📍 NOTE
RubyGems (the GitHub org, not the website) suffered a hostile takeover in September 2025.
Ultimately 4 maintainers were hard removed and a reason has been given for only 1 of those, while 2 others resigned in protest.
It is a complicated story which is difficult to parse quickly.
Simply put - there was active policy for adding or removing maintainers/owners of rubygems and bundler, and those policies were not followed.
I'm adding notes like this to gems because I don't condone theft of repositories or gems from their rightful owners.
If a similar theft happened with my repos/gems, I'd hope some would stand up for me.
Disenfranchised former-maintainers have started gem.coop.
Once available I will publish there exclusively; unless RubyCentral makes amends with the community.
The "Technology for Humans: Joel Draper" podcast episode by reinteractive is the most cogent summary I'm aware of.
See here, here and here for more info on what comes next.
What I'm doing: A (WIP) proposal for bundler/gem scopes, and a (WIP) proposal for a federated gem server.

☯️ Psych::Merge

if ci_badges.map(&:color).detect { it != "green"} ☝️ let me know, as I may have missed the discord notification.

---

if ci_badges.map(&:color).all? { it == "green"} 👇️ send money so I can do more of this. FLOSS maintenance is now my full-time job.

🌻 Synopsis

Psych::Merge is a standalone Ruby module that intelligently merges two versions of a YAML file using Psych AST analysis. It's like a smart "git merge" specifically designed for YAML configuration files. Built on top of ast-merge, it shares the same architecture as prism-merge for Ruby source files.

Key Features

• Psych-Powered: Uses Ruby's built-in Psych parser for YAML AST analysis
• YAML-Aware: Understands YAML structure including mappings, sequences, and scalars
• Intelligent: Matches nodes by structural signatures
• Fuzzy Key Matching: MappingMatchRefiner matches similar keys (e.g., database_url ↔ db_url) using Levenshtein distance for typos and naming convention differences
• Comment-Preserving: Comments are preserved in their context
• Freeze Block Support: Respects freeze markers (default: psych-merge:freeze / psych-merge:unfreeze) for merge control - customizable to match your project's conventions
• Full Provenance: Tracks origin of every node
• Standalone: Minimal dependencies - just ast-merge and Ruby's built-in psych
• Customizable:
  • signature_generator - callable custom signature generators
  • preference - setting of :template, :destination, or a Hash for per-node-type preferences
  • node_splitter - Hash mapping node types to callables for per-node-type merge customization (see ast-merge docs)
  • add_template_only_nodes - setting to retain nodes that do not exist in destination
  • freeze_token - customize freeze block markers (default: "psych-merge")
  • match_refiners - array of refiners for fuzzy matching (e.g., MappingMatchRefiner)

Supported Node Types

Node Type	Signature Format	Matching Behavior
Mapping	[:mapping, key_signatures...]	Mappings match by their key structure
Sequence	[:sequence, element_count]	Sequences match by position and type
Scalar	[:scalar, value]	Scalars match by value
Alias	[:alias, anchor]	Aliases match by anchor name

Example

require "psych/merge"

template = File.read("template.yml")
destination = File.read("destination.yml")

merger = Psych::Merge::SmartMerger.new(template, destination)
result = merger.merge

File.write("merged.yml", result.to_yaml)

The *-merge Gem Family

This gem is part of a family of gems that provide intelligent merging for various file formats:

Gem	Format	Parser	Description
ast-merge	Text	internal	Shared infrastructure for all *-merge gems
prism-merge	Ruby	Prism	Smart merge for Ruby source files
psych-merge	YAML	Psych	Smart merge for YAML files
json-merge	JSON	tree-sitter-json	Smart merge for JSON files
jsonc-merge	JSONC	tree-sitter-jsonc	⚠️ Proof of concept; Smart merge for JSON with Comments
bash-merge	Bash	tree-sitter-bash	Smart merge for Bash scripts
rbs-merge	RBS	RBS	Smart merge for Ruby type signatures
dotenv-merge	Dotenv	internal (dotenv)	Smart merge for .env files
toml-merge	TOML	tree-sitter-toml	Smart merge for TOML files
markdown-merge	Markdown	base classes	Shared foundation for Markdown mergers
markly-merge	Markdown	Markly	Smart merge for Markdown (CommonMark via libcmark-gfm)
commonmarker-merge	Markdown	Commonmarker	Smart merge for Markdown (CommonMark via comrak)

