Provided by: libyaml-pp-perl_0.034-1_all 
NAME
YAML::PP - YAML 1.2 processor
SYNOPSIS
WARNING: Most of the inner API is not stable yet.
Here are a few examples of the basic load and dump methods:
use YAML::PP;
my $ypp = YAML::PP->new;
my $yaml = <<'EOM';
--- # Document one is a mapping
name: Tina
age: 29
favourite language: Perl
--- # Document two is a sequence
- plain string
- 'in single quotes'
- "in double quotes we have escapes! like \t and \n"
- | # a literal block scalar
line1
line2
- > # a folded block scalar
this is all one
single line because the
linebreaks will be folded
EOM
my @documents = $ypp->load_string($yaml);
my @documents = $ypp->load_file($filename);
my $yaml = $ypp->dump_string($data1, $data2);
$ypp->dump_file($filename, $data1, $data2);
# The loader offers JSON::PP::Boolean, boolean.pm or
# perl 1/'' (currently default) for booleans
my $ypp = YAML::PP->new(boolean => 'JSON::PP');
my $ypp = YAML::PP->new(boolean => 'boolean');
my $ypp = YAML::PP->new(boolean => 'perl');
# Enable perl data types and objects
my $ypp = YAML::PP->new(schema => [qw/ + Perl /]);
my $yaml = $yp->dump_string($data_with_perl_objects);
# Legacy interface
use YAML::PP qw/ Load Dump LoadFile DumpFile /;
my @documents = Load($yaml);
my @documents = LoadFile($filename);
my @documents = LoadFile($filehandle);
my $yaml = = Dump(@documents);
DumpFile($filename, @documents);
DumpFile($filenhandle @documents);
Some utility scripts, mostly useful for debugging:
# Load YAML into a data structure and dump with Data::Dumper
yamlpp-load < file.yaml
# Load and Dump
yamlpp-load-dump < file.yaml
# Print the events from the parser in yaml-test-suite format
yamlpp-events < file.yaml
# Parse and emit events directly without loading
yamlpp-parse-emit < file.yaml
# Create ANSI colored YAML. Can also be useful for invalid YAML, showing
# you the exact location of the error
yamlpp-highlight < file.yaml
DESCRIPTION
YAML::PP is a modular YAML processor.
It aims to support "YAML 1.2" and "YAML 1.1". See <https://yaml.org/>. Some (rare) syntax
elements are not yet supported and documented below.
YAML is a serialization language. The YAML input is called "YAML Stream". A stream
consists of one or more "Documents", separated by a line with a document start marker
"---". A document optionally ends with the document end marker "...".
This allows one to process continuous streams additionally to a fixed input file or
string.
The YAML::PP frontend will currently load all documents, and return only the first if
called with scalar context.
The YAML backend is implemented in a modular way that allows one to add custom handling of
YAML tags, perl objects and data types. The inner API is not yet stable. Suggestions
welcome.
You can check out all current parse and load results from the yaml-test-suite here:
<https://perlpunk.github.io/YAML-PP-p5/test-suite.html>
METHODS
new
my $ypp = YAML::PP->new;
# load booleans via boolean.pm
my $ypp = YAML::PP->new( boolean => 'boolean' );
# load booleans via JSON::PP::true/false
my $ypp = YAML::PP->new( boolean => 'JSON::PP' );
# use YAML 1.2 Failsafe Schema
my $ypp = YAML::PP->new( schema => ['Failsafe'] );
# use YAML 1.2 JSON Schema
my $ypp = YAML::PP->new( schema => ['JSON'] );
# use YAML 1.2 Core Schema
my $ypp = YAML::PP->new( schema => ['Core'] );
# Die when detecting cyclic references
my $ypp = YAML::PP->new( cyclic_refs => 'fatal' );
my $ypp = YAML::PP->new(
boolean => 'JSON::PP',
schema => ['Core'],
cyclic_refs => 'fatal',
indent => 4,
header => 1,
footer => 1,
version_directive => 1,
);
Options:
boolean
Values: "perl" (currently default), "JSON::PP", "boolean", "perl_experimental"
This option is for loading and dumping.
