NAME¶
YAML - YAML Ain't Markup LanguageX
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, $string) = Load(<<'...');
---
name: ingy
age: old
weight: heavy
# I should comment that I also like pink, but don't tell anybody.
favorite colors:
- red
- green
- blue
---
- Clark Evans
- Oren Ben-Kiki
- Ingy doet Net
--- >
You probably think YAML stands for "Yet Another Markup Language". It
ain't! YAML is really a data serialization language. But if you want
to think of it as a markup, that's OK with me. A lot of people try
to use XML as a serialization format.
"YAML" is catchy and fun to say. Try it. "YAML, YAML, YAML!!!"
...
# Dump the Perl data structures back into YAML.
print Dump($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.
- 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.
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 serialization (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 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.
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 parser loader 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://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 1.0 specification.
<
http://yaml.kwiki.org> is the official YAML wiki.
SEE ALSO¶
See YAML::XS. Fast!
AUTHOR¶
Ingy doet Net <ingy@cpan.org>
is responsible for YAML.pm.
The YAML serialization language is the result of years of collaboration between
Oren Ben-Kiki, Clark Evans and Ingy doet Net. Several others have added help
along the way.
COPYRIGHT¶
Copyright 2001-2014. Ingy doet Net
Some parts copyright 2009 - 2010 Adam Kennedy
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>