++ed by:

3 PAUSE users

Tina Müller


YAML::PP - YAML 1.2 processor


WARNING: This is highly experimental.

Here are a few examples of what you can do right now:

    use YAML::PP;
    my $ypp = YAML::PP->new;

    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');

    # Legacy interface
    use YAML::PP qw/ Load Dump LoadFile DumpFile /;
    my @documents = Load($yaml);
    my @documents = LoadFile($filename);
    my $yaml = = Dump(@documents);
    DumpFile($filename, @documents);

Some utility scripts:

    # Load YAML into a data structure and dump with Data::Dumper
    yamlpp5-load < file.yaml

    # Load and Dump
    yamlpp5-load-dump < file.yaml

    # Print the events from the parser in yaml-test-suite format
    yamlpp5-events < file.yaml

    # Create ANSI colored YAML
    yamlpp5-highlight < file.yaml

    # Parse and emit events directly without loading
    yamlpp5-parse-emit < file.yaml


This is Yet Another YAML Framework. For why this project was started, see "WHY".

The parser aims to parse YAML 1.2. See http://yaml.org/.

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


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.


The Parser retrieves the tokens from the Lexer. The main YAML content is then parsed with the Grammar.


The Constructor creates a data structure from the Parser events.


The Loader combines the constructor and parser.


The Dumper will delegate to the Representer


The Representer will create Emitter events from the given data structure.


The Emitter creates a YAML stream.


Still TODO:

Flow Style

Flow style is partially implemented.

Not yet working: Implicit flow collection keys, implicit keys in flow sequences, content directly after the colon, empty nodes, explicit keys

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


The Constructor now supports all three YAML 1.2 Schemas, Failsafe, JSON and Core.

You can choose the Schema, however, the API for that is not yet fixed. Currently it looks like this:

    my $ypp = YAML::PP->new(schema => ['JSON']); # default is 'Core' for now

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.

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 are created as real numbers instead of strings, so that they are dumped correctly by modules like JSON::PP or JSON::XS, for example.

See "NUMBERS" for an 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).


    use YAML::PP;
    use JSON::PP;
    my $ypp = YAML::PP->new;
    my $coder = JSON::PP->new->ascii->pretty->allow_nonref->canonical;
    my $yaml = <<'EOM';
                a: 1
                c: 2
            : 23
        : 42
    my $data = $yppl->load_string($yaml);
    say $coder->encode($data);
       "complex" : {
          "{'{a => 1,c => 2}' => 23}" : 42

Tags are completely ignored.

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.

TODO: Correctly recognize numbers which also have a string flag, like:

    my $int = 23;
    say "int: $int"; # $int will now also have a PV flag

The layout is like libyaml output:

    - a
    - b
    - c
    - key1: 1
      key2: 2
      key3: 3
    - - a1
      - a2
    - - b1
      - b2


    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' );
    # Other values:
    # warn   - Just warn about them and replace with undef
    # ignore - replace with undef
    # allow  - Default
    my $ypp = YAML::PP->new(
        boolean => 'JSON::PP',
        schema => ['JSON'],
        cyclic_refs => 'fatal',
    my $doc = $ypp->load_string("foo: bar");
    my @docs = $ypp->load_string("foo: bar\n---\n- a");

Input should be utf-8 decoded.

    my $doc = $ypp->load_file("file.yaml");
    my @docs = $ypp->load_file("file.yaml");

UTF-8 decoding will be done automatically

    my $yaml = $ypp->dump_string($doc);
    my $yaml = $ypp->dump_string($doc1, $doc2);
    my $yaml = $ypp->dump_string(@docs);

Output will be UTF-8 decoded

    $ypp->dump_file("file.yaml", $doc);
    $ypp->dump_file("file.yaml", $doc1, $doc2);
    $ypp->dump_file("file.yaml", @docs);

File will be written UTF-8 encoded


Compare the output of the following YAML Loaders and JSON::PP dump:

    use JSON::PP;
    use Devel::Peek;

    use YAML::XS ();
    use YAML ();
        $YAML::Numify = 1; # since version 1.23
    use YAML::Syck ();
        $YAML::Syck::ImplicitTyping = 1;
    use YAML::Tiny ();
    use YAML::PP;

    my $yaml = "foo: 23";

    my $d1 = YAML::XS::Load($yaml);
    my $d2 = YAML::Load($yaml);
    my $d3 = YAML::Syck::Load($yaml);
    my $d4 = YAML::Tiny->read_string($yaml)->[0];
    my $d5 = YAML::PP->new->load_string($yaml);

    Dump $d1->{foo};
    Dump $d2->{foo};
    Dump $d3->{foo};
    Dump $d4->{foo};
    Dump $d5->{foo};

    say encode_json($d1);
    say encode_json($d2);
    say encode_json($d3);
    say encode_json($d4);
    say encode_json($d5);

    SV = PVIV(0x55bbaff2bae0) at 0x55bbaff26518
      REFCNT = 1
      IV = 23
      PV = 0x55bbb06e67a0 "23"\0
      CUR = 2
      LEN = 10
    SV = PVMG(0x55bbb08959b0) at 0x55bbb08fc6e8
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 23
      NV = 0
      PV = 0
    SV = IV(0x55bbaffcb3b0) at 0x55bbaffcb3c0
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 23
    SV = PVMG(0x55bbaff2f1f0) at 0x55bbb08fc8c8
      REFCNT = 1
      FLAGS = (POK,pPOK,UTF8)
      IV = 0
      NV = 0
      PV = 0x55bbb0909d00 "23"\0 [UTF8 "23"]
      CUR = 2
      LEN = 10
    SV = PVMG(0x55bbaff2f6d0) at 0x55bbb08b2c10
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 23
      NV = 0
      PV = 0



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 216, 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:


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 to get a quick overview of which frameworks support which YAML features:




It contains about 230 test cases and expected parsing events and more. There will be more tests coming. This test suite allows 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 seperate 'data' branch you will find the content in seperate 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.


This is a tool to play around with several YAML parsers and loaders in vim.


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.


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.


You can find the latest build at http://matrix.yaml.io

As of this writing, the test matrix only contains valid test cases. Invalid ones will be added.


Copyright 2017 by Tina Müller

This library is free software and may be distributed under the same terms as perl itself.