You can also specify more than one class, comma separated. This is important for
dumping.
Examples:
boolean => 'JSON::PP,boolean'
Booleans will be loaded as JSON::PP::Booleans, but when dumping, also
'boolean' objects will be recognized
boolean => 'JSON::PP,*'
Booleans will be loaded as JSON::PP::Booleans, but when dumping, all
currently supported boolean classes will be recognized
boolean => '*'
Booleans will be loaded as perl booleans, but when dumping, all
currently supported boolean classes will be recognized
If you have perl >= 5.36 then you might want to try out the experimental boolean
support, see builtin.
YAML::PP supports that by using the "perl_experimental" value for the boolean option.
Rules are the same as for the experimental builtin class: It's not guaranteed to work
in the future.
As soon as the builtin boolean support leaves experimental status, I will update
YAML::PP to support this via the default "perl" value.
boolean => 'perl_experimental'
Booleans will be loaded as perl booleans, and they will be recognized
as such when dumping also
schema
Default: "['Core']"
This option is for loading and dumping.
Array reference. Here you can define what schema to use. Supported standard Schemas
are: "Failsafe", "JSON", "Core", "YAML1_1".
To get an overview how the different Schemas behave, see
<https://perlpunk.github.io/YAML-PP-p5/schemas.html>
Additionally you can add further schemas, for example "Merge".
cyclic_refs
Default: 'allow' but will be switched to fatal in the future for safety!
This option is for loading only.
Defines what to do when a cyclic reference is detected when loading.
# fatal - die
# warn - Just warn about them and replace with undef
# ignore - replace with undef
# allow - Default
duplicate_keys
Default: 0
Since version 0.027
This option is for loading.
The YAML Spec says duplicate mapping keys should be forbidden.
When set to true, duplicate keys in mappings are allowed (and will overwrite the
previous key).
When set to false, duplicate keys will result in an error when loading.
This is especially useful when you have a longer mapping and don't see the duplicate
key in your editor:
---
a: 1
b: 2
# .............
a: 23 # error
indent
Default: 2
This option is for dumping.
Use that many spaces for indenting
width
Since version 0.025
Default: 80
This option is for dumping.
Maximum columns when dumping.
This is only respected when dumping flow collections right now.
in the future it will be used also for wrapping long strings.
header
Default: 1
This option is for dumping.
Print document header "---"
footer
Default: 0
This option is for dumping.
Print document footer "..."
yaml_version
Since version 0.020
This option is for loading and dumping.
Default: 1.2
Note that in this case, a directive "%YAML 1.1" will basically be ignored and
everything loaded with the "1.2 Core" Schema.
If you want to support both YAML 1.1 and 1.2, you have to specify that, and the schema
("Core" or "YAML1_1") will be chosen automatically.
my $yp = YAML::PP->new(
yaml_version => ['1.2', '1.1'],
);
This is the same as
my $yp = YAML::PP->new(
schema => ['+'],
yaml_version => ['1.2', '1.1'],
);
because the "+" stands for the default schema per version.
When loading, and there is no %YAML directive, 1.2 will be considered as default, and
the "Core" schema will be used.
If there is a "%YAML 1.1" directive, the "YAML1_1" schema will be used.
Of course, you can also make 1.1 the default:
my $yp = YAML::PP->new(
yaml_version => ['1.1', '1.2'],
);
You can also specify 1.1 only:
my $yp = YAML::PP->new(
yaml_version => ['1.1'],
);
In this case also documents with "%YAML 1.2" will be loaded with the "YAML1_1" schema.
version_directive
Since version 0.020
This option is for dumping.
Default: 0
Print Version Directive "%YAML 1.2" (or "%YAML 1.1") on top of each YAML document. It
will use the first version specified in the "yaml_version" option.
preserve
Since version 0.021
Default: false
This option is for loading and dumping.
