Ruby Next

Ruby Next is a transpiler and a collection of polyfills for supporting the latest and upcoming Ruby features (APIs and syntax) in older versions and alternative implementations. For example, you can use pattern matching and Kernel#then in Ruby 2.5 or mruby.

Who might be interested in Ruby Next?

• Ruby gems maintainers who want to write code using the latest Ruby version but still support older ones.
• Application developers who want to give new features a try without waiting for the final release (or, more often, for the first patch).
• Users of non-MRI implementations such as mruby, JRuby, TruffleRuby, Opal, RubyMotion, Artichoke, Prism.

Ruby Next also aims to help the community to assess new, experimental, MRI features by making it easier to play with them. That's why Ruby Next implements the master features as fast as possible.

Read more about the motivation behind the Ruby Next in this post: Ruby Next: Make all Rubies quack alike.

Posts

• Ruby Next: Make all Rubies quack alike

Talks

• Ruby Next: Make old Rubies quack like a new one (RubyConf 2019)

Examples

• Ruby gems
  • action_policy
  • anyway_config
  • graphql-fragment_cache
• Rails applications
  • anycable_rails_demo
• mruby
  • ACLI

Please, submit a PR to add your project to the list!

Table of contents

• Overview
• Quick Start
• Polyfills
• Transpiling
  • Modes
  • CLI
  • Using in gems
  • Runtime usage
  • Bootsnap integration
  • ruby -ruby-next
  • Logging & Debugging
• RuboCop
• Using with IRB
• Using with Pry
• Using with EOL Rubies
• Proposed & edge features
• Known limitations

Overview

Ruby Next consists of two parts: core and language.

Core provides polyfills for Ruby core classes APIs via Refinements (default strategy) or core extensions (optionally or for refinement-less environments).

Language is responsible for transpiling edge Ruby syntax into older versions. It could be done programmatically or via CLI. It also could be done in runtime.

Currently, Ruby Next supports Ruby versions 2.2+, including JRuby 9.2.8+ and TruffleRuby 20.1+ (with some limitations). Support for EOL versions (<2.5) slightly differs though (see below).

Please, open an issue or join the discussion in the existing ones if you would like us to support older Ruby versions.

Quick start

The quickest way to start experimenting with Ruby Next is to install the gem and run a sample script. For example:

# Install Ruby Next globally
$ gem install ruby-next

# Call ruby with -ruby-next flag
$ ruby -ruby-next -e "
def greet(val) =
  case val
    in hello: hello if hello =~ /human/i
      '🙂'
    in hello: 'martian'
      '👽'
    end

puts greet(hello: 'martian')
"

=> 👽

Using only polyfills

First, install a gem:

# Gemfile
gem "ruby-next-core"

# gemspec
spec.add_dependency "ruby-next-core"

NOTE: we use the different distribution gem, ruby-next-core, to provide zero-dependency, polyfills-only version.

Then, all you need is to load the Ruby Next:

require "ruby-next"

And activate the refinement in every file where you want to use it*:

using RubyNext

Ruby Next only refines core classes if necessary; thus, this line wouldn't have any effect in the edge Ruby.

NOTE: Even if the runtime already contains a monkey-patch with the backported functionality, we consider the method as dirty and activate the refinement for it. Thus, you always have predictable behaviour. That's why we recommend using refinements for gems development.

Alternatively, you can go with monkey-patches. Just add this line:

require "ruby-next/core_ext"

The following rule of thumb is recommended when choosing between refinements and monkey-patches:

• Use refinements for libraries development (to avoid conflicts with others code)
• Using core extensions could be considered for application development (no need to think about using RubyNext); this approach could potentially lead to conflicts with dependencies (if these dependencies are not using refinements 🙂)
• Use core extensions if refinements are not supported by your platform

NOTE: Edge APIs (i.e., from the Ruby's master branch) are included by default.

The list of supported APIs.

Transpiling

Ruby Next allows you to transpile* edge Ruby syntax to older versions.

Transpiler relies on two libraries: parser and unparser.

NOTE: The "official" parser gem only supports the latest stable Ruby version, while Ruby Next aims to support edge and experimental Ruby features. To enable them, you should use our version of Parser (see instructions below).

Installation:

# Gemfile
gem "ruby-next"

# gemspec
spec.add_dependency "ruby-next"

# or install globally
gem install ruby-next

The list of supported syntax features.

Transpiler modes

Ruby Next currently provides two different modes of generating transpiled code: AST and rewrite.

In the AST mode, we parse the source code into AST, modifies this AST and generate a new code from AST (using unparser). Thus, the transpiled code being identical in terms of functionality has different formatting.

In the rewrite mode, we apply changes to the source code itself, thus, keeping the original formatting of the unaffected code (in a similar way to RuboCop's autocorrect feature).

