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NAME

    YAML - YAML Ain't Markup Language™

VERSION

    This document describes YAML version 1.30.

NOTE

    This module has been released to CPAN as YAML::Old, and soon YAML.pm
    will be changed to just be a frontend interface module for all the
    various Perl YAML implementation modules, including YAML::Old.

    If you want robust and fast YAML processing using the normal Dump/Load
    API, please consider switching to YAML::XS. It is by far the best Perl
    module for YAML at this time. It requires that you have a C compiler,
    since it is written in C.

    If you really need to use this version of YAML.pm it will always be
    available as YAML::Old.

    The rest of this documentation is left unchanged, until YAML.pm is
    switched over to the new UI-only version.

SYNOPSIS

        use YAML;
    
        # Load a YAML stream of 3 YAML documents into Perl data structures.
        my ($hashref, $arrayref, $string) = Load(<<'...');
        ---
        name: ingy       # A Mapping
        age: old
        weight: heavy
        # I should comment that I also like pink, but don't tell anybody.
        favorite colors:
          - red
          - green
          - blue
        ---
        - Clark Evans    # A Sequence
        - Oren Ben-Kiki
        - Ingy döt Net
        --- >            # A Block Scalar
        You probably think YAML stands for "Yet Another Markup Language". It
        ain't! YAML is really a data serialization language. But if you want
        to think of it as a markup, that's OK with me. A lot of people try
        to use XML as a serialization format.
    
        "YAML" is catchy and fun to say. Try it. "YAML, YAML, YAML!!!"
        ...
    
        # Dump the Perl data structures back into YAML.
        print Dump($string, $arrayref, $hashref);
    
        # YAML::Dump is used the same way you'd use Data::Dumper::Dumper
        use Data::Dumper;
        print Dumper($string, $arrayref, $hashref);
    
        Since version 1.25 YAML.pm supports trailing comments.

DESCRIPTION

    The YAML.pm module implements a YAML Loader and Dumper based on the
    YAML 1.0 specification. http://www.yaml.org/spec/

    YAML is a generic data serialization language that is optimized for
    human readability. It can be used to express the data structures of
    most modern programming languages. (Including Perl!!!)

    For information on the YAML syntax, please refer to the YAML
    specification.

WHY YAML IS COOL

    YAML is readable for people.

      It makes clear sense out of complex data structures. You should find
      that YAML is an exceptional data dumping tool. Structure is shown
      through indentation, YAML supports recursive data, and hash keys are
      sorted by default. In addition, YAML supports several styles of
      scalar formatting for different types of data.

    YAML is editable.

      YAML was designed from the ground up to be an excellent syntax for
      configuration files. Almost all programs need configuration files, so
      why invent a new syntax for each one? And why subject users to the
      complexities of XML or native Perl code?

    YAML is multilingual.

      Yes, YAML supports Unicode. But I'm actually referring to programming
      languages. YAML was designed to meet the serialization needs of Perl,
      Python, Ruby, Tcl, PHP, Javascript and Java. It was also designed to
      be interoperable between those languages. That means YAML
      serializations produced by Perl can be processed by Python.

    YAML is taint safe.

      Using modules like Data::Dumper for serialization is fine as long as
      you can be sure that nobody can tamper with your data files or
      transmissions. That's because you need to use Perl's eval() built-in
      to deserialize the data. Somebody could add a snippet of Perl to
      erase your files.

      YAML's parser does not need to eval anything.

    YAML is full featured.

      YAML can accurately serialize all of the common Perl data structures
      and deserialize them again without losing data relationships.
      Although it is not 100% perfect (no serializer is or can be perfect),
      it fares as well as the popular current modules: Data::Dumper,
      Storable, XML::Dumper and Data::Denter.

      YAML.pm also has the ability to handle code (subroutine) references
      and typeglobs. (Still experimental) These features are not found in
      Perl's other serialization modules.

    YAML is extensible.

      The YAML language has been designed to be flexible enough to solve
      it's own problems. The markup itself has 3 basic construct which
      resemble Perl's hash, array and scalar. By default, these map to
      their Perl equivalents. But each YAML node also supports a tagging
      mechanism (type system) which can cause that node to be interpreted
      in a completely different manner. That's how YAML can support object
      serialization and oddball structures like Perl's typeglob.

