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NAME
    YAML - YAML Ain't Markup Language (tm)

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.

If you don't care which YAML module use, as long as it's the best one
    installed on your system, use YAML::Any.

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
        age: old
        weight: heavy
        # I should comment that I also like pink, but don't tell anybody.
        favorite colors:
            - red
            - green
            - blue
        ---
        - Clark Evans
        - Oren Ben-Kiki
        - Ingy döt Net
        --- >
        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);

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 strucures 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.

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.

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.

DumpCode can also be set to a subroutine reference so that you can
        write your own serializing routine. YAML.pm passes you the code ref.
        You pass back the serialization (as a string) and a format
        indicator. The format indicator is a simple string like: 'deparse'
        or 'bytecode'.

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 can also be set to a subroutine reference so that you can
        write your own deserializing routine. YAML.pm passes the
        serialization (as a string) and a format indicator. You pass back
        the code reference.

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
        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.

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 stucture.
        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 parser/loader and the dumper/emitter 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 distribution ships with a 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 1.0 specification.

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