Example implementations for the gem templating use case:

Gem	Purpose	Description
kettle-dev	Gem Development	Gem templating tool using *-merge gems
kettle-jem	Gem Templating	Gem template library with smart merge support

💡 Info you can shake a stick at

Tokens to Remember
Works with JRuby
Works with Truffle Ruby
Works with MRI Ruby 3
Support & Community
Source
Documentation
Compliance
Style
Maintainer 🎖️
... 💖	🧊 🐙 🛖 🧪

Compatibility

Compatible with MRI Ruby 3.2.0+, and concordant releases of JRuby, and TruffleRuby.

🚚 Amazing test matrix was brought to you by	🔎 appraisal2 🔎 and the color 💚 green 💚
👟 Check it out!	✨ github.com/appraisal-rb/appraisal2 ✨

Federated DVCS

Find this repo on federated forges (Coming soon!)

Federated DVCS Repository	Status	Issues	PRs	Wiki	CI	Discussions
🧪 kettle-rb/psych-merge on GitLab	The Truth	💚	💚	💚	🐭 Tiny Matrix	➖
🧊 kettle-rb/psych-merge on CodeBerg	An Ethical Mirror (Donate)	💚	💚	➖	⭕️ No Matrix	➖
🐙 kettle-rb/psych-merge on GitHub	Another Mirror	💚	💚	💚	💯 Full Matrix	💚
🎮️ Discord Server		Let's	talk	about	this	library!

Enterprise Support

Available as part of the Tidelift Subscription.

Need enterprise-level guarantees?

The maintainers of this and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source packages you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact packages you use.

• 💡Subscribe for support guarantees covering all your FLOSS dependencies
• 💡Tidelift is part of Sonar
• 💡Tidelift pays maintainers to maintain the software you depend on!
  📊@Pointy Haired Boss: An enterprise support subscription is "never gonna let you down", and supports open source maintainers

Alternatively:

• 
• 
• 

✨ Installation

Install the gem and add to the application's Gemfile by executing:

bundle add psych-merge

If bundler is not being used to manage dependencies, install the gem by executing:

gem install psych-merge

🔒 Secure Installation

For Medium or High Security Installations

This gem is cryptographically signed, and has verifiable SHA-256 and SHA-512 checksums by stone_checksums. Be sure the gem you install hasn’t been tampered with by following the instructions below.

Add my public key (if you haven’t already, expires 2045-04-29) as a trusted certificate:

gem cert --add <(curl -Ls https://raw.github.com/galtzo-floss/certs/main/pboling.pem)

You only need to do that once. Then proceed to install with:

gem install psych-merge -P HighSecurity

The HighSecurity trust profile will verify signed gems, and not allow the installation of unsigned dependencies.

If you want to up your security game full-time:

bundle config set --global trust-policy MediumSecurity

MediumSecurity instead of HighSecurity is necessary if not all the gems you use are signed.

NOTE: Be prepared to track down certs for signed gems and add them the same way you added mine.

⚙️ Configuration

Signature Match Preference

Control which version to use when nodes have matching signatures but different content:

# Use template version (for config updates)
merger = Psych::Merge::SmartMerger.new(
  template,
  destination,
  preference: :template,
)

# Use destination version (default - preserve customizations)
merger = Psych::Merge::SmartMerger.new(
  template,
  destination,
  preference: :destination,
)

Template-Only Nodes

Control whether to add nodes that only exist in the template:

# Add template-only nodes
merger = Psych::Merge::SmartMerger.new(
  template,
  destination,
  add_template_only_nodes: true,
)