The main benefit of the rewrite mode is that it preserves the original code line numbers and layout, which is especially useful in debugging.

By default, we use the rewrite mode. If you found a bug with rewrite mode which is not reproducible in the AST mode, please, let us know.

You can change the transpiler mode:

• From code by setting RubyNext::Language.mode = :ast or RubyNext::Language.mode = :rewrite.
• Via environmental variable RUBY_NEXT_TRANSPILE_MODE=ast.
• Via CLI option (see below).

CLI

Ruby Next ships with the command-line interface (ruby-next) which provides the following functionality:

ruby-next nextify

This command allows you to transpile a file or directory into older Rubies (see, for example, the "Integrating into a gem development" section above).

It has the following interface:

$ ruby-next nextify
Usage: ruby-next nextify DIRECTORY_OR_FILE [options]
    -o, --output=OUTPUT              Specify output directory or file or stdout
        --min-version=VERSION        Specify the minimum Ruby version to support
        --single-version             Only create one version of a file (for the earliest Ruby version)
        --edge                       Enable edge (master) Ruby features
        --proposed                   Enable proposed/experimental Ruby features
        --transpile-mode=MODE        Transpiler mode (ast or rewrite). Default: ast
        --[no-]refine                Do not inject `using RubyNext`
        --list-rewriters             List available rewriters
        --rewrite=REWRITERS...       Specify particular Ruby features to rewrite
    -h, --help                       Print help
    -V                               Turn on verbose mode
        --dry-run                    Print verbose output without generating files

The behaviour depends on whether you transpile a single file or a directory:

• When transpiling a directory, the .rbnext subfolder is created within the target folder with subfolders for each supported Ruby versions (e.g., .rbnext/2.6, .rbnext/2.7, .rbnext/3.0, etc.). If you want to create only a single version (the smallest), you can also pass --single-version flag. In that case, no version directory is created (i.e., transpiled files go into .rbnext).

• When transpiling a file and providing the output path as a file path, only a single version is created. For example:

$ ruby-next nextify my_ruby.rb -o my_ruby_next.rb -V
Ruby Next core strategy: refine
Generated: my_ruby_next.rb

ruby-next core_ext

This command could be used to generate a Ruby file with a configurable set of core extensions.

Use this command if you want to backport new Ruby features to Ruby implementations not compatible with RubyGems.

It has the following interface:

$ ruby-next core_ext
Usage: ruby-next core_ext [options]
    -o, --output=OUTPUT              Specify output file or stdout (default: ./core_ext.rb)
    -l, --list                       List all available extensions
        --min-version=VERSION        Specify the minimum Ruby version to support
    -n, --name=NAME                  Filter extensions by name
    -h, --help                       Print help
    -V                               Turn on verbose mode
        --dry-run                    Print verbose output without generating files

The most common use-case is to backport the APIs required by pattern matching. You can do this, for example, by including only monkey-patches containing the "deconstruct" in their names:

ruby-next core_ext -n deconstruct -o pattern_matching_core_ext.rb

To list all available (are matching if --min-version or --name specified) monkey-patches, use the -l switch:

$ ruby-next core_ext -l --name=filter --name=deconstruct
2.6 extensions:
  - ArrayFilter
  - EnumerableFilter
  - HashFilter

2.7 extensions:
  - ArrayDeconstruct
  - EnumerableFilterMap
  - EnumeratorLazyFilterMap
  - HashDeconstructKeys
  - StructDeconstruct

...

CLI configuration file

You can define CLI options in the .rbnextrc file located in the root of your project to avoid adding them every time you run ruby-next.

Configuration file is a YAML with commands as keys and options as multiline strings:

# ./.rbnextrc

nextify: |
  --transpiler-mode=rewrite
  --edge

Integrating into a gem development

We recommend pre-transpiling source code to work with older versions before releasing it.

This is how you can do that with Ruby Next:

• Write source code using the modern/edge Ruby syntax.

• Generate transpiled code by calling ruby-next nextify ./lib (e.g., before releasing or pushing to VCS).

This will produce lib/.rbnext folder containing the transpiled files, lib/.rbnext/2.6, lib/.rbnext/2.7. The version in the path indicates which Ruby version is required for the original functionality. Only the source files containing new syntax are added to this folder.

NOTE: Do not edit these files manually, either run linters/type checkers/whatever against these files.

• Add the following code to your gem's entrypoint (the file that is required first and contains other require-s):

require "ruby-next/language/setup"

RubyNext::Language.setup_gem_load_path

The setup_gem_load_path does the following:

• Resolves the current ruby version.
• Checks whether there are directories corresponding to the current and earlier* Ruby versions within the .rbnext folder.
• Add the path to this directory to the $LOAD_PATH before the path to the gem's directory.

That's why need an entrypoint: all the subsequent require calls will load the transpiled files instead of the original ones due to the way feature resolving works in Ruby (scanning the $LOAD_PATH and halting as soon as the matching file is found).

