/usr/man/cat.3/YAML.3.Z(/usr/man/cat.3/YAML.3.Z)
NAME
YAML - YAML Ain't Markup Language (tm)
SYNOPSIS
use YAML;
# Load a YAML stream of 3 YAML documents into Perl data structures.
my ($hashref, $arrayref, $string) = Load(<<'...');
---
name: ingy
age: old
weight: heavy
# I should comment that I also like pink, but don't tell anybody.
favorite colors:
- red
- white
- blue
---
- Clark Evans
- Oren Ben-Kiki
- Brian Ingerson
--- >
You probably think YAML stands for "Yet Another Markup Language". It
ain't! YAML is really a data serialization language. But if you want
to think of it as a markup, that's OK with me. A lot of people try
to use XML as a serialization format.
"YAML" is catchy and fun to say. Try it. "YAML, YAML, YAML!!!"
...
# Dump the Perl data structures back into YAML.
print Dump($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 specifica-
tion.
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 program-
ming 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
any YAML serialization produced by Perl can be processed by Python,
and be guaranteed to return the data structure intact. (Even if it
contained Perl specific structures like GLOBs)
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 struc-
tures and deserialize them again without losing data relationships.
Although it is not 100% perfect (no serializer is or can be per-
fect), 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 type (or
"transfer method") which can cause that node to be interpreted in a
completely different manner. That's how YAML can support oddball
structures like Perl's typeglob.
USAGE
Exported Functions
The following functions are exported by YAML.pm by default when you use
YAML.pm like this:
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
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 contain-
ing 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:
--- #YAML:1.0
apple: good
banana: bad
cauliflower: ugly
--- #YAML:1.0
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(:all);
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;
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.
Dumper()
Alias to Dump(). For Data::Dumper fans.
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 serializa-
tion.
Exportable Function Groups
This is a list of the various groups of exported functions that you can
import using the following syntax:
use YAML ':groupname';
all Imports Dump(), Load(), DumpFile(), LoadFile(), Bless() and
Blessed().
POE Imports freeze() and thaw().
Storable
Imports freeze() and thaw().
Class Methods
YAML can also be used in an object oriented manner. At this point it
offers no real advantage. This interface will be improved in a later
release.
new()
New returns a new YAML object. For example:
my $y = YAML->new;
$y->Indent(4);
$y->dump($foo, $bar);
Object Methods
dump()
OO version of Dump().
load()
OO version of Load().
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:
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.
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 1. (true)
Tells YAML.pm whether to include the YAML version on the separa-
tor/header.
The canonical form is:
--- YAML:1.0
SortKeys
Default is 1. (true)
Tells YAML.pm whether or not to sort hash keys when storing a docu-
ment.
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.
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. Safe deserialization is one of the core goals of
YAML.
DumpCode
Determines if and how YAML.pm should serialize Perl code refer-
ences. By default YAML.pm will dump code references as dummy place-
holders (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 serializa-
tion (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 multi-
line 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 pro-
cessing 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 it's 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 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 bytes 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.
--- YAML:1.0
This: top level mapping
is:
- a
- YAML
- document
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.)
transfer method
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 transfer '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 key/value pairs. 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 ele-
ments. 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
This is a single line of unquoted text. All plain scalars are auto-
matic candidates for "implicit transferring". This means that their
type is determined automatically by examination. Unless they match
a set of predetermined YAML regex patterns, they will raise a
parser exception. 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 escap-
ing and no implicit transfer. It must be used on a single line.
- '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. There is no implicit transfer and it must still be
single line.
- "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 closing brace. It is unescaped like the sin-
gle 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 struc-
ture.
dumper
The Dump() function consists of a dumper and an emitter. The dumper
walks through each Perl data structure and gives info to the emit-
ter.
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. When libyaml is written (in
C) there will be a definite separation. libyaml will contain a
parser and emitter, and YAML.pm (and YAML.py etc) will supply the
loader and dumper.
For more information please refer to the immensely helpful YAML speci-
fication 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 ysh 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 repro-
duced the bug, please mail the LOG file to the author (ingy@cpan.org).
WARNING: This is *ALPHA* code.
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 code 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.
Circular Leaves
YAML is quite capable of serializing circular references. And for
the most part it can deserialize them correctly too. One notable
exception is a reference to a leaf node containing itself. This is
hard to do from pure Perl in any elegant way. The "canonical" exam-
ple is:
$foo = \$foo;
This serializes fine, but I can't parse it correctly yet. Unfortu-
nately, every wiseguy programmer in the world seems to try this
first when you ask them to test your serialization module. Even
though it is of almost no real world value. So please don't report
this bug unless you have a pure Perl patch to fix it for me.
By the way, similar non-leaf structures Dump and Load just fine:
$foo->[0] = $foo;
You can test these examples using 'ysh -r'. This option makes sure
that the example can be deserialized after it is serialized. We
call that "roundtripping", thus the '-r'.
Unicode
Unicode is not yet supported. The YAML specification dictates that
all strings be unicode, but this early implementation just uses
ASCII.
Structured Keys
Python, Java and perhaps others support using any data type as the
key to a hash. YAML also supports this. Perl5 only uses strings as
hash keys.
YAML.pm can currently parse structured keys, but their meaning gets
lost when they are loaded into a Perl hash. Consider this example
using the YAML Shell:
ysh > ---
yaml> ?
yaml> foo: bar
yaml> : baz
yaml> ...
$VAR1 = {
'HASH(0x1f1d20)' => 'baz'
};
ysh >
YAML.pm will need to be fixed to preserve these keys somehow. Why?
Because if YAML.pm gets a YAML document from YAML.py it must be
able to return it with the Python data intact.
Globs, Subroutines, Regexes and File Handles
As far as I know, other Perl serialization modules are not capable
of serializing and deserializing typeglobs, subroutines (code
refs), regexes and file handles. YAML.pm has dumping capabilities
for all of these. Loading them may produce wild results. Take
care.
NOTE: For a (huge) dump of Perl's global guts, try:
perl -MYAML -e '$YAML::UseCode=1; print Dump *::'
To limit this to a single namespace try:
perl -MCGI -MYAML -e '$YAML::UseCode=1; print Dump \%CGI::'
Speed
This is a pure Perl implementation that has been optimized for pro-
grammer readability, not for computational speed.
Oren Ben-Kiki and Clark Evans are currently developing libyaml, the
official C implementation of the YAML parser and emitter. YAML.pm
will be refactoring to use this library once it is stable. Other
languages like Python, Tcl, PHP, Ruby, JavaScript and Java can make
use of the same core library.
Please join us on the YAML mailing list if you are interested in
implementing something.
<https://lists.sourceforge.net/lists/listinfo/yaml-core>
Streaming Access
This module Dumps and Loads in one operation. There is no interface
for parsing or emitting a YAML stream one node at a time. It's all
or nothing.
An upcoming release will have support for incremental parsing and
dumping. Stay tuned.
RESOURCES
Please read YAML::Node for advanced YAML features.
<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.
YAML has been registered as a Source Forge project.
(<http://www.sourceforge.net>) Currently we are only using the mailing
list facilities there.
AUTHOR
Brian Ingerson <INGY@cpan.org> is resonsible for YAML.pm.
The YAML language is the result of a ton of collaboration between Oren
Ben-Kiki, Clark Evans and Brian Ingerson. Several others have added
help along the way.
COPYRIGHT
Copyright (c) 2001, 2002, 2005. Brian Ingerson. All rights reserved.
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 v5.8.6 2002-06-24 YAML(3)
See also YAML::Node(3): man 3 YAML::Node
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