Custom Freeze Token

Use a custom freeze token to avoid conflicts with other tools:

merger = Psych::Merge::SmartMerger.new(
  template,
  destination,
  freeze_token: "my-project",
)
# Now looks for: # my-project:freeze and # my-project:unfreeze

Mapping Match Refiner

When YAML mapping entries (key-value pairs) don't match by exact key name, the MappingMatchRefiner uses fuzzy matching to pair entries with:

• Similar key names (e.g., database_url vs db_url)
• Keys with typos or naming convention differences
• Renamed keys that contain similar values

# Enable mapping fuzzy matching
merger = Psych::Merge::SmartMerger.new(
  template,
  destination,
  match_refiners: [
    Psych::Merge::MappingMatchRefiner.new(threshold: 0.5),
  ],
)

MappingMatchRefiner Options

Option	Default	Description
threshold	0.5	Minimum similarity score (0.0-1.0) to accept a match
key_weight	0.7	Weight for key name similarity
value_weight	0.3	Weight for value similarity

# Custom weights for key-centric matching
refiner = Psych::Merge::MappingMatchRefiner.new(
  threshold: 0.6,
  key_weight: 0.8,   # Focus more on key names
  value_weight: 0.2,  # Less focus on values
)

Debug Logging

Enable debug logging to see merge decisions:

export PSYCH_MERGE_DEBUG=1

🔧 Basic Usage

Merging Two YAML Files

require "psych/merge"

template_content = File.read("template.yml")
dest_content = File.read("destination.yml")

merger = Psych::Merge::SmartMerger.new(template_content, dest_content)
result = merger.merge

File.write("merged.yml", result.to_yaml)

Analyzing a YAML File

require "psych/merge"

source = File.read("config.yml")
analysis = Psych::Merge::FileAnalysis.new(source)

# Iterate over all top-level nodes
analysis.statements.each do |node|
  sig = analysis.generate_signature(node)
  puts "#{node.class}: #{sig.inspect}"
end

# Get freeze blocks
analysis.freeze_blocks.each do |freeze_node|
  puts "Protected: lines #{freeze_node.start_line}-#{freeze_node.end_line}"
end

Fuzzy Key Matching

When keys are renamed between template and destination, use the MappingMatchRefiner:

require "psych/merge"

template = <<~YAML
  database_url: postgres://localhost/app
  cache_ttl: 3600
  api_endpoint: https://api.example.com
YAML

destination = <<~YAML
  db_url: postgres://localhost/custom
  cache_timeout: 7200
  service_endpoint: https://custom.example.com
YAML

# Default merge won't match keys (names differ)
# Use MappingMatchRefiner for fuzzy matching
merger = Psych::Merge::SmartMerger.new(
  template,
  destination,
  match_refiners: [
    Psych::Merge::MappingMatchRefiner.new(threshold: 0.5),
  ],
)
result = merger.merge

# Keys are matched despite name differences:
# - database_url ↔ db_url (similar: "database" ~ "db")
# - cache_ttl ↔ cache_timeout (similar: "ttl" ~ "timeout")
# - api_endpoint ↔ service_endpoint (similar: "endpoint")

Freeze Block Example

# Application configuration
app_name: MyApp

# psych-merge:freeze Secret configuration
database:
  host: production-db.example.com
  password: super-secret-password
# psych-merge:unfreeze

logging:
  level: info
  format: json

🦷 FLOSS Funding

While kettle-rb tools are free software and will always be, the project would benefit immensely from some funding. Raising a monthly budget of... "dollars" would make the project more sustainable.

We welcome both individual and corporate sponsors! We also offer a wide array of funding channels to account for your preferences (although currently Open Collective is our preferred funding platform).

If you're working in a company that's making significant use of kettle-rb tools we'd appreciate it if you suggest to your company to become a kettle-rb sponsor.

You can support the development of kettle-rb tools via GitHub Sponsors, Liberapay, PayPal, Open Collective and Tidelift.

