.  This only applies to the first document in a stream.
339           Subsequent documents must have a YAML header by definition.
340
341       UseVersion
342           Default is 0. (false)
343
344           Tells YAML.pm whether to include the YAML version on the
345           separator/header.
346
347               --- %YAML:1.0
348
349       AnchorPrefix
350           Default is ''.
351
352           Anchor names are normally numeric. YAML.pm simply starts with '1'
353           and increases by one for each new anchor. This option allows you to
354           specify a string to be prepended to each anchor number.
355
356       UseCode
357           Setting the UseCode option is a shortcut to set both the DumpCode
358           and LoadCode options at once. Setting UseCode to '1' tells YAML.pm
359           to dump Perl code references as Perl (using B::Deparse) and to load
360           them back into memory using eval(). The reason this has to be an
361           option is that using eval() to parse untrusted code is, well,
362           untrustworthy.
363
364       DumpCode
365           Determines if and how YAML.pm should serialize Perl code
366           references. By default YAML.pm will dump code references as dummy
367           placeholders (much like Data::Dumper). If DumpCode is set to '1' or
368           'deparse', code references will be dumped as actual Perl code.
369
370       LoadCode
371           LoadCode is the opposite of DumpCode. It tells YAML if and how to
372           deserialize code references. When set to '1' or 'deparse' it will
373           use "eval()". Since this is potentially risky, only use this option
374           if you know where your YAML has been.
375
376           LoadCode must be enabled also to use the feature of evaluating
377           typeglobs (because with the typeglob feature you would be able to
378           set the variable $YAML::LoadCode from a YAML file).
379
380       Preserve
381           When set to true, this option tells the Loader to load hashes into
382           YAML::Node objects. These are tied hashes. This has the effect of
383           remembering the key order, thus it will be preserved when the hash
384           is dumped again. See YAML::Node for more information.
385
386       UseBlock
387           YAML.pm uses heuristics to guess which scalar style is best for a
388           given node.  Sometimes you'll want all multiline scalars to use the
389           'block' style. If so, set this option to 1.
390
391           NOTE: YAML's block style is akin to Perl's here-document.
392
393       UseFold (Not supported anymore since v0.60)
394           If you want to force YAML to use the 'folded' style for all
395           multiline scalars, then set $UseFold to 1.
396
397           NOTE: YAML's folded style is akin to the way HTML folds text,
398           except smarter.
399
400       UseAliases
401           YAML has an alias mechanism such that any given structure in memory
402           gets serialized once. Any other references to that structure are
403           serialized only as alias markers. This is how YAML can serialize
404           duplicate and recursive structures.
405
406           Sometimes, when you KNOW that your data is nonrecursive in nature,
407           you may want to serialize such that every node is expressed in
408           full. (ie as a copy of the original). Setting $YAML::UseAliases to
409           0 will allow you to do this. This also may result in faster
410           processing because the lookup overhead is by bypassed.
411
412           THIS OPTION CAN BE DANGEROUS. If your data is recursive, this
413           option will cause Dump() to run in an endless loop, chewing up your
414           computers memory. You have been warned.
415
416       CompressSeries
417           Default is 1.
418
419           Compresses the formatting of arrays of hashes:
420
421               -
422                 foo: bar
423               -
424                 bar: foo
425
426           becomes:
427
428               - foo: bar
429               - bar: foo
430
431           Since this output is usually more desirable, this option is turned
432           on by default.
433
434       QuoteNumericStrings
435           Default is 0. (false)
436
437           Adds detection mechanisms to encode strings that resemble numbers
438           with mandatory quoting.
439
440           This ensures leading that things like leading/trailing zeros and
441           other formatting are preserved.
442

