Ingy dot Net - Port PyYAML to Perl

Title: Port PyYAML to Perl

Name: Ingy döt Net

Grant Manager: Adrian Howard

Duration: One to two months (Deadline: Christmas 2008)

Started: August, 2006

PyYAML(1) is a pure Python implementation of the YAML 1.1 specification(2). It is considered the highest quality YAML implementation to date. It has an excellent API (including streaming SAX-like support), a comprehensive test suite, full unicode support, very good tagging/type support, follows the spec precisely and is written in a style that makes it easy to port. It has already been ported to pure Ruby(3), and is considered by the YAML community to be the standard reference implementation. It would greatly benefit both the YAML and Perl communities to port this code to a Perl module.

Benefits to the Perl community:

  • A solid, dependable pure Perl implementation of YAML
  • Ability to share YAML reliably with Ruby and Python
  • A streaming interface as well as the Dump/Load API
  • Full support for YAML implicit types
  • More control for how YAML objects get serialized
  • Better error messages

YAML is currently used for many important Perl projects such as SVK(4), Plagger(5), Catalyst(6), Jifty(7) and Kwiki(8) to name a few. It is used to store CPAN metadata(9). Currently most of these use cases are small and simple config files.

YAML was designed to scale to much more elaborate use cases. For instance, the Perl 6(10) community, where it is used as a kind of Perl 6 bytecode(11) to transport a Pugs AST from Haskell to Perl 6, to Perl 5 and back.

For YAML to be used as a solid platform for flawless data transport, it needs rock solid implementations. This project will deliver that.


  • Port all the classes of PyYAML to Perl
  • Port all of the PyYAML tests and make them pass
  • Write full documentation for all the classes and the API
  • Release this module as, replacing the legacy code

Relevant History:

The YAML serialization project was started in May 2001 with the intent of bringing a simple yet complete, plain text, human friendly data serialization language to all programming languages. It was designed with an information module that very closely follows the Perl data model of potentially tagged (blessed) scalars, arrays and hashes with potentially cyclic references. This makes it an excellent choice for languages like Perl, Python, Ruby and PHP.

The language took several years to design, has a very complete specification document and has had two major versions, 1.0 and 1.1. The YAML language specification 1.1 has been fairly stable for over a year and a half now.

In late 2001, Brian Ingerson wrote the original Perl implementation. This is still the primary implementation used by the Perl world today. Although it is generally useful it has many problems. Its parser is very incomplete. It parses a subset of YAML 1.0. It generally can parse what it emits, but there are bugs in that regard too. Its type resolution is severely lacking. The code is rather poorly written, and lacks significantly in its test suite.

In early 2002, Brian talked coworker Steve Howell into writing an implementation for Python. Dubbed PyYaml, it was indeed written, but was generally considered even worse than the For years, Python has had very lacking YAML support.

Later in 2002, Why The Lucky Stiff began writing a Ruby implementation. The Ruby community rallied behind him, and YAML started showing up all over the place in Ruby projects. In 2003, Why decided to write a C implementation called Syck(13), aka libsyck. This library was written following the YAML 1.0 spec. It was amazingly fast, and was soon ported to Python, PHP and OCaml. Syck became the YAML codebase that is distributed the Ruby itself.

In 2004, the YAML spec went through some significant change, and was thus dubbed YAML 1.1. While the basic syntax remained the same, the tagging system and directive declaration was revamped.

In early 2006, Audrey Tang ported Syck to Perl as YAML::Syck(14). The code is very fast and offers improvement over, but it too has many problems. It is still YAML 1.0, with its own set of bugs. It doesn't produce good error messages. It ocassionally segfaults. This is almost entirely the fault of libsyck, not YAML::Syck. It also lacks most of the advanced features of, which allow more control of how objects get serialized.

In Spring of 2006, Kirill Simonov, began a project code named PyYAML 3000. It later simply took on the name PyYAML, replacing the old code base. This new Python code followed the spec in every way, and even exposed a few bugs in the spec. It was intended from the start to be a reference implementation, and is currently considered the best implementation available.

Also in Spring of 2006, Ola Bini ported PyYAML to Ruby, giving Ruby the same benefits that Python was now enjoying. This left Perl as the language with the worst YAML support. A sad thing.

In June 2006, Kirill Simonov was awarded a Google Summer of Code grant(15) to write a C implementation of YAML tools and bind them to Python. This code will eventually become libyaml, and will be bound to many languages, including Perl.

NOTE: My proposal does not include porting libyaml (which doesn't even exist yet) to Perl. Although when it is ready, I will likely distribute a binding of it with

Project details:

Most users of YAML and are only aware of the simple Dump/Load interface, but in reality there is a stack of processing that happens. PyYAML gives programming access to each of those layers of the YAML stack(16).

The Dump stack looks like this:

  • Dumper
  • Representer
  • Serializer
  • Emitter

The Load stack looks like this:

  • Loader
  • Constructor
  • Composer
  • Parser
  • Scanner

PyYAML has classes for all these processes. These classes will be ported as accurately as possible to Perl while taking defacto standard Perl coding styles into account.

The resulting module distribution will have no dependency modules/distributions if possible. It will likely use Test::Base for testing, but will be packaged using the Module::Install method that does not require Test::Base to be installed.

The current has a YAML::Node generic node class that is used as a meta object under certain circumstances, to acheive certain goals such as customized serialization. The new project will attempt to make this, and other details like this, backwards compatible with the current

This work is intended to be done in the open and will have a publically readable Subversion repository(17), a Trac, a mailing list(18) and an irc channel(19).

Ingy döt Net is a prolific Perl module author and one of the three people who designed the YAML serialization language. He wrote the original, which has served the community fairly well, but is severely lacking in many ways. Ingy is intimately aware of both the serialization needs of Perl and of the multilayered architecture of a properly written YAML implementation.

Note: I realize this proposal seemingly contradicts what I once said in my journal(20), but... what the heck!


  19. irc://