☯️ Psych::Merge

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👣 How will this project approach the September 2025 hostile takeover of RubyGems? 🚑️

I've summarized my thoughts in this blog post.

🌻 Synopsis

Psych::Merge intelligently merges two versions of a YAML file using Psych-backed AST analysis. It is built on ast-merge and tree_haver, and shares YAML-family behavior with yaml-merge.

psych-merge is a YAML provider gem, not an alternate home for YAML merge semantics. Psych-specific code should be isolated behind its TreeHaver backend adapter. Merge behavior should consume normalized YAML-family owners and source-preserving edit plans supplied by the StructuredMerge stack. Partial YAML insertion, replacement, and removal should route through ast-crispr rather than converting documents to Ruby objects and serializing them with Psych.dump.

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
YAML Family Behavior: Delegates shared YAML merge semantics to yaml-merge
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
StructuredMerge Native: Depends on ast-merge, tree_haver, yaml-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][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)

💡 Info you can shake a stick at

Tokens to Remember
Works with MRI Ruby 4
Support & Community
Source
Documentation
Compliance
Style
Maintainer 🎖️
`...` 💖	🧊 🐙 🛖 🧪

Compatibility

Compatible with MRI Ruby 4.0.0+, and concordant releases of JRuby, and TruffleRuby. CI workflows and Appraisals are generated for MRI Ruby 4.0.0+. This test floor is configured by ruby.test_minimum in .kettle-jem.yml and may be higher than the gem's runtime compatibility floor when legacy Rubies are not practical for the current toolchain.

The amazing test matrix is powered by the kettle-dev stack.

How kettle-dev manages complexity in tests

Gem	Source	Role
appraisal2	GitHub	multi-dependency Appraisal matrix generation
appraisal2-rubocop	GitHub	RuboCop Appraisal generator integration
kettle-dev	GitHub	development, release, and CI workflow tooling
kettle-jem	GitHub	Appraisals & CI workflow templates
kettle-soup-cover	GitHub	SimpleCov coverage policy and reporting
kettle-test	GitHub	standard test runner and coverage harness
rubocop-lts	GitHub	Ruby-version-aware linting
turbo_tests2	GitHub	parallel test execution

✨ 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

⚙️ 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

🔐 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 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.

Code Coverage

Coverage service badges

📌 Versioning

This library follows for its public API where practical. For most applications, prefer the Pessimistic Version Constraint with two digits of precision.

For example:

spec.add_dependency("psych-merge", "~> 7.0")

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

Dropping support for a platform can be a breaking change for affected users. If a release changes supported platforms, it should be called out clearly in the changelog and versioned with that impact in mind.

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 under the following licenses: AGPL-3.0-only, PolyForm-Small-Business-1.0.0. See LICENSE.md for details.

If none of the available licenses suit your use case, please contact us to discuss a custom commercial license.