📍 NOTE
If doing a sponsorship in the form of donation is problematic for your company from an accounting standpoint, we'd recommend the use of Tidelift, where you can get a support-like subscription instead.

Open Collective for Individuals

Support us with a monthly donation and help us continue our activities. [Become a backer]

NOTE: kettle-readme-backers updates this list every day, automatically.

No backers yet. Be the first!

Open Collective for Organizations

Become a sponsor and get your logo on our README on GitHub with a link to your site. [Become a sponsor]

NOTE: kettle-readme-backers updates this list every day, automatically.

No sponsors yet. Be the first!

Another way to support open-source

I’m driven by a passion to foster a thriving open-source community – a space where people can tackle complex problems, no matter how small. Revitalizing libraries that have fallen into disrepair, and building new libraries focused on solving real-world challenges, are my passions. I was recently affected by layoffs, and the tech jobs market is unwelcoming. I’m reaching out here because your support would significantly aid my efforts to provide for my family, and my farm (11 🐔 chickens, 2 🐶 dogs, 3 🐰 rabbits, 8 🐈‍ cats).

If you work at a company that uses my work, please encourage them to support me as a corporate sponsor. My work on gems you use might show up in bundle fund.

I’m developing a new library, floss_funding, designed to empower open-source developers like myself to get paid for the work we do, in a sustainable way. Please give it a look.

Floss-Funding.dev: 👉️ No network calls. 👉️ No tracking. 👉️ No oversight. 👉️ Minimal crypto hashing. 💡 Easily disabled nags

🔐 Security

See SECURITY.md.

🤝 Contributing

If you need some ideas of where to help, you could work on adding more code coverage, or if it is already 💯 (see below) check reek, issues, or PRs, or use the gem and think about how it could be better.

We so if you make changes, remember to update it.

See CONTRIBUTING.md for more detailed instructions.

🚀 Release Instructions

See CONTRIBUTING.md.

Code Coverage

🪇 Code of Conduct

Everyone interacting with this project's codebases, issue trackers, chat rooms and mailing lists agrees to follow the .

🌈 Contributors

Made with contributors-img.

Also see GitLab Contributors: https://gitlab.com/kettle-rb/psych-merge/-/graphs/main

⭐️ Star History

📌 Versioning

This Library adheres to . Violations of this scheme should be reported as bugs. Specifically, if a minor or patch version is released that breaks backward compatibility, a new version should be immediately released that restores compatibility. Breaking changes to the public API will only be introduced with new major versions.

dropping support for a platform is both obviously and objectively a breaking change
—Jordan Harband (@ljharb, maintainer of SemVer) in SemVer issue 716

I understand that policy doesn't work universally ("exceptions to every rule!"), but it is the policy here. As such, in many cases it is good to specify a dependency on this library using the Pessimistic Version Constraint with two digits of precision.

For example:

spec.add_dependency("psych-merge", "~> 1.0")

📌 Is "Platform Support" part of the public API? More details inside.

SemVer should, IMO, but doesn't explicitly, say that dropping support for specific Platforms is a breaking change to an API, and for that reason the bike shedding is endless.

To get a better understanding of how SemVer is intended to work over a project's lifetime, read this article from the creator of SemVer:

• "Major Version Numbers are Not Sacred"

See CHANGELOG.md for a list of releases.

📄 License

The gem is available as open source under the terms of the MIT License . See LICENSE.txt for the official Copyright Notice.

© Copyright

• Copyright (c) 2023, 2025 Peter H. Boling, of Galtzo.com , and psych-merge contributors.

🤑 A request for help

Maintainers have teeth and need to pay their dentists. After getting laid off in an RIF in March, and encountering difficulty finding a new one, I began spending most of my time building open source tools. I'm hoping to be able to pay for my kids' health insurance this month, so if you value the work I am doing, I need your support. Please consider sponsoring me or the project.

To join the community or get help 👇️ Join the Discord.

To say "thanks!" ☝️ Join the Discord or 👇️ send money.

💌 💌 💌

Please give the project a star ⭐ ♥.

Thanks for RTFM. ☺️