YAML IMPLEMENTATIONS IN PERL

    This module, YAML.pm, is really just the interface module for YAML
    modules written in Perl. The basic interface for YAML consists of two
    functions: Dump and Load. The real work is done by the modules
    YAML::Dumper and YAML::Loader.

    Different YAML module distributions can be created by subclassing
    YAML.pm and YAML::Loader and YAML::Dumper. For example, YAML-Simple
    consists of YAML::Simple YAML::Dumper::Simple and YAML::Loader::Simple.

    Why would there be more than one implementation of YAML? Well, despite
    YAML's offering of being a simple data format, YAML is actually very
    deep and complex. Implementing the entirety of the YAML specification
    is a daunting task.

    For this reason I am currently working on 3 different YAML
    implementations.

    YAML

      The main YAML distribution will keeping evolving to support the
      entire YAML specification in pure Perl. This may not be the fastest
      or most stable module though. Currently, YAML.pm has lots of known
      bugs. It is mostly a great tool for dumping Perl data structures to a
      readable form.

    YAML::Tiny

      The point of YAML::Tiny is to strip YAML down to the 90% that people
      use most and offer that in a small, fast, stable, pure Perl form.
      YAML::Tiny will simply die when it is asked to do something it can't.

    YAML::Syck

      libsyck is the C based YAML processing library used by the Ruby
      programming language (and also Python, PHP and Pugs). YAML::Syck is
      the Perl binding to libsyck. It should be very fast, but may have
      problems of its own. It will also require C compilation.

      NOTE: Audrey Tang has actually completed this module and it works
      great and is 10 times faster than YAML.pm.

    In the future, there will likely be even more YAML modules. Remember,
    people other than Ingy are allowed to write YAML modules!

FUNCTIONAL USAGE

    YAML is completely OO under the hood. Still it exports a few useful top
    level functions so that it is dead simple to use. These functions just
    do the OO stuff for you. If you want direct access to the OO API see
    the documentation for YAML::Dumper and YAML::Loader.

 Exported Functions

    The following functions are exported by YAML.pm by default. The reason
    they are exported is so that YAML works much like Data::Dumper. If you
    don't want functions to be imported, just use YAML with an empty import
    list:

        use YAML ();

    Dump(list-of-Perl-data-structures)

      Turn Perl data into YAML. This function works very much like
      Data::Dumper::Dumper(). It takes a list of Perl data structures and
      dumps them into a serialized form. It returns a string containing the
      YAML stream. The structures can be references or plain scalars.

    Load(string-containing-a-YAML-stream)

      Turn YAML into Perl data. This is the opposite of Dump. Just like
      Storable's thaw() function or the eval() function in relation to
      Data::Dumper. It parses a string containing a valid YAML stream into
      a list of Perl data structures.

 Exportable Functions

    These functions are not exported by default but you can request them in
    an import list like this:

        use YAML qw'freeze thaw Bless';

    freeze() and thaw()

      Aliases to Dump() and Load() for Storable fans. This will also allow
      YAML.pm to be plugged directly into modules like POE.pm, that use the
      freeze/thaw API for internal serialization.

    DumpFile(filepath, list)

      Writes the YAML stream to a file instead of just returning a string.

    LoadFile(filepath)

      Reads the YAML stream from a file instead of a string.

    Bless(perl-node, [yaml-node | class-name])

      Associate a normal Perl node, with a yaml node. A yaml node is an
      object tied to the YAML::Node class. The second argument is either a
      yaml node that you've already created or a class (package) name that
      supports a yaml_dump() function. A yaml_dump() function should take a
      perl node and return a yaml node. If no second argument is provided,
      Bless will create a yaml node. This node is not returned, but can be
      retrieved with the Blessed() function.

