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 1.28.
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 1 (true).
280
281           When set to true, YAML nodes with special tags will be
282           automatocally blessed into objects:
283
284               - !perl/hash:Foo::Bar
285                   foo: 42
286
287           When loading untrusted YAML, you should disable this option by
288           setting it to 0. This will also disable setting typeglobs when
289           loading them.
290
291       LoaderClass
292           You can override which module/class YAML uses for Loading data.
293
294       Indent
295           This is the number of space characters to use for each indentation
296           level when doing a Dump(). The default is 2.
297
298           By the way, YAML can use any number of characters for indentation
299           at any level. So if you are editing YAML by hand feel free to do it
300           anyway that looks pleasing to you; just be consistent for a given
301           level.
302
303       SortKeys
304           Default is 1. (true)
305
306           Tells YAML.pm whether or not to sort hash keys when storing a
307           document.
308
309           YAML::Node objects can have their own sort order, which is usually
310           what you want. To override the YAML::Node order and sort the keys
311           anyway, set SortKeys to 2.
312
313       Stringify
314           Default is 0. (false)
315
316           Objects with string overloading should honor the overloading and
317           dump the stringification of themselves, rather than the actual
318           object's guts.
319
320       Numify
321           Default is 0. (false)
322
323           Values that look like numbers (integers, floats) will be numified
324           when loaded.
325
326       UseHeader
327           Default is 1. (true)
328
329           This tells YAML.pm whether to use a separator string for a Dump
330           operation.  This only applies to the first document in a stream.
331           Subsequent documents must have a YAML header by definition.
332
333       UseVersion
334           Default is 0. (false)
335
336           Tells YAML.pm whether to include the YAML version on the
337           separator/header.
338
339               --- %YAML:1.0
340
341       AnchorPrefix
342           Default is ''.
343
344           Anchor names are normally numeric. YAML.pm simply starts with '1'
345           and increases by one for each new anchor. This option allows you to
346           specify a string to be prepended to each anchor number.
347
348       UseCode
349           Setting the UseCode option is a shortcut to set both the DumpCode
350           and LoadCode options at once. Setting UseCode to '1' tells YAML.pm
351           to dump Perl code references as Perl (using B::Deparse) and to load
352           them back into memory using eval(). The reason this has to be an
353           option is that using eval() to parse untrusted code is, well,
354           untrustworthy.
355
356       DumpCode
357           Determines if and how YAML.pm should serialize Perl code
358           references. By default YAML.pm will dump code references as dummy
359           placeholders (much like Data::Dumper). If DumpCode is set to '1' or
360           'deparse', code references will be dumped as actual Perl code.
361
362       LoadCode
363           LoadCode is the opposite of DumpCode. It tells YAML if and how to
364           deserialize code references. When set to '1' or 'deparse' it will
365           use "eval()". Since this is potentially risky, only use this option
366           if you know where your YAML has been.
367
368           LoadCode must be enabled also to use the feature of evaluating
369           typeglobs (because with the typeglob feature you would be able to
370           set the variable $YAML::LoadCode from a YAML file).
371
372       Preserve
373           When set to true, this option tells the Loader to load hashes into
374           YAML::Node objects. These are tied hashes. This has the effect of
375           remembering the key order, thus it will be preserved when the hash
376           is dumped again. See YAML::Node for more information.
377
378       UseBlock
379           YAML.pm uses heuristics to guess which scalar style is best for a
380           given node.  Sometimes you'll want all multiline scalars to use the
381           'block' style. If so, set this option to 1.
382
383           NOTE: YAML's block style is akin to Perl's here-document.
384
385       UseFold (Not supported anymore since v0.60)
386           If you want to force YAML to use the 'folded' style for all
387           multiline scalars, then set $UseFold to 1.
388
389           NOTE: YAML's folded style is akin to the way HTML folds text,
390           except smarter.
391
392       UseAliases
393           YAML has an alias mechanism such that any given structure in memory
394           gets serialized once. Any other references to that structure are
395           serialized only as alias markers. This is how YAML can serialize
396           duplicate and recursive structures.
397
398           Sometimes, when you KNOW that your data is nonrecursive in nature,
399           you may want to serialize such that every node is expressed in
400           full. (ie as a copy of the original). Setting $YAML::UseAliases to
401           0 will allow you to do this. This also may result in faster
402           processing because the lookup overhead is by bypassed.
403
404           THIS OPTION CAN BE DANGEROUS. If your data is recursive, this
405           option will cause Dump() to run in an endless loop, chewing up your
406           computers memory. You have been warned.
407
408       CompressSeries
409           Default is 1.
410
411           Compresses the formatting of arrays of hashes:
412
413               -
414                 foo: bar
415               -
416                 bar: foo
417
418           becomes:
419
420               - foo: bar
421               - bar: foo
422
423           Since this output is usually more desirable, this option is turned
424           on by default.
425
426       QuoteNumericStrings
427           Default is 0. (false)
428
429           Adds detection mechanisms to encode strings that resemble numbers
430           with mandatory quoting.
431
432           This ensures leading that things like leading/trailing zeros and
433           other formatting are preserved.
434

YAML TERMINOLOGY

436       YAML is a full featured data serialization language, and thus has its
437       own terminology.
438
439       It is important to remember that although YAML is heavily influenced by
440       Perl and Python, it is a language in its own right, not merely just a
441       representation of Perl structures.
442
443       YAML has three constructs that are conspicuously similar to Perl's
444       hash, array, and scalar. They are called mapping, sequence, and string
445       respectively.  By default, they do what you would expect. But each
446       instance may have an explicit or implicit tag (type) that makes it
447       behave differently. In this manner, YAML can be extended to represent
448       Perl's Glob or Python's tuple, or Ruby's Bigint.
449
450       stream
451               A YAML stream is the full sequence of Unicode characters that a YAML
452               parser would read or a YAML emitter would write. A stream may contain
453               one or more YAML documents separated by YAML headers.
454
455               ---
456               a: mapping
457               foo: bar
458               ---
459               - a
460               - sequence
461
462       document
463           A YAML document is an independent data structure representation
464           within a stream. It is a top level node. Each document in a YAML
465           stream must begin with a YAML header line. Actually the header is
466           optional on the first document.
467
468               ---
469               This: top level mapping
470               is:
471                   - a
472                   - YAML
473                   - document
474
475       header
476           A YAML header is a line that begins a YAML document. It consists of
477           three dashes, possibly followed by more info. Another purpose of
478           the header line is that it serves as a place to put top level tag
479           and anchor information.
480
481               --- !recursive-sequence &001
482               - * 001
483               - * 001
484
485       node
486           A YAML node is the representation of a particular data structure.
487           Nodes may contain other nodes. (In Perl terms, nodes are like
488           scalars. Strings, arrayrefs and hashrefs. But this refers to the
489           serialized format, not the in- memory structure.)
490
491       tag This is similar to a type. It indicates how a particular YAML node
492           serialization should be transferred into or out of memory. For
493           instance a Foo::Bar object would use the tag 'perl/Foo::Bar':
494
495               - !perl/Foo::Bar
496                   foo: 42
497                   bar: stool
498
499       collection
500           A collection is the generic term for a YAML data grouping. YAML has
501           two types of collections: mappings and sequences. (Similar to
502           hashes and arrays)
503
504       mapping
505           A mapping is a YAML collection defined by unordered key/value pairs
506           with unique keys. By default YAML mappings are loaded into Perl
507           hashes.
508
509               a mapping:
510                   foo: bar
511                   two: times two is 4
512
513       sequence
514           A sequence is a YAML collection defined by an ordered list of
515           elements. By default YAML sequences are loaded into Perl arrays.
516
517               a sequence:
518                   - one bourbon
519                   - one scotch
520                   - one beer
521
522       scalar
523           A scalar is a YAML node that is a single value. By default YAML
524           scalars are loaded into Perl scalars.
525
526               a scalar key: a scalar value
527
528           YAML has many styles for representing scalars. This is important
529           because varying data will have varying formatting requirements to
530           retain the optimum human readability.
531
532       plain scalar
533           A plain scalar is unquoted. All plain scalars are automatic
534           candidates for "implicit tagging". This means that their tag may be
535           determined automatically by examination. The typical uses for this
536           are plain alpha strings, integers, real numbers, dates, times and
537           currency.
538
539               - a plain string
540               - -42
541               - 3.1415
542               - 12:34
543               - 123 this is an error
544
545       single quoted scalar
546           This is similar to Perl's use of single quotes. It means no
547           escaping except for single quotes which are escaped by using two
548           adjacent single quotes.
549
550               - 'When I say ''\n'' I mean "backslash en"'
551
552       double quoted scalar
553           This is similar to Perl's use of double quotes. Character escaping
554           can be used.
555
556               - "This scalar\nhas two lines, and a bell -->\a"
557
558       folded scalar
559           This is a multiline scalar which begins on the next line. It is
560           indicated by a single right angle bracket. It is unescaped like the
561           single quoted scalar.  Line folding is also performed.
562
563               - >
564                This is a multiline scalar which begins on
565                the next line. It is indicated by a single
566                carat. It is unescaped like the single
567                quoted scalar. Line folding is also
568                performed.
569
570       block scalar
571           This final multiline form is akin to Perl's here-document except
572           that (as in all YAML data) scope is indicated by indentation.
573           Therefore, no ending marker is required. The data is verbatim. No
574           line folding.
575
576               - |
577                   QTY  DESC          PRICE  TOTAL
578                   ---  ----          -----  -----
579                     1  Foo Fighters  $19.95 $19.95
580                     2  Bar Belles    $29.95 $59.90
581
582       parser
583           A YAML processor has four stages: parse, load, dump, emit.
584
585           A parser parses a YAML stream. YAML.pm's Load() function contains a
586           parser.
587
588       loader
589           The other half of the Load() function is a loader. This takes the
590           information from the parser and loads it into a Perl data
591           structure.
592
593       dumper
594           The Dump() function consists of a dumper and an emitter. The dumper
595           walks through each Perl data structure and gives info to the
596           emitter.
597
598       emitter
599           The emitter takes info from the dumper and turns it into a YAML
600           stream.
601
602           NOTE: In YAML.pm the parserloader and the dumperemitter code are
603           currently
604                 very closely tied together. In the future they may be broken
605           into
606                 separate stages.
607
608       For more information please refer to the immensely helpful YAML
609       specification available at <http://www.yaml.org/spec/>.
610

YSH - THE YAML SHELL

612       The YAML::Shell distribution provides script called 'ysh', the YAML
613       shell.  ysh provides a simple, interactive way to play with YAML. If
614       you type in Perl code, it displays the result in YAML. If you type in
615       YAML it turns it into Perl code.
616
617       To run ysh, (assuming you installed it along with YAML.pm) simply type:
618
619           ysh [options]
620
621       Please read the "ysh" documentation for the full details. There are
622       lots of options.
623

BUGS & DEFICIENCIES

625       If you find a bug in YAML, please try to recreate it in the YAML Shell
626       with logging turned on ('ysh -L'). When you have successfully
627       reproduced the bug, please mail the LOG file to the author
628       (ingy@cpan.org).
629
630       WARNING: This is still ALPHA code. Well, most of this code has been
631       around for years...
632
633       BIGGER WARNING: YAML.pm has been slow in the making, but I am committed
634       to having top notch YAML tools in the Perl world. The YAML team is
635       close to finalizing the YAML 1.1 spec. This version of YAML.pm is based
636       off of a very old pre 1.0 spec. In actuality there isn't a ton of
637       difference, and this YAML.pm is still fairly useful. Things will get
638       much better in the future.
639

RESOURCES

641       <http://lists.sourceforge.net/lists/listinfo/yaml-core> is the mailing
642       list.  This is where the language is discussed and designed.
643
644       <http://www.yaml.org> is the official YAML website.
645
646       <http://www.yaml.org/spec/> is the YAML 1.2 specification.
647
648       <http://yaml.kwiki.org> is the official YAML wiki.
649

SEE ALSO

651       ·   YAML::XS
652

AUTHOR

654       Ingy döt Net <ingy@cpan.org>
655
657       Copyright 2001-2019. Ingy döt Net.
658
659       This program is free software; you can redistribute it and/or modify it
660       under the same terms as Perl itself.
661
662       See <http://www.perl.com/perl/misc/Artistic.html>
663
664
665
666perl v5.28.1                      2019-04-28                           YAML(3)
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