Preserving scalar styles is still experimental.
use YAML::PP::Common qw/ :PRESERVE /;
# Preserve the order of hash keys
my $yp = YAML::PP->new( preserve => PRESERVE_ORDER );
# Preserve the quoting style of scalars
my $yp = YAML::PP->new( preserve => PRESERVE_SCALAR_STYLE );
# Preserve block/flow style (since 0.024)
my $yp = YAML::PP->new( preserve => PRESERVE_FLOW_STYLE );
# Preserve alias names (since 0.027)
my $yp = YAML::PP->new( preserve => PRESERVE_ALIAS );
# Combine, e.g. preserve order and scalar style
my $yp = YAML::PP->new( preserve => PRESERVE_ORDER | PRESERVE_SCALAR_STYLE );
Do NOT rely on the internal implementation of it.
If you load the following input:
---
z: 1
a: 2
---
- plain
- 'single'
- "double"
- |
literal
---
block mapping: &alias
flow sequence: [a, b]
same mapping: *alias
flow mapping: {a: b}
with this code:
my $yp = YAML::PP->new(
preserve => PRESERVE_ORDER | PRESERVE_SCALAR_STYLE
| PRESERVE_FLOW_STYLE | PRESERVE_ALIAS
);
my ($hash, $styles, $flow) = $yp->load_file($file);
$yp->dump_file($hash, $styles, $flow);
Then dumping it will return the same output. Only folded block scalars '>' cannot
preserve the style yet.
Note that YAML allows repeated definition of anchors. They cannot be preserved with
YAML::PP right now. Example:
---
- &seq [a]
- *seq
- &seq [b]
- *seq
Because the data could be shuffled before dumping again, the anchor definition could
be broken. In this case repeated anchor names will be discarded when loading and
dumped with numeric anchors like usual.
Implementation:
When loading, hashes will be tied to an internal class ("YAML::PP::Preserve::Hash")
that keeps the key order.
Scalars will be returned as objects of an internal class
("YAML::PP::Preserve::Scalar") with overloading. If you assign to such a scalar, the
object will be replaced by a simple scalar.
# assignment, style gets lost
$styles->[1] .= ' append';
You can also pass 1 as a value. In this case all preserving options will be enabled,
also if there are new options added in the future.
There are also methods to create preserved nodes from scratch. See the
"preserved_(scalar|mapping|sequence)" "METHODS" below.
load_string
my $doc = $ypp->load_string("foo: bar");
my @docs = $ypp->load_string("foo: bar\n---\n- a");
Input should be Unicode characters.
So if you read from a file, you should decode it, for example with "Encode::decode()".
Note that in scalar context, "load_string" and "load_file" return the first document (like
YAML::Syck), while YAML and YAML::XS return the last.
load_file
my $doc = $ypp->load_file("file.yaml");
my @docs = $ypp->load_file("file.yaml");
Strings will be loaded as unicode characters.
dump_string
my $yaml = $ypp->dump_string($doc);
my $yaml = $ypp->dump_string($doc1, $doc2);
my $yaml = $ypp->dump_string(@docs);
Input strings should be Unicode characters.
Output will return Unicode characters.
So if you want to write that to a file (or pass to YAML::XS, for example), you typically
encode it via "Encode::encode()".
dump_file
$ypp->dump_file("file.yaml", $doc);
$ypp->dump_file("file.yaml", $doc1, $doc2);
$ypp->dump_file("file.yaml", @docs);
Input data should be Unicode characters.
dump
This will dump to a predefined writer. By default it will just use the YAML::PP::Writer
and output a string.
my $writer = MyWriter->new(\my $output);
my $yp = YAML::PP->new(
writer => $writer,
);
$yp->dump($data);
preserved_scalar
Since version 0.024
Experimental. Please report bugs or let me know this is useful and works.