      Here's an example of how to use Bless. Say you have a hash containing
      three keys, but you only want to dump two of them. Furthermore the
      keys must be dumped in a certain order. Here's how you do that:

          use YAML qw(Dump Bless);
          $hash = {apple => 'good', banana => 'bad', cauliflower => 'ugly'};
          print Dump $hash;
          Bless($hash)->keys(['banana', 'apple']);
          print Dump $hash;

      produces:

          ---
          apple: good
          banana: bad
          cauliflower: ugly
          ---
          banana: bad
          apple: good

      Bless returns the tied part of a yaml-node, so that you can call the
      YAML::Node methods. This is the same thing that YAML::Node::ynode()
      returns. So another way to do the above example is:

          use YAML qw(Dump Bless);
          use YAML::Node;
          $hash = {apple => 'good', banana => 'bad', cauliflower => 'ugly'};
          print Dump $hash;
          Bless($hash);
          $ynode = ynode(Blessed($hash));
          $ynode->keys(['banana', 'apple']);
          print Dump $hash;

      Note that Blessing a Perl data structure does not change it anyway.
      The extra information is stored separately and looked up by the
      Blessed node's memory address.

    Blessed(perl-node)

      Returns the yaml node that a particular perl node is associated with
      (see above). Returns undef if the node is not (YAML) Blessed.

GLOBAL OPTIONS

    YAML options are set using a group of global variables in the YAML
    namespace. This is similar to how Data::Dumper works.

    For example, to change the indentation width, do something like:

        local $YAML::Indent = 3;

    The current options are:

    DumperClass

      You can override which module/class YAML uses for Dumping data.

    LoadBlessed (since 1.25)

      Default is undef (false)

      The default was changed in version 1.30.

      When set to true, YAML nodes with special tags will be automatocally
      blessed into objects:

          - !perl/hash:Foo::Bar
              foo: 42

      When loading untrusted YAML, you should disable this option by
      setting it to 0. This will also disable setting typeglobs when
      loading them.

      You can create any kind of object with YAML. The creation itself is
      not the critical part. If the class has a DESTROY method, it will be
      called once the object is deleted. An example with File::Temp
      removing files can be found at
      https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=862373

    LoaderClass

      You can override which module/class YAML uses for Loading data.

    Indent

      This is the number of space characters to use for each indentation
      level when doing a Dump(). The default is 2.

      By the way, YAML can use any number of characters for indentation at
      any level. So if you are editing YAML by hand feel free to do it
      anyway that looks pleasing to you; just be consistent for a given
      level.

    SortKeys

      Default is 1. (true)

      Tells YAML.pm whether or not to sort hash keys when storing a
      document.

      YAML::Node objects can have their own sort order, which is usually
      what you want. To override the YAML::Node order and sort the keys
      anyway, set SortKeys to 2.

    Stringify

      Default is 0. (false)

      Objects with string overloading should honor the overloading and dump
      the stringification of themselves, rather than the actual object's
      guts.

    Numify

      Default is 0. (false)

      Values that look like numbers (integers, floats) will be numified
      when loaded.

    UseHeader

      Default is 1. (true)

      This tells YAML.pm whether to use a separator string for a Dump
      operation. This only applies to the first document in a stream.
      Subsequent documents must have a YAML header by definition.

    UseVersion

      Default is 0. (false)

      Tells YAML.pm whether to include the YAML version on the
      separator/header.

          --- %YAML:1.0

    AnchorPrefix

      Default is ''.

      Anchor names are normally numeric. YAML.pm simply starts with '1' and
      increases by one for each new anchor. This option allows you to
      specify a string to be prepended to each anchor number.

    UseCode

      Setting the UseCode option is a shortcut to set both the DumpCode and
      LoadCode options at once. Setting UseCode to '1' tells YAML.pm to
      dump Perl code references as Perl (using B::Deparse) and to load them
      back into memory using eval(). The reason this has to be an option is
      that using eval() to parse untrusted code is, well, untrustworthy.

    DumpCode

      Determines if and how YAML.pm should serialize Perl code references.
      By default YAML.pm will dump code references as dummy placeholders
      (much like Data::Dumper). If DumpCode is set to '1' or 'deparse',
      code references will be dumped as actual Perl code.

    LoadCode

      LoadCode is the opposite of DumpCode. It tells YAML if and how to
      deserialize code references. When set to '1' or 'deparse' it will use
      eval(). Since this is potentially risky, only use this option if you
      know where your YAML has been.

      LoadCode must be enabled also to use the feature of evaluating
      typeglobs (because with the typeglob feature you would be able to set
      the variable $YAML::LoadCode from a YAML file).

    Preserve

      When set to true, this option tells the Loader to load hashes into
      YAML::Node objects. These are tied hashes. This has the effect of
      remembering the key order, thus it will be preserved when the hash is
      dumped again. See YAML::Node for more information.

    UseBlock

      YAML.pm uses heuristics to guess which scalar style is best for a
      given node. Sometimes you'll want all multiline scalars to use the
      'block' style. If so, set this option to 1.

      NOTE: YAML's block style is akin to Perl's here-document.

    UseFold (Not supported anymore since v0.60)

      If you want to force YAML to use the 'folded' style for all multiline
      scalars, then set $UseFold to 1.

      NOTE: YAML's folded style is akin to the way HTML folds text, except
      smarter.

    UseAliases

      YAML has an alias mechanism such that any given structure in memory
      gets serialized once. Any other references to that structure are
      serialized only as alias markers. This is how YAML can serialize
      duplicate and recursive structures.

      Sometimes, when you KNOW that your data is nonrecursive in nature,
      you may want to serialize such that every node is expressed in full.
      (ie as a copy of the original). Setting $YAML::UseAliases to 0 will
      allow you to do this. This also may result in faster processing
      because the lookup overhead is by bypassed.

      THIS OPTION CAN BE DANGEROUS. If your data is recursive, this option
      will cause Dump() to run in an endless loop, chewing up your
      computers memory. You have been warned.

    CompressSeries

      Default is 1.

      Compresses the formatting of arrays of hashes:

          -
            foo: bar
          -
            bar: foo

      becomes:

          - foo: bar
          - bar: foo

      Since this output is usually more desirable, this option is turned on
      by default.

    QuoteNumericStrings

      Default is 0. (false)

      Adds detection mechanisms to encode strings that resemble numbers
      with mandatory quoting.

      This ensures leading that things like leading/trailing zeros and
      other formatting are preserved.

YAML TERMINOLOGY

    YAML is a full featured data serialization language, and thus has its
    own terminology.

    It is important to remember that although YAML is heavily influenced by
    Perl and Python, it is a language in its own right, not merely just a
    representation of Perl structures.

    YAML has three constructs that are conspicuously similar to Perl's
    hash, array, and scalar. They are called mapping, sequence, and string
    respectively. By default, they do what you would expect. But each
    instance may have an explicit or implicit tag (type) that makes it
    behave differently. In this manner, YAML can be extended to represent
    Perl's Glob or Python's tuple, or Ruby's Bigint.

    stream

          A YAML stream is the full sequence of Unicode characters that a YAML
          parser would read or a YAML emitter would write. A stream may contain
          one or more YAML documents separated by YAML headers.
      
          ---
          a: mapping
          foo: bar
          ---
          - a
          - sequence

    document

      A YAML document is an independent data structure representation
      within a stream. It is a top level node. Each document in a YAML
      stream must begin with a YAML header line. Actually the header is
      optional on the first document.

          ---
          This: top level mapping
          is:
              - a
              - YAML
              - document

    header

      A YAML header is a line that begins a YAML document. It consists of
      three dashes, possibly followed by more info. Another purpose of the
      header line is that it serves as a place to put top level tag and
      anchor information.

          --- !recursive-sequence &001
          - * 001
          - * 001

    node

      A YAML node is the representation of a particular data structure.
      Nodes may contain other nodes. (In Perl terms, nodes are like
      scalars. Strings, arrayrefs and hashrefs. But this refers to the
      serialized format, not the in- memory structure.)

    tag

      This is similar to a type. It indicates how a particular YAML node
      serialization should be transferred into or out of memory. For
      instance a Foo::Bar object would use the tag 'perl/Foo::Bar':

          - !perl/Foo::Bar
              foo: 42
              bar: stool

    collection

      A collection is the generic term for a YAML data grouping. YAML has
      two types of collections: mappings and sequences. (Similar to hashes
      and arrays)

    mapping

      A mapping is a YAML collection defined by unordered key/value pairs
      with unique keys. By default YAML mappings are loaded into Perl
      hashes.

          a mapping:
              foo: bar
              two: times two is 4

    sequence

      A sequence is a YAML collection defined by an ordered list of
      elements. By default YAML sequences are loaded into Perl arrays.

          a sequence:
              - one bourbon
              - one scotch
              - one beer

    scalar

      A scalar is a YAML node that is a single value. By default YAML
      scalars are loaded into Perl scalars.

          a scalar key: a scalar value

      YAML has many styles for representing scalars. This is important
      because varying data will have varying formatting requirements to
      retain the optimum human readability.

    plain scalar

      A plain scalar is unquoted. All plain scalars are automatic
      candidates for "implicit tagging". This means that their tag may be
      determined automatically by examination. The typical uses for this
      are plain alpha strings, integers, real numbers, dates, times and
      currency.

          - a plain string
          - -42
          - 3.1415
          - 12:34
          - 123 this is an error

    single quoted scalar

      This is similar to Perl's use of single quotes. It means no escaping
      except for single quotes which are escaped by using two adjacent
      single quotes.

          - 'When I say ''\n'' I mean "backslash en"'

    double quoted scalar

      This is similar to Perl's use of double quotes. Character escaping
      can be used.

          - "This scalar\nhas two lines, and a bell -->\a"

    folded scalar

      This is a multiline scalar which begins on the next line. It is
      indicated by a single right angle bracket. It is unescaped like the
      single quoted scalar. Line folding is also performed.

          - >
           This is a multiline scalar which begins on
           the next line. It is indicated by a single
           carat. It is unescaped like the single
           quoted scalar. Line folding is also
           performed.

    block scalar

      This final multiline form is akin to Perl's here-document except that
      (as in all YAML data) scope is indicated by indentation. Therefore,
      no ending marker is required. The data is verbatim. No line folding.

          - |
              QTY  DESC          PRICE  TOTAL
              ---  ----          -----  -----
                1  Foo Fighters  $19.95 $19.95
                2  Bar Belles    $29.95 $59.90

    parser

      A YAML processor has four stages: parse, load, dump, emit.

      A parser parses a YAML stream. YAML.pm's Load() function contains a
      parser.

    loader

      The other half of the Load() function is a loader. This takes the
      information from the parser and loads it into a Perl data structure.

    dumper

      The Dump() function consists of a dumper and an emitter. The dumper
      walks through each Perl data structure and gives info to the emitter.

    emitter

      The emitter takes info from the dumper and turns it into a YAML
      stream.

      NOTE: In YAML.pm the parserloader and the dumperemitter code are
      currently very closely tied together. In the future they may be
      broken into separate stages.

    For more information please refer to the immensely helpful YAML
    specification available at http://www.yaml.org/spec/.

YSH - THE YAML SHELL

    The YAML::Shell distribution provides script called 'ysh', the YAML
    shell. ysh provides a simple, interactive way to play with YAML. If you
    type in Perl code, it displays the result in YAML. If you type in YAML
    it turns it into Perl code.

    To run ysh, (assuming you installed it along with YAML.pm) simply type:

        ysh [options]

    Please read the ysh documentation for the full details. There are lots
    of options.

BUGS & DEFICIENCIES

    If you find a bug in YAML, please try to recreate it in the YAML Shell
    with logging turned on ('ysh -L'). When you have successfully
    reproduced the bug, please mail the LOG file to the author
    (ingy@cpan.org).

    WARNING: This is still ALPHA code. Well, most of this code has been
    around for years...

    BIGGER WARNING: YAML.pm has been slow in the making, but I am committed
    to having top notch YAML tools in the Perl world. The YAML team is
    close to finalizing the YAML 1.1 spec. This version of YAML.pm is based
    off of a very old pre 1.0 spec. In actuality there isn't a ton of
    difference, and this YAML.pm is still fairly useful. Things will get
    much better in the future.

RESOURCES

    http://lists.sourceforge.net/lists/listinfo/yaml-core is the mailing
    list. This is where the language is discussed and designed.

    http://www.yaml.org is the official YAML website.

    http://www.yaml.org/spec/ is the YAML 1.2 specification.

    http://yaml.kwiki.org is the official YAML wiki.

SEE ALSO

      * YAML::XS

AUTHOR

    Ingy döt Net <ingy@cpan.org>

COPYRIGHT AND LICENSE

    Copyright 2001-2020. Ingy döt Net.

    This program is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.

    See http://www.perl.com/perl/misc/Artistic.html