| YAML - YAML Ain't Markup Language™ |
YAML - YAML Ain't Markup Language™
This document describes the YAML manpage version 1.29.
This module has been released to CPAN as the YAML::Old manpage, 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 the YAML::XS manpage. 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.
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.
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.
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.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.
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 the YAML::Dumper manpage and
the YAML::Loader manpage.
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.
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!
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.
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)Load(string-containing-a-YAML-stream)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.
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()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.
LoadFile(filepath)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)
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:
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.
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.
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.
Objects with string overloading should honor the overloading and dump the stringification of themselves, rather than the actual object's guts.
Values that look like numbers (integers, floats) will be numified when loaded.
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.
Tells YAML.pm whether to include the YAML version on the separator/header.
--- %YAML:1.0
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.
eval() to parse
untrusted code is, well, untrustworthy.
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).
NOTE: YAML's block style is akin to Perl's here-document.
NOTE: YAML's folded style is akin to the way HTML folds text, except smarter.
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.
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.
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 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.
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
---
This: top level mapping
is:
- a
- YAML
- document
--- !recursive-sequence &001
- * 001
- * 001
- !perl/Foo::Bar
foo: 42
bar: stool
a mapping:
foo: bar
two: times two is 4
a sequence:
- one bourbon
- one scotch
- one beer
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.
- a plain string
- -42
- 3.1415
- 12:34
- 123 this is an error
- 'When I say ''\n'' I mean "backslash en"'
- "This scalar\nhas two lines, and a bell -->\a"
- >
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.
- |
QTY DESC PRICE TOTAL
--- ---- ----- -----
1 Foo Fighters $19.95 $19.95
2 Bar Belles $29.95 $59.90
A parser parses a YAML stream. YAML.pm's Load() function contains a parser.
Load() function is a loader. This takes the information
from the parser and loads it into a Perl data structure.
Dump() function consists of a dumper and an emitter. The dumper walks
through each Perl data structure and gives info to the emitter.
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/.
The the YAML::Shell manpage 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.
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.
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.
Ingy döt Net <ingy@cpan.org>
Copyright 2001-2019. 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
| YAML - YAML Ain't Markup Language™ |