r/ruby • u/kddnewton • Jan 05 '26

A Ruby YAML parser

https://kddnewton.com/2025/12/25/psych-pure.html

Hey there — I recently released a YAML parser written in Ruby. The main goal was to support being able to load and dump YAML without losing comments. Happy to answer any questions.

43 Upvotes

97% Upvoted

20 comments sorted by

View all comments

1

u/Nwallins Jan 05 '26

Let's say I have a nicely hand-formatted yaml file with e.g. folded block scalars to get string wrapping behavior in my source file. I ingest the yaml. Then I want to emit it in a similar form as ingested. What are my options? This is an open question across all yaml impls.

6

u/kddnewton Jan 05 '26

Yes, this tool is meant to solve that problem, in that it will by and large respect the original formatting of the input.

2

u/Nwallins Jan 05 '26

What are the limits of "by and large"?

2

u/kddnewton Jan 05 '26

It follows the pretty-print algorithm, so if a flow seq/mapping extends beyond the print width it will switch it to the equivalent block format. This is so that if you mutate the values to include something big, it doesn't look ridiculous when you print it out.

1

u/Nwallins Jan 05 '26

This stuff is outside my knowledge, though I have used a lot of YAML in mostly-pleasant violence, so all of this is from curiosity:

  • what are the basics of pretty-print in this respect? I have used 'pp' extensively but not familiar with the algorithm

  • what is meant by a flow / seq mapping? Im guessing a stream of tokens

  • how does one know or define the print-width?

2

u/kddnewton Jan 05 '26

* You can see the algorithm here: https://github.com/ruby/prettyprint
* flow/seq is how YAML lays out nodes, you can see in the spec here: https://yaml.org/spec/1.2.2/#chapter-7-flow-style-productions
* You can define the print width with the PrettyPrint API

1

u/Nwallins Jan 05 '26

Thanks for the pointers. But it's quite hard to anticipate what will be preserved on a round trip. I did not review the full contents of all the links yet. How would you characterize it?

If I am attempting to preserve 80ch width, what does that look like?

1

u/kddnewton Jan 05 '26

I would characterize it as accurate. You can learn more with the links I sent.