Provided by: libyaml-perl_1.31-1_all 
NAME
YAML - YAML Ain't Markup Language™
VERSION
This document describes YAML version 1.31.
IMPORTANT - PLEASE READ THIS FIRST
If you need to use YAML with Perl, it is likely that you will have a look at this module ("YAML.pm")
first. There are several YAMLmodules <https://metacpan.org/search?q=yaml> in Perl and they all support
the simple "Load()" and "Dump()" API. Since this one has the obvious name "YAML", it may seem obvious to
pick this one.
As the author of this module, I humbly ask you to choose another. YAML.pm was the very first YAML
implementation in the world, released in 2001. It was originally made as a prototype, over 2 years before
the YAML 1.0 spec was published. Although it may work for your needs, it has numerous bugs and is barely
maintained.
Please consider using these first:
• YAML::PP - Pure Perl, Full Featured, Well Maintained
• YAML::PP::LibYAML - A "libyaml" Perl binding like YAML::XS but with the YAML::PP API.
The rest of this documentation is left unchanged...
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
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@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://www.yaml.org> is the official YAML website.
<http://www.yaml.org/spec/> is the YAML 1.2 specification.
SEE ALSO
• YAML::PP - This is almost certainly the YAML module you are looking for. It is full-featured and well
maintained.
• YAML::PP::LibYAML - Same overall API as YAML::PP but uses the libyaml shared library for speed.
AUTHOR
Ingy döt Net <ingy@cpan.org>
COPYRIGHT AND LICENSE
Copyright 2001-2023. 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>
perl v5.36.0 2024-01-01 YAML(3pm)