You can define a certain scalar style when dumping data. Figuring out the best style is a
hard task and practically impossible to get it right for all cases. It's also a matter of
taste.
use YAML::PP::Common qw/ PRESERVE_SCALAR_STYLE YAML_LITERAL_SCALAR_STYLE /;
my $yp = YAML::PP->new(
preserve => PRESERVE_SCALAR_STYLE,
);
# a single linebreak would normally be dumped with double quotes: "\n"
my $scalar = $yp->preserved_scalar("\n", style => YAML_LITERAL_SCALAR_STYLE );
my $data = { literal => $scalar };
my $dump = $yp->dump_string($data);
# output
---
literal: |+
...
preserved_mapping, preserved_sequence
Since version 0.024
Experimental. Please report bugs or let me know this is useful and works.
With this you can define which nodes are dumped with the more compact flow style instead
of block style.
If you add "PRESERVE_ORDER" to the "preserve" option, it will also keep the order of the
keys in a hash.
use YAML::PP::Common qw/
PRESERVE_ORDER PRESERVE_FLOW_STYLE
YAML_FLOW_MAPPING_STYLE YAML_FLOW_SEQUENCE_STYLE
/;
my $yp = YAML::PP->new(
preserve => PRESERVE_FLOW_STYLE | PRESERVE_ORDER
);
my $hash = $yp->preserved_mapping({}, style => YAML_FLOW_MAPPING_STYLE);
# Add values after initialization to preserve order
%$hash = (z => 1, a => 2, y => 3, b => 4);
my $array = $yp->preserved_sequence([23, 24], style => YAML_FLOW_SEQUENCE_STYLE);
my $data = $yp->preserved_mapping({});
%$data = ( map => $hash, seq => $array );
my $dump = $yp->dump_string($data);
# output
---
map: {z: 1, a: 2, y: 3, b: 4}
seq: [23, 24]
loader
Returns or sets the loader object, by default YAML::PP::Loader
dumper
Returns or sets the dumper object, by default YAML::PP::Dumper
schema
Returns or sets the schema object
default_schema
Creates and returns the default schema
FUNCTIONS
The functions "Load", "LoadFile", "Dump" and "DumpFile" are provided as a drop-in
replacement for other existing YAML processors. No function is exported by default.
Note that in scalar context, "Load" and "LoadFile" return the first document (like
YAML::Syck), while YAML and YAML::XS return the last.
Load
use YAML::PP qw/ Load /;
my $doc = Load($yaml);
my @docs = Load($yaml);
Works like "load_string".
LoadFile
use YAML::PP qw/ LoadFile /;
my $doc = LoadFile($file);
my @docs = LoadFile($file);
my @docs = LoadFile($filehandle);
Works like "load_file".
Dump
use YAML::PP qw/ Dump /;
my $yaml = Dump($doc);
my $yaml = Dump(@docs);
Works like "dump_string".
DumpFile
use YAML::PP qw/ DumpFile /;
DumpFile($file, $doc);
DumpFile($file, @docs);
DumpFile($filehandle, @docs);
Works like "dump_file".
PLUGINS
You can alter the behaviour of YAML::PP by using the following schema classes:
YAML::PP::Schema::Failsafe
One of the three YAML 1.2 official schemas
YAML::PP::Schema::JSON
One of the three YAML 1.2 official schemas.
YAML::PP::Schema::Core
One of the three YAML 1.2 official schemas. Default
YAML::PP::Schema::YAML1_1
Schema implementing the most common YAML 1.1 types
YAML::PP::Schema::Perl
Serializing Perl objects and types
YAML::PP::Schema::Binary
Serializing binary data
YAML::PP::Schema::Tie::IxHash
Deprecated. See option "preserve"
YAML::PP::Schema::Merge
YAML 1.1 merge keys for mappings
YAML::PP::Schema::Include
Include other YAML files via "!include" tags
To make the parsing process faster, you can plugin the libyaml parser with
YAML::PP::LibYAML.
IMPLEMENTATION
The process of loading and dumping is split into the following steps:
Load:
YAML Stream Tokens Event List Data Structure
---------> ---------> --------->
lex parse construct
Dump:
Data Structure Event List YAML Stream
---------> --------->
represent emit
You can dump basic perl types like hashes, arrays, scalars (strings, numbers). For
dumping blessed objects and things like coderefs have a look at
YAML::PP::Perl/YAML::PP::Schema::Perl.
YAML::PP::Lexer
The Lexer is reading the YAML stream into tokens. This makes it possible to generate
syntax highlighted YAML output.
Note that the API to retrieve the tokens will change.
YAML::PP::Parser
The Parser retrieves the tokens from the Lexer. The main YAML content is then parsed
with the Grammar.
YAML::PP::Grammar
YAML::PP::Constructor
The Constructor creates a data structure from the Parser events.
YAML::PP::Loader
The Loader combines the constructor and parser.
YAML::PP::Dumper
The Dumper will delegate to the Representer
YAML::PP::Representer
The Representer will create Emitter events from the given data structure.
YAML::PP::Emitter
The Emitter creates a YAML stream.
YAML::PP::Parser
Still TODO:
Implicit collection keys
---
[ a, b, c ]: value
Implicit mapping in flow style sequences
This is supported since 0.029 (except some not relevant cases):
---
[ a, b, c: d ]
# equals
[ a, b, { c: d } ]
Plain mapping keys ending with colons
---
key ends with two colons::: value
Supported Characters
If you have valid YAML that's not parsed, or the other way round, please create an
issue.
Line and Column Numbers
You will see line and column numbers in the error message. The column numbers might
still be wrong in some cases.
Error Messages
The error messages need to be improved.
Unicode Surrogate Pairs
Currently loaded as single characters without validating
Possibly more
YAML::PP::Constructor
The Constructor now supports all three YAML 1.2 Schemas, Failsafe, JSON and Core.
Additionally you can choose the schema for YAML 1.1 as "YAML1_1".
Too see what strings are resolved as booleans, numbers, null etc. look at
<https://perlpunk.github.io/YAML-PP-p5/schema-examples.html>.
You can choose the Schema like this:
my $ypp = YAML::PP->new(schema => ['JSON']); # default is 'Core'
The Tags "!!seq" and "!!map" are still ignored for now.
It supports:
Handling of Anchors/Aliases
Like in modules like YAML, the Constructor will use references for mappings and
sequences, but obviously not for scalars.
YAML::XS uses real aliases, which allows also aliasing scalars. I might add an option
for that since aliasing is now available in pure perl.
Boolean Handling
You can choose between 'perl' (1/'', currently default), 'JSON::PP' and 'boolean'.pm
for handling boolean types. That allows you to dump the data structure with one of
the JSON modules without losing information about booleans.
Numbers
Numbers are created as real numbers instead of strings, so that they are dumped
correctly by modules like JSON::PP or JSON::XS, for example.
Complex Keys
Mapping Keys in YAML can be more than just scalars. Of course, you can't load that
into a native perl structure. The Constructor will stringify those keys with
Data::Dumper instead of just returning something like "HASH(0x55dc1b5d0178)".
Example:
use YAML::PP;
use JSON::PP;
my $ypp = YAML::PP->new;
my $coder = JSON::PP->new->ascii->pretty->allow_nonref->canonical;
my $yaml = <<'EOM';
complex:
?
?
a: 1
c: 2
: 23
: 42
EOM
my $data = $yppl->load_string($yaml);
say $coder->encode($data);
__END__
{
"complex" : {
"{'{a => 1,c => 2}' => 23}" : 42
}
}
TODO:
Parse Tree
I would like to generate a complete parse tree, that allows you to manipulate the data
structure and also dump it, including all whitespaces and comments. The spec says
that this is throwaway content, but I read that many people wish to be able to keep
the comments.
YAML::PP::Dumper, YAML::PP::Emitter
The Dumper should be able to dump strings correctly, adding quotes whenever a plain scalar
would look like a special string, like "true", or when it contains or starts with
characters that are not allowed.
Most strings will be dumped as plain scalars without quotes. If they contain special
characters or have a special meaning, they will be dumped with single quotes. If they
contain control characters, including <"\n">, they will be dumped with double quotes.
It will recognize JSON::PP::Boolean and boolean.pm objects and dump them correctly.
Numbers which also have a "PV" flag will be recognized as numbers and not as strings:
my $int = 23;
say "int: $int"; # $int will now also have a PV flag
That means that if you accidentally use a string in numeric context, it will also be
recognized as a number:
my $string = "23";
my $something = $string + 0;
print $yp->dump_string($string);
# will be emitted as an integer without quotes!
The layout is like libyaml output:
key:
- a
- b
- c
---
- key1: 1
key2: 2
key3: 3
---
- - a1
- a2
- - b1
- b2
WHY
All the available parsers and loaders for Perl are behaving differently, and more
important, aren't conforming to the spec. YAML::XS is doing pretty well, but "libyaml"
only handles YAML 1.1 and diverges a bit from the spec. The pure perl loaders lack support
for a number of features.
I was going over YAML.pm issues end of 2016, integrating old patches from rt.cpan.org and
creating some pull requests myself. I realized that it would be difficult to patch YAML.pm
to parse YAML 1.1 or even 1.2, and it would also break existing usages relying on the
current behaviour.
In 2016 Ingy döt Net initiated two really cool projects:
"YAML TEST SUITE"
"YAML EDITOR"
These projects are a big help for any developer. So I got the idea to write my own parser
and started on New Year's Day 2017. Without the test suite and the editor I would have
never started this.
I also started another YAML Test project which allows one to get a quick overview of which
frameworks support which YAML features:
"YAML TEST MATRIX"
YAML TEST SUITE
<https://github.com/yaml/yaml-test-suite>
It contains almost 400 test cases and expected parsing events and more. There will be
more tests coming. This test suite allows you to write parsers without turning the
examples from the Specification into tests yourself. Also the examples aren't completely
covering all cases - the test suite aims to do that.
The suite contains .tml files, and in a separate 'data' release you will find the content
in separate files, if you can't or don't want to use TestML.
Thanks also to Felix Krause, who is writing a YAML parser in Nim. He turned all the spec
examples into test cases.
YAML EDITOR
This is a tool to play around with several YAML parsers and loaders in vim.
<https://github.com/yaml/yaml-editor>
The project contains the code to build the frameworks (16 as of this writing) and put it
into one big Docker image.
It also contains the yaml-editor itself, which will start a vim in the docker container.
It uses a lot of funky vimscript that makes playing with it easy and useful. You can
choose which frameworks you want to test and see the output in a grid of vim windows.
Especially when writing a parser it is extremely helpful to have all the test cases and be
able to play around with your own examples to see how they are handled.
YAML TEST MATRIX
I was curious to see how the different frameworks handle the test cases, so, using the
test suite and the docker image, I wrote some code that runs the tests, manipulates the
output to compare it with the expected output, and created a matrix view.
<https://github.com/perlpunk/yaml-test-matrix>
You can find the latest build at <https://matrix.yaml.info>
CONTRIBUTORS
Ingy döt Net
Ingy is one of the creators of YAML. In 2016 he started the YAML Test Suite and the
YAML Editor. He also made useful suggestions on the class hierarchy of YAML::PP.
Felix "flyx" Krause
Felix answered countless questions about the YAML Specification.
SEE ALSO
YAML
YAML::XS
YAML::Syck
YAML::Tiny
YAML::PP::LibYAML
YAML::LibYAML::API
<https://www.yaml.info>
SPONSORS
The Perl Foundation <https://www.perlfoundation.org/> sponsored this project (and the YAML
Test Suite) with a grant of 2500 USD in 2017-2018.
COPYRIGHT AND LICENSE
Copyright 2017-2022 by Tina Müller
This library is free software and may be distributed under the same terms as perl itself.