YAML(3) User Contributed Perl Documentation YAML(3)
NAME
YAML - YAML Ain't Markup Languagetm
VERSION
This document describes YAML version 1.24.
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, $string) = Load(<<'...');
---
name: ingy
age: old
weight: heavy
# I should comment that I also like pink, but don't tell anybody.
favorite colors:
- red
- green
- blue
---
- Clark Evans
- Oren Ben-Kiki
- Ingy dot Net
--- >
You probably think YAML stands for "Yet Another Markup Language". It
ain't! YAML is really a data serialization language. But if you want
to think of it as a markup, that's OK with me. A lot of people try
to use XML as a serialization format.
"YAML" is catchy and fun to say. Try it. "YAML, YAML, YAML!!!"
...
# Dump the Perl data structures back into YAML.
print Dump($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 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.
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.
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
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
The YAML::Shell distribution provides 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.
SEE ALSO
o YAML::XS
AUTHOR
Ingy dot Net <ingy AT cpan.org>
COPYRIGHT AND LICENSE
Copyright 2001-2017. Ingy dot 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 v5.26.3 2017-10-30 YAML(3)
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