YAML(3) - phpMan

YAML(3pm) User Contributed Perl Documentation YAML(3pm)

NAME
YAML - YAML Ain't Markup Languagetm

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 dot 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
libyaml-shell-perl contains 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 AT 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.