YAML TERMINOLOGY

444       YAML is a full featured data serialization language, and thus has its
445       own terminology.
446
447       It is important to remember that although YAML is heavily influenced by
448       Perl and Python, it is a language in its own right, not merely just a
449       representation of Perl structures.
450
451       YAML has three constructs that are conspicuously similar to Perl's
452       hash, array, and scalar. They are called mapping, sequence, and string
453       respectively.  By default, they do what you would expect. But each
454       instance may have an explicit or implicit tag (type) that makes it
455       behave differently. In this manner, YAML can be extended to represent
456       Perl's Glob or Python's tuple, or Ruby's Bigint.
457
458       stream
459               A YAML stream is the full sequence of Unicode characters that a YAML
460               parser would read or a YAML emitter would write. A stream may contain
461               one or more YAML documents separated by YAML headers.
462
463               ---
464               a: mapping
465               foo: bar
466               ---
467               - a
468               - sequence
469
470       document
471           A YAML document is an independent data structure representation
472           within a stream. It is a top level node. Each document in a YAML
473           stream must begin with a YAML header line. Actually the header is
474           optional on the first document.
475
476               ---
477               This: top level mapping
478               is:
479                   - a
480                   - YAML
481                   - document
482
483       header
484           A YAML header is a line that begins a YAML document. It consists of
485           three dashes, possibly followed by more info. Another purpose of
486           the header line is that it serves as a place to put top level tag
487           and anchor information.
488
489               --- !recursive-sequence &001
490               - * 001
491               - * 001
492
493       node
494           A YAML node is the representation of a particular data structure.
495           Nodes may contain other nodes. (In Perl terms, nodes are like
496           scalars. Strings, arrayrefs and hashrefs. But this refers to the
497           serialized format, not the in- memory structure.)
498
499       tag This is similar to a type. It indicates how a particular YAML node
500           serialization should be transferred into or out of memory. For
501           instance a Foo::Bar object would use the tag 'perl/Foo::Bar':
502
503               - !perl/Foo::Bar
504                   foo: 42
505                   bar: stool
506
507       collection
508           A collection is the generic term for a YAML data grouping. YAML has
509           two types of collections: mappings and sequences. (Similar to
510           hashes and arrays)
511
512       mapping
513           A mapping is a YAML collection defined by unordered key/value pairs
514           with unique keys. By default YAML mappings are loaded into Perl
515           hashes.
516
517               a mapping:
518                   foo: bar
519                   two: times two is 4
520
521       sequence
522           A sequence is a YAML collection defined by an ordered list of
523           elements. By default YAML sequences are loaded into Perl arrays.
524
525               a sequence:
526                   - one bourbon
527                   - one scotch
528                   - one beer
529
530       scalar
531           A scalar is a YAML node that is a single value. By default YAML
532           scalars are loaded into Perl scalars.
533
534               a scalar key: a scalar value
535
536           YAML has many styles for representing scalars. This is important
537           because varying data will have varying formatting requirements to
538           retain the optimum human readability.
539
540       plain scalar
541           A plain scalar is unquoted. All plain scalars are automatic
542           candidates for "implicit tagging". This means that their tag may be
543           determined automatically by examination. The typical uses for this
544           are plain alpha strings, integers, real numbers, dates, times and
545           currency.
546
547               - a plain string
548               - -42
549               - 3.1415
550               - 12:34
551               - 123 this is an error
552
553       single quoted scalar
554           This is similar to Perl's use of single quotes. It means no
555           escaping except for single quotes which are escaped by using two
556           adjacent single quotes.
557
558               - 'When I say ''\n'' I mean "backslash en"'
559
560       double quoted scalar
561           This is similar to Perl's use of double quotes. Character escaping
562           can be used.
563
564               - "This scalar\nhas two lines, and a bell -->\a"
565
566       folded scalar
567           This is a multiline scalar which begins on the next line. It is
568           indicated by a single right angle bracket. It is unescaped like the
569           single quoted scalar.  Line folding is also performed.
570
571               - >
572                This is a multiline scalar which begins on
573                the next line. It is indicated by a single
574                carat. It is unescaped like the single
575                quoted scalar. Line folding is also
576                performed.
577
578       block scalar
579           This final multiline form is akin to Perl's here-document except
580           that (as in all YAML data) scope is indicated by indentation.
581           Therefore, no ending marker is required. The data is verbatim. No
582           line folding.
583
584               - |
585                   QTY  DESC          PRICE  TOTAL
586                   ---  ----          -----  -----
587                     1  Foo Fighters  $19.95 $19.95
588                     2  Bar Belles    $29.95 $59.90
589
590       parser
591           A YAML processor has four stages: parse, load, dump, emit.
592
593           A parser parses a YAML stream. YAML.pm's Load() function contains a
594           parser.
595
596       loader
597           The other half of the Load() function is a loader. This takes the
598           information from the parser and loads it into a Perl data
599           structure.
600
601       dumper
602           The Dump() function consists of a dumper and an emitter. The dumper
603           walks through each Perl data structure and gives info to the
604           emitter.
605
606       emitter
607           The emitter takes info from the dumper and turns it into a YAML
608           stream.
609
610           NOTE: In YAML.pm the parserloader and the dumperemitter code are
611           currently
612                 very closely tied together. In the future they may be broken
613           into
614                 separate stages.
615
616       For more information please refer to the immensely helpful YAML
617       specification available at <http://www.yaml.org/spec/>.
618

YSH - THE YAML SHELL

620       The YAML::Shell distribution provides script called 'ysh', the YAML
621       shell.  ysh provides a simple, interactive way to play with YAML. If
622       you type in Perl code, it displays the result in YAML. If you type in
623       YAML it turns it into Perl code.
624
625       To run ysh, (assuming you installed it along with YAML.pm) simply type:
626
627           ysh [options]
628
629       Please read the "ysh" documentation for the full details. There are
630       lots of options.
631

BUGS & DEFICIENCIES

633       If you find a bug in YAML, please try to recreate it in the YAML Shell
634       with logging turned on ('ysh -L'). When you have successfully
635       reproduced the bug, please mail the LOG file to the author
636       (ingy@cpan.org).
637
638       WARNING: This is still ALPHA code. Well, most of this code has been
639       around for years...
640
641       BIGGER WARNING: YAML.pm has been slow in the making, but I am committed
642       to having top notch YAML tools in the Perl world. The YAML team is
643       close to finalizing the YAML 1.1 spec. This version of YAML.pm is based
644       off of a very old pre 1.0 spec. In actuality there isn't a ton of
645       difference, and this YAML.pm is still fairly useful. Things will get
646       much better in the future.
647

RESOURCES

649       <http://lists.sourceforge.net/lists/listinfo/yaml-core> is the mailing
650       list.  This is where the language is discussed and designed.
651
652       <http://www.yaml.org> is the official YAML website.
653
654       <http://www.yaml.org/spec/> is the YAML 1.2 specification.
655
656       <http://yaml.kwiki.org> is the official YAML wiki.
657

SEE ALSO

659       •   YAML::XS
660

AUTHOR

662       Ingy döt Net <ingy@cpan.org>
663
665       Copyright 2001-2020. Ingy döt Net.
666
667       This program is free software; you can redistribute it and/or modify it
668       under the same terms as Perl itself.
669
670       See <http://www.perl.com/perl/misc/Artistic.html>
671
672
673
674perl v5.36.0                      2022-07-22                           YAML(3)
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