JOjorams·Aug 1, 2025
> If you assume that YAML 1.2 is the default, you don't need that nasty %YAML header.
Indeed the YAML 1.2 spec says a document without a YAML directive should be assumed to be 1.2[1]:
> A version 1.2 YAML processor must accept documents with an explicit “%YAML 1.2” directive, as well as documents lacking a “YAML” directive. Such documents are assumed to conform to the 1.2 version specification.
It's only the YAML 1.2 spec that says it's a superset of JSON. The YAML authors weren't aware of JSON when publishing version 1.1[2]:
> The YAML 1.1 specification was published in 2005. Around this time, the developers became aware of JSON. By sheer coincidence, JSON was almost a complete subset of YAML (both syntactically and semantically).
> In 2006, Kyrylo Simonov produced PyYAML and LibYAML. A lot of the YAML frameworks in various programming l
Read the rest
anguages are built over LibYAML and many others have looked to PyYAML as a solid reference for their implementations.
> The YAML 1.2 specification was published in 2009. Its primary focus was making YAML a strict superset of JSON. It also removed many of the problematic implicit typing recommendations.
The middle paragraph there is the reason this is a problem people keep running into: Most implementations are based on LibYAML, which is an implementation of YAML 1.1 that does not really support 1.2[3]. Indeed the last example from the post doesn't actually work for me on Ruby 3.4.4 with LibYAML 0.2.5. It produces the exact same output as the one before it.
[1]: https://yaml.org/spec/1.2.2/#681-yaml-directives
[2]: https://yaml.org/spec/1.2.2/#12-yaml-history
[3]: https://github.com/yaml/libyaml/issues/20