☯️ Rbs::Merge

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Rbs::Merge is a standalone Ruby module that intelligently merges two versions of an RBS (Ruby Signature) file using the official RBS parser. It's like a smart "git merge" specifically designed for RBS type definitions. Built on top of ast-merge, it shares the same architecture as prism-merge for Ruby source files.

• RBS-Aware: Uses the official RBS parser to understand type signature structure
• Intelligent: Matches declarations by structural signatures (class names, method names, type aliases)
• Recursive Merge: Automatically merges class and module bodies recursively, intelligently combining nested method definitions and members
• Comment-Preserving: Comments are properly attached to relevant declarations
• Freeze Block Support: Respects freeze markers (default: rbs-merge:freeze / rbs-merge:unfreeze) for merge control - customizable to match your project's conventions
• Full Provenance: Tracks origin of every declaration
• Standalone: Minimal dependencies - just rbs and ast-merge
• 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 declarations that do not exist in destination
  • freeze_token - customize freeze block markers (default: "rbs-merge")

require "rbs/merge"

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

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

File.write("merged.rbs", result.to_s)

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

Example implementations for the gem templating use case:

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Works with JRuby
Works with Truffle Ruby
Works with MRI Ruby 3
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Compatible with MRI Ruby 3.2.0+, and concordant releases of JRuby, and TruffleRuby.

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Alternatively:

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

bundle add rbs-merge

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

gem install rbs-merge

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 rbs-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.

Rbs::Merge works out of the box with zero configuration, but offers customization options for advanced use cases.

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

# Use template version (for updating type definitions from a canonical source)
merger = Rbs::Merge::SmartMerger.new(
  template,
  destination,
  preference: :template,
)

# Use destination version (for preserving local type customizations)
merger = Rbs::Merge::SmartMerger.new(
  template,
  destination,
  preference: :destination,  # This is the default
)

When to use each:

• :template - Template contains canonical/updated type definitions

  • Generated RBS files from rbs prototype that should replace older versions
  • Type definition updates from upstream libraries
  • Standardized type signatures that should be enforced

• :destination (default) - Destination contains customizations

  • Hand-tuned type definitions with more specific types
  • Local overrides for library types
  • Project-specific type annotations

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

# Add template-only declarations (for merging new type definitions)
merger = Rbs::Merge::SmartMerger.new(
  template,
  destination,
  add_template_only_nodes: true,
)

# Skip template-only declarations (for templates with placeholder types)
merger = Rbs::Merge::SmartMerger.new(
  template,
  destination,
  add_template_only_nodes: false,  # This is the default
)

When to use each:

• true - Template has new type definitions to add

  • New classes/modules from updated code
  • New method signatures that need type annotations
  • Required type aliases or constants

• false (default) - Template has placeholder/example types

  • Example type definitions that shouldn't be added
  • Generated types that may not apply to destination

For different merge scenarios:

# Scenario 1: Update types from generated RBS (template wins, add new types)
merger = Rbs::Merge::SmartMerger.new(
  generated_rbs,
  existing_rbs,
  preference: :template,
  add_template_only_nodes: true,
)
# Result: All type definitions updated to match generated, new types added

# Scenario 2: Preserve custom types (destination wins, skip template-only)
merger = Rbs::Merge::SmartMerger.new(
  library_types,
  custom_types,
  preference: :destination,  # default
  add_template_only_nodes: false,             # default
)
# Result: Custom type refinements preserved, template-only types skipped

# Scenario 3: Merge new types but keep customizations
merger = Rbs::Merge::SmartMerger.new(
  template_types,
  project_types,
  preference: :destination,  # Keep custom type refinements
  add_template_only_nodes: true,              # But add new type definitions
)
# Result: Existing types keep destination definitions, new types added from template

You can provide a custom signature generator to control how declarations are matched between template and destination files:

signature_generator = lambda do |node|
  case node
  when RBS::AST::Declarations::Class
    # Match classes by name only
    [:class, node.name.to_s]
  when RBS::AST::Declarations::TypeAlias
    # Match type aliases by name
    [:type_alias, node.name.to_s]
  when RBS::AST::Members::MethodDefinition
    # Match methods by name and kind
    [:method, node.name.to_s, node.kind]
  else
    # Return node to fall through to default signature computation
    node
  end
end

merger = Rbs::Merge::SmartMerger.new(
  template,
  destination,
  signature_generator: signature_generator,
)

Protect sections in the destination file from being overwritten by the template using freeze markers.