NOTE: require_relative should be avoided due to the way we hijack the features loading mechanism.

If you're using runtime mode a long with setup_gem_load_path (e.g., in tests), the transpiled files are ignored (i.e., we do not modify $LOAD_PATH).

* Ruby Next avoids storing duplicates; instead, only the code for the earlier version is created and is assumed to be used with other versions. For example, if the transpiled code is the same for Ruby 2.5 and Ruby 2.6, only the .rbnext/2.7/path/to/file.rb is kept. That's why multiple entries are added to the $LOAD_PATH (.rbnext/2.6, .rbnext/2.7, and .rbnext/3.0 in the specified order for Ruby 2.5, and .rbnext/2.7 and .rbnext/3.0 for Ruby 2.6).

Transpiled files vs. VCS vs. installing from source

It's a best practice to not keep generated files in repositories. In case of Ruby Next, it's a lib/.rbnext folder.

We recommend adding this folder only to the gem package (i.e., it should be added to your spec.files) and ignore it in your VCS (e.g., echo ".rbnext/" >> .gitignore). That would make transpiled files available in releases without polluting your repository.

What if someone decides to install your gem from the VCS source? They would likely face some syntax errors due to the missing transpiled files.

To solve this problem, Ruby Next tries to transpile the source code when you call #setup_gem_load_path. It does this by calling bundle exec ruby-next nextify <lib_dir> -o <next_dir>. We make the following assumptions:

• We are in the Bundler context (since that's the most common way of installing gems from source).
• Our Gemfile contains ruby-next gem.
• We use .rbnextrc for transpiling options.

If the command fails we warn the end user.

This feature, auto-transpiling, is disabled by default (will likely be enabled in future versions). You can enable it by calling RubyNext::Language.setup_gem_load_path(transpile: true).

Runtime usage

It is also possible to transpile Ruby source code in run-time via Ruby Next.

All you need is to require "ruby-next/language/runtime" as early as possible to hijack Kernel#require and friends. You can also automatically inject using RubyNext to every* loaded file by also adding require "ruby-next/core/runtime".

Since the runtime mode requires Kernel monkey-patching, it should be used carefully. For example, we use it in Ruby Next tests—works perfectly. But think twice before enabling it in production.

Consider using Bootsnap integration, 'cause its monkey-patching has been bullet-proofed 😉.

* Ruby Next doesn't hijack every required file but watches only the configured directories: ./app/, ./lib/, ./spec/, ./test/ (relative to the pwd). You can configure the watch dirs:

RubyNext::Language.watch_dirs << "path/to/other/dir"

Eval & similar

By default, we do not hijack Kernel.eval and similar methods due to some limitations (e.g., there is no easy and efficient way to access the caller's scope, or binding, and some evaluations relies on local variables).

If you want to support transpiling in eval-like methods, opt-in explicitly by activating the refinement:

using RubyNext::Language::Eval

Using with Bootsnap

Bootsnap is a great tool to speed-up your application load and it's included into the default Rails Gemfile. It patches Ruby mechanism of loading source files to make it possible to cache the intermediate representation (iseq).

Ruby Next provides a specific integration which allows to add a transpiling step to this process, thus making the transpiler overhead as small as possible, because the cached and already transpiled version is used if no changes were made.

To enable this integration, add the following line after the require "bootsnap/setup":

require "ruby-next/language/bootsnap"

NOTE: There is no way to invalidate the cache when you upgrade Ruby Next (e.g., due to the bug fixes), so you should do this manually.

uby-next

This is not a typo, that’s the way ruby -ruby-next works: it’s equal to ruby -r uby-next, and uby-next.rb is a special file that activates the runtime mode.

You can also enable runtime mode by requiring uby-next while running a Ruby executable:

ruby -ruby-next my_ruby_script.rb

# or
RUBYOPT="-ruby-next" ruby my_ruby_script.rb

# or
ruby -ruby-next -e "puts [2, 4, 5].tally"

NOTE: running Ruby scripts directly or executing code via -e option is not supported in TruffleRuby. You can still use -ruby-next to transpile required files, e.g.:

ruby -ruby-next -r my_ruby_script.rb -e "puts my_method"

Logging and debugging

Ruby Next prints some debugging information when fails to load a file in the runtime mode (and fallbacks to the built-in loading mechanism).

You can disable these warnings either by providing the RUBY_NEXT_WARN=false env variable or by setting RubyNext.silence_warnings = true in your code.

You can also enable transpiled source code debugging by setting the RUBY_NEXT_DEBUG=true env variable. When it's set, Ruby Next prints the transpiled code before loading it.

You can use a file pattern as the value for the env var to limit the output: for example, RUBY_NEXT_DEBUG=my_script.rb.

RuboