📍 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.

☯️ Ast::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

Ast::Merge is not typically used directly - instead, use one of the format-specific gems built on top of it.

The `*-merge` Gem Family

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

What Ast::Merge Provides

Base Classes: FreezeNode, MergeResult base classes with unified constructors
Shared Modules: FileAnalysisBase, MergerConfig, DebugLogger
Freeze Block Support: Configurable marker patterns for multiple comment syntaxes
Error Classes: ParseError, TemplateParseError, DestinationParseError
RSpec Shared Examples: Test helpers for implementing new merge gems

Creating a New Merge Gem

require "ast/merge"

module MyFormat
  module Merge
    class FreezeNode < Ast::Merge::FreezeNode
      # Override methods as needed for your format
    end

    class MergeResult < Ast::Merge::MergeResult
      # Add format-specific output methods
      def to_my_format
        to_s
      end
    end

    class FileAnalysis
      include Ast::Merge::FileAnalysisBase

      # Implement required methods:
      # - compute_node_signature(node)
      # - extract_freeze_blocks
    end

    class SmartMerger
      include Ast::Merge::MergerConfig

      # Implement merge logic
    end
  end
end

💡 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][💎d-in-dvcs] Repository | Status | Issues | PRs | Wiki | CI | Discussions | |-------------------------------------------------|-----------------------------------------------------------------------|---------------------------|--------------------------|---------------------------|--------------------------|------------------------------| | 🧪 [kettle-rb/ast-merge on GitLab][📜src-gl] | The Truth | [💚][🤝gl-issues] | [💚][🤝gl-pulls] | [💚][📜gl-wiki] | 🐭 Tiny Matrix | ➖ | | 🧊 [kettle-rb/ast-merge on CodeBerg][📜src-cb] | An Ethical Mirror ([Donate][🤝cb-donate]) | [💚][🤝cb-issues] | [💚][🤝cb-pulls] | ➖ | ⭕️ No Matrix | ➖ | | 🐙 [kettle-rb/ast-merge on GitHub][📜src-gh] | Another Mirror | [💚][🤝gh-issues] | [💚][🤝gh-pulls] | [💚][📜gh-wiki] | 💯 Full Matrix | [💚][gh-discussions] | | 🎮️ [Discord Server][✉️discord-invite] | [![Live Chat on Discord][✉️discord-invite-img-ftb]][✉️discord-invite] | [Let's][✉️discord-invite] | [talk][✉️discord-invite] | [about][✉️discord-invite] | [this][✉️discord-invite] | [library!][✉️discord-invite] |

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. [![Get help from me on Tidelift][🏙️entsup-tidelift-img]][🏙️entsup-tidelift] - 💡Subscribe for support guarantees covering _all_ your FLOSS dependencies - 💡Tidelift is part of [Sonar][🏙️entsup-tidelift-sonar] - 💡Tidelift pays maintainers to maintain the software you depend on!
📊`@`Pointy Haired Boss: An [enterprise support][🏙️entsup-tidelift] subscription is "[never gonna let you down][🧮kloc]", and *supports* open source maintainers Alternatively: - [![Live Chat on Discord][✉️discord-invite-img-ftb]][✉️discord-invite] - [![Get help from me on Upwork][👨🏼‍🏫expsup-upwork-img]][👨🏼‍🏫expsup-upwork] - [![Get help from me on Codementor][👨🏼‍🏫expsup-codementor-img]][👨🏼‍🏫expsup-codementor]

✨ Installation

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

bundle add ast-merge

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

gem install ast-merge

🔒 Secure Installation

For Medium or High Security Installations

This gem is cryptographically signed, and has verifiable [SHA-256 and SHA-512][💎SHA_checksums] checksums by [stone_checksums][💎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: ```console 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: ```console gem install ast-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: ```console 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

ast-merge provides base classes and shared interfaces for building format-specific merge tools. Each implementation (like prism-merge, psych-merge, etc.) has its own SmartMerger with format-specific configuration.

Common Configuration Options

All SmartMerger implementations share these configuration options:

merger = SomeFormat::Merge::SmartMerger.new(
  template,
  destination,
  # When conflicts occur, prefer template or destination values
  preference: :template,            # or :destination (default), or a Hash for per-node-type
  # Add nodes that only exist in template
  add_template_only_nodes: true,    # default: false
  # Custom node type handling
  node_typing: {},                # optional, for per-node-type preference
)

Signature Match Preference

Control which source wins when both files have the same structural element:

:template - Template values replace destination values
:destination (default) - Destination values are preserved
Hash - Per-node-type preference (see Advanced Configuration)

Template-Only Nodes

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

true - Add new nodes from template
false (default) - Skip template-only nodes

🔧 Basic Usage

Using Shared Examples in Tests

# spec/spec_helper.rb
require "ast/merge/rspec/shared_examples"

# spec/my_format/merge/freeze_node_spec.rb
RSpec.describe(MyFormat::Merge::FreezeNode) do
  it_behaves_like "Ast::Merge::FreezeNode" do
    let(:freeze_node_class) { described_class }
    let(:default_pattern_type) { :hash_comment }
    let(:build_freeze_node) do
      lambda { |start_line:, end_line:, **opts|
        # Build a freeze node for your format
      }
    end
  end
end

Available Shared Examples

"Ast::Merge::FreezeNode" - Tests for FreezeNode implementations
"Ast::Merge::MergeResult" - Tests for MergeResult implementations
"Ast::Merge::DebugLogger" - Tests for DebugLogger implementations
"Ast::Merge::FileAnalysisBase" - Tests for FileAnalysis implementations
"Ast::Merge::MergerConfig" - Tests for SmartMerger implementations

🎛️ Advanced Configuration

Freeze Blocks

Freeze blocks are special comment-delimited regions in your files that tell the merge tool to preserve content exactly as-is, preventing any changes from the template. This is useful for hand-edited customizations you never want overwritten.