By default, Rbs::Merge uses rbs-merge as the freeze token:

# In your destination.rbs file
# rbs-merge:freeze
type custom_config = { api_key: String, timeout: Integer }
# rbs-merge:unfreeze

You can customize the freeze token to match your project's conventions:

# Use a custom freeze token
merger = Rbs::Merge::SmartMerger.new(
  template,
  destination,
  freeze_token: "my-project",  # Now uses # my-project:freeze / # my-project:unfreeze
)

Freeze blocks are always preserved from the destination file during merge, regardless of template content. They can be placed around:

• Type alias definitions
• Class/module declarations
• Interface definitions
• Method signatures within classes

This allows you to protect custom type definitions that should never be overwritten by template updates.

The most basic usage merges two RBS files:

require "rbs/merge"

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

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

File.write("merged.rbs", result.to_s)

Rbs::Merge intelligently combines files by:

1. Parsing RBS: Uses the official RBS parser to understand type structure
2. Finding Matches: Identifies matching declarations between files by signature
3. Resolving Conflicts: Uses configurable preference to choose between versions
4. Preserving Context: Maintains comments and freeze blocks

Example:

# template.rbs
class Foo
  def bar: (String) -> Integer
  def new_method: () -> void
end

type my_type = String

# destination.rbs
class Foo
  def bar: (Integer) -> String  # Custom signature
  def custom_method: () -> void
end

type my_type = Integer | String  # Expanded type

# After merge with default settings (destination preference):
# - Foo#bar keeps destination signature: (Integer) -> String
# - Foo#custom_method preserved (destination-only)
# - Foo#new_method NOT added (add_template_only_nodes: false)
# - my_type keeps destination definition: Integer | String

When merging RBS files generated by rbs prototype:

require "rbs/merge"

# Generate new type signatures
generated = `rbs prototype rb lib/my_class.rb`
existing = File.read("sig/my_class.rbs")

merger = Rbs::Merge::SmartMerger.new(
  generated,
  existing,
  preference: :template,  # Use generated signatures
  add_template_only_nodes: true,          # Add new methods
)
result = merger.merge

File.write("sig/my_class.rbs", result.to_s)

Use freeze blocks to protect hand-crafted type definitions:

# destination.rbs
class MyAPI
  # rbs-merge:freeze
  # These types are carefully tuned and should not be overwritten
  def fetch: [T] (String path, Class[T] type) -> T
  def post: [T, U] (String path, T body, Class[U] response_type) -> U
  # rbs-merge:unfreeze

  def version: () -> String
end

The generic method signatures in the freeze block will be preserved even if the template has simpler signatures.

The merge result provides detailed information about decisions made:

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

# Get the merged content
puts result.to_s

# Check if anything was merged
puts "Empty result" if result.empty?

# Get summary of decisions
summary = result.summary
puts "Total decisions: #{summary[:total_decisions]}"
puts "Total lines: #{summary[:total_lines]}"
puts "By decision type: #{summary[:by_decision]}"

Rbs::Merge provides specific error types for different failure modes:

require "rbs/merge"

begin
  merger = Rbs::Merge::SmartMerger.new(template, destination)
  result = merger.merge
rescue Rbs::Merge::TemplateParseError => e
  puts "Template has syntax errors: #{e.message}"
rescue Rbs::Merge::DestinationParseError => e
  puts "Destination has syntax errors: #{e.message}"
rescue Rbs::Merge::FreezeNode::InvalidStructureError => e
  puts "Invalid freeze block structure: #{e.message}"
  puts "  Start line: #{e.start_line}"
  puts "  End line: #{e.end_line}"
end

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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.

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See SECURITY.md.

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.

See CONTRIBUTING.md.

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

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

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("rbs-merge", "~> 1.0")

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.

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

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

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.

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