new¶

    my $ypp = YAML::PP->new;
    
    # 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 => 'perl',
        schema => ['Core'],
        cyclic_refs => 'fatal',
        indent => 4,
        header => 1,
        footer => 0,
        version_directive => 0,
    );

Options:

boolean

Values: "perl" (currently default), "JSON::PP", "boolean", "perl_experimental"

This option is for loading and dumping.

In case of perl 5.36 and later, builtin booleans should work out of the box (since YAML::PP >= 0.38.0).

    print YAML::PP->new->dump_string([ builtin::true, !1 ]);
    # ---
    # - true
    # - false
    

For earlier perl versions, you can use "pseudo" booleans like documented in the following examples.

    # load/dump booleans via boolean.pm
    my $ypp = YAML::PP->new( boolean => 'boolean' );
    # load/dump booleans via JSON::PP::true/false
    my $ypp = YAML::PP->new( boolean => 'JSON::PP' );
    

You can also specify more than one class, comma separated. This is important for dumping.

    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
    boolean => ''
    Booleans will be loaded as perl booleans, but when dumping, nothing
    will be recognized as booleans.
    This option is for backwards compatibility for perl versions < 5.36,
    if you rely on [!!1, !1] being dumped as [1, ''].
    

The option "perl_experimental" was introduced when experimental boolean support was added to perl 5.36. Since it will not be experimental anymore in perl 5.40 \o/ the option is deprecated and the same as "perl".

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: "fatal" (since 0.037)

Before the default was "allow", but this can lead to memory leaks when loading on untrusted data, so it was changed to "fatal" by default.

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
    - >
      folded
    ---
    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.

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

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
    

This was implemented in 0.037.

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

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.

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>