A freeze block consists of:

A start marker comment (e.g., # mytoken:freeze)
The protected content
An end marker comment (e.g., # mytoken:unfreeze)

# In a Ruby file with prism-merge:
class MyApp
  # prism-merge:freeze
  # Custom configuration that should never be overwritten
  CUSTOM_SETTING = "my-value"
  # prism-merge:unfreeze

  VERSION = "1.0.0"  # This can be updated by template
end

The FreezeNode class represents these protected regions internally. Each format-specific merge gem (like prism-merge, psych-merge, etc.) configures its own freeze token (the token in token:freeze), which defaults to the gem name (e.g., prism-merge).

Supported Comment Patterns

Different file formats use different comment syntaxes. The merge tools detect freeze markers using the appropriate pattern for each format:

Pattern Type	Start Marker	End Marker	Languages
`:hash_comment`	`# token:freeze`	`# token:unfreeze`	Ruby, Python, YAML, Bash, Shell
`:html_comment`	`<!-- token:freeze -->`	`<!-- token:unfreeze -->`	HTML, XML, Markdown
`:c_style_line`	`// token:freeze`	`// token:unfreeze`	C (C99+), C++, JavaScript, TypeScript, Java, C#, Go, Rust, Swift, Kotlin, PHP, JSONC
`:c_style_block`	`/* token:freeze */`	`/* token:unfreeze */`	C, C++, JavaScript, TypeScript, Java, C#, Go, Rust, Swift, Kotlin, PHP, CSS

📍 NOTE
CSS only supports block comments (`/* */`), not line comments.
JSON does not support comments; use JSONC for JSON with comments.

Per-Node-Type Preference with `node_typing`

The node_typing option allows you to customize merge behavior on a per-node-type basis. When combined with a Hash-based preference, you can specify different merge preferences for different types of nodes (e.g., prefer template for linter configs but destination for everything else).

How It Works

Define a node_typing: A Hash mapping node type symbols to callables that receive a node and return either:
- The original node (no special handling)
- A wrapped node with a merge_type attribute (via Ast::Merge::NodeTyping::Wrapper)
Use a Hash-based preference: Instead of a simple :destination or :template Symbol, pass a Hash with:
- :default key for the fallback preference
- Custom keys matching the merge_type values from your node_typing

# Example: Prefer template for lint gem configs, destination for everything else
node_typing = {
  call_node: ->(node) {
    if node.name == :gem && node.arguments&.arguments&.first&.unescaped&.match?(/rubocop|standard|reek/)
      Ast::Merge::NodeTyping::Wrapper.new(node, :lint_gem)
    else
      node
    end
  },
}

merger = Prism::Merge::SmartMerger.new(
  template_content,
  dest_content,
  node_typing: node_typing,
  preference: {
    default: :destination,
    lint_gem: :template,
  },
)

NodeTyping::Wrapper

The Ast::Merge::NodeTyping::Wrapper class wraps an AST node and adds a merge_type attribute. It delegates all method calls to the wrapped node, so it can be used transparently in place of the original node.

# Wrap a node with a custom merge_type
wrapped = Ast::Merge::NodeTyping::Wrapper.new(original_node, :special_config)
wrapped.merge_type  # => :special_config
wrapped.class       # => Ast::Merge::NodeTyping::Wrapper
wrapped.location    # => delegates to original_node.location

NodeTyping Utility Methods

# Process a node through the node_typing configuration
processed = Ast::Merge::NodeTyping.process(node, node_typing_config)

# Check if a node has been wrapped with a merge_type
Ast::Merge::NodeTyping.typed_node?(node)  # => true/false

# Get the merge_type from a wrapped node (or nil)
Ast::Merge::NodeTyping.merge_type_for(node)  # => Symbol or nil

# Unwrap a node type wrapper to get the original
Ast::Merge::NodeTyping.unwrap(wrapped_node)  # => original_node

Hash-Based Preference (without node_typing)

Even without node_typing, you can use a Hash-based preference to set a default and document your intention for future per-type customization:

# Simple Hash preference (functionally equivalent to preference: :destination)
merger = MyMerger.new(
  template_content,
  dest_content,
  preference: {default: :destination},
)

MergerConfig Factory Methods

The MergerConfig class provides factory methods that support all options:

# Create config preferring destination
config = Ast::Merge::MergerConfig.destination_wins(
  freeze_token: "my-freeze",
  signature_generator: my_generator,
  node_typing: my_typing,
)

# Create config preferring template
config = Ast::Merge::MergerConfig.template_wins(
  freeze_token: "my-freeze",
  signature_generator: my_generator,
  node_typing: my_typing,
)

🦷 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/ast-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("ast-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"][📌major-versions-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

🤑 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. ☺️