yaml.3

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.\" ========================================================================
.\"
.IX Title "YAML 3"
.TH YAML 3 "2007-09-27" "perl v5.10.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
YAML \- YAML Ain't Markup Language (tm)
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use YAML;
\&    
\&    # Load a YAML stream of 3 YAML documents into Perl data structures.
\&    my ($hashref, $arrayref, $string) = Load(<<\*(Aq...\*(Aq);
\&    \-\-\-
\&    name: ingy
\&    age: old
\&    weight: heavy
\&    # I should comment that I also like pink, but don\*(Aqt tell anybody.
\&    favorite colors:
\&        \- red
\&        \- green
\&        \- blue
\&    \-\-\-
\&    \- Clark Evans
\&    \- Oren Ben\-Kiki
\&    \- Ingy dA\*~Xt Net
\&    \-\-\- >
\&    You probably think YAML stands for "Yet Another Markup Language". It
\&    ain\*(Aqt! YAML is really a data serialization language. But if you want
\&    to think of it as a markup, that\*(Aqs OK with me. A lot of people try
\&    to use XML as a serialization format.
\&    
\&    "YAML" is catchy and fun to say. Try it. "YAML, YAML, YAML!!!"
\&    ...
\&    
\&    # Dump the Perl data structures back into YAML.
\&    print Dump($string, $arrayref, $hashref); 
\&    
\&    # YAML::Dump is used the same way you\*(Aqd use Data::Dumper::Dumper
\&    use Data::Dumper;
\&    print Dumper($string, $arrayref, $hashref);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \s-1YAML\s0.pm module implements a \s-1YAML\s0 Loader and Dumper based on the \s-1YAML\s0
1.0 specification. <http://www.yaml.org/spec/>
.PP
\&\s-1YAML\s0 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!!!)
.PP
For information on the \s-1YAML\s0 syntax, please refer to the \s-1YAML\s0
specification.
.SH "WHY YAML IS COOL"
.IX Header "WHY YAML IS COOL"
.IP "\s-1YAML\s0 is readable for people." 4
.IX Item "YAML is readable for people."
It makes clear sense out of complex data structures. You should find
that \s-1YAML\s0 is an exceptional data dumping tool. Structure is shown
through indentation, \s-1YAML\s0 supports recursive data, and hash keys are
sorted by default. In addition, \s-1YAML\s0 supports several styles of scalar
formatting for different types of data.
.IP "\s-1YAML\s0 is editable." 4
.IX Item "YAML is editable."
\&\s-1YAML\s0 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 \s-1XML\s0 or native Perl code?
.IP "\s-1YAML\s0 is multilingual." 4
.IX Item "YAML is multilingual."
Yes, \s-1YAML\s0 supports Unicode. But I'm actually referring to programming
languages. \s-1YAML\s0 was designed to meet the serialization needs of Perl,
Python, Ruby, Tcl, \s-1PHP\s0, Javascript and Java. It was also designed to be
interoperable between those languages. That means \s-1YAML\s0 serializations
produced by Perl can be processed by Python.
.IP "\s-1YAML\s0 is taint safe." 4
.IX Item "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 \f(CW\*(C`eval()\*(C'\fR built-in
to deserialize the data. Somebody could add a snippet of Perl to erase
your files.
.Sp
\&\s-1YAML\s0's parser does not need to eval anything.
.IP "\s-1YAML\s0 is full featured." 4
.IX Item "YAML is full featured."
\&\s-1YAML\s0 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.
.Sp
\&\s-1YAML\s0.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.
.IP "\s-1YAML\s0 is extensible." 4
.IX Item "YAML is extensible."
The \s-1YAML\s0 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 \s-1YAML\s0 node also supports a tagging mechanism (type
system) which can cause that node to be interpreted in a completely
different manner. That's how \s-1YAML\s0 can support object serialization and
oddball structures like Perl's typeglob.
.SH "YAML IMPLEMENTATIONS IN PERL"
.IX Header "YAML IMPLEMENTATIONS IN PERL"
This module, \s-1YAML\s0.pm, is really just the interface module for \s-1YAML\s0
modules written in Perl. The basic interface for \s-1YAML\s0 consists of two
functions: \f(CW\*(C`Dump\*(C'\fR and \f(CW\*(C`Load\*(C'\fR. The real work is done by the modules
YAML::Dumper and YAML::Loader.
.PP
Different \s-1YAML\s0 module distributions can be created by subclassing
\&\s-1YAML\s0.pm and YAML::Loader and YAML::Dumper. For example, YAML-Simple
consists of YAML::Simple YAML::Dumper::Simple and YAML::Loader::Simple.
.PP
Why would there be more than one implementation of \s-1YAML\s0? Well, despite
\&\s-1YAML\s0's offering of being a simple data format, \s-1YAML\s0 is actually very
deep and complex. Implementing the entirety of the \s-1YAML\s0 specification is
a daunting task.
.PP
For this reason I am currently working on 3 different \s-1YAML\s0 implementations.
.IP "\s-1YAML\s0" 4
.IX Item "YAML"
The main \s-1YAML\s0 distribution will keeping evolving to support the entire
\&\s-1YAML\s0 specification in pure Perl. This may not be the fastest or most
stable module though. Currently, \s-1YAML\s0.pm has lots of known bugs. It is
mostly a great tool for dumping Perl data structures to a readable form.
.IP "YAML::Lite" 4
.IX Item "YAML::Lite"
The point of YAML::Lite is to strip \s-1YAML\s0 down to the 90% that people
use most and offer that in a small, fast, stable, pure Perl form.
YAML::Lite will simply die when it is asked to do something it can't.
.IP "YAML::Syck" 4
.IX Item "YAML::Syck"
\&\f(CW\*(C`libsyck\*(C'\fR is the C based \s-1YAML\s0 processing library used by the Ruby
programming language (and also Python, \s-1PHP\s0 and Pugs). YAML::Syck is the
Perl binding to \f(CW\*(C`libsyck\*(C'\fR. It should be very fast, but may have
problems of its own. It will also require C compilation.
.Sp
\&\s-1NOTE:\s0 Audrey Tang has actually completed this module and it works great
      and is 10 times faster than \s-1YAML\s0.pm.
.PP
In the future, there will likely be even more \s-1YAML\s0 modules. Remember,
people other than Ingy are allowed to write \s-1YAML\s0 modules!
.SH "FUNCTIONAL USAGE"
.IX Header "FUNCTIONAL USAGE"
\&\s-1YAML\s0 is completely \s-1OO\s0 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 \s-1OO\s0 stuff for you. If you want direct access to the \s-1OO\s0 \s-1API\s0 see the
documentation for YAML::Dumper and YAML::Loader.
.Sh "Exported Functions"
.IX Subsection "Exported Functions"
The following functions are exported by \s-1YAML\s0.pm by default. The reason
they are exported is so that \s-1YAML\s0 works much like Data::Dumper. If you
don't want functions to be imported, just use \s-1YAML\s0 with an empty
import list:
.PP
.Vb 1
\&    use YAML ();
.Ve
.IP "Dump(list\-of\-Perl\-data\-structures)" 4
.IX Item "Dump(list-of-Perl-data-structures)"
Turn Perl data into \s-1YAML\s0. This function works very much like
\&\fIData::Dumper::Dumper()\fR. It takes a list of Perl data strucures and
dumps them into a serialized form. It returns a string containing the
\&\s-1YAML\s0 stream. The structures can be references or plain scalars.
.IP "Load(string\-containing\-a\-YAML\-stream)" 4
.IX Item "Load(string-containing-a-YAML-stream)"
Turn \s-1YAML\s0 into Perl data. This is the opposite of Dump. Just like
Storable's \fIthaw()\fR function or the \fIeval()\fR function in relation to
Data::Dumper. It parses a string containing a valid \s-1YAML\s0 stream into a
list of Perl data structures.
.Sh "Exportable Functions"
.IX Subsection "Exportable Functions"
These functions are not exported by default but you can request them in
an import list like this:
.PP
.Vb 1
\&    use YAML qw\*(Aqfreeze thaw Bless\*(Aq;
.Ve
.IP "\fIfreeze()\fR and \fIthaw()\fR" 4
.IX Item "freeze() and thaw()"
Aliases to \fIDump()\fR and \fILoad()\fR for Storable fans. This will also allow
\&\s-1YAML\s0.pm to be plugged directly into modules like \s-1POE\s0.pm, that use the
freeze/thaw \s-1API\s0 for internal serialization.
.IP "DumpFile(filepath, list)" 4
.IX Item "DumpFile(filepath, list)"
Writes the \s-1YAML\s0 stream to a file instead of just returning a string.
.IP "LoadFile(filepath)" 4
.IX Item "LoadFile(filepath)"
Reads the \s-1YAML\s0 stream from a file instead of a string.
.IP "Bless(perl\-node, [yaml\-node | class\-name])" 4
.IX Item "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
\&\fIyaml_dump()\fR function. A \fIyaml_dump()\fR 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
\&\fIBlessed()\fR function.
.Sp
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:
.Sp
.Vb 5
\&    use YAML qw(Dump Bless);
\&    $hash = {apple => \*(Aqgood\*(Aq, banana => \*(Aqbad\*(Aq, cauliflower => \*(Aqugly\*(Aq};
\&    print Dump $hash;
\&    Bless($hash)\->keys([\*(Aqbanana\*(Aq, \*(Aqapple\*(Aq]);
\&    print Dump $hash;
.Ve
.Sp
produces:
.Sp
.Vb 7
\&    \-\-\-
\&    apple: good
\&    banana: bad
\&    cauliflower: ugly
\&    \-\-\-
\&    banana: bad
\&    apple: good
.Ve
.Sp
Bless returns the tied part of a yaml-node, so that you can call the
YAML::Node methods. This is the same thing that \fIYAML::Node::ynode()\fR
returns. So another way to do the above example is:
.Sp
.Vb 8
\&    use YAML qw(Dump Bless);
\&    use YAML::Node;
\&    $hash = {apple => \*(Aqgood\*(Aq, banana => \*(Aqbad\*(Aq, cauliflower => \*(Aqugly\*(Aq};
\&    print Dump $hash;
\&    Bless($hash);
\&    $ynode = ynode(Blessed($hash));
\&    $ynode\->keys([\*(Aqbanana\*(Aq, \*(Aqapple\*(Aq]);
\&    print Dump $hash;
.Ve
.Sp
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.
.IP "Blessed(perl\-node)" 4
.IX Item "Blessed(perl-node)"
Returns the yaml node that a particular perl node is associated with
(see above). Returns undef if the node is not (\s-1YAML\s0) Blessed.
.SH "GLOBAL OPTIONS"
.IX Header "GLOBAL OPTIONS"
\&\s-1YAML\s0 options are set using a group of global variables in the \s-1YAML\s0
namespace. This is similar to how Data::Dumper works.
.PP
For example, to change the indentation width, do something like:
.PP
.Vb 1
\&    local $YAML::Indent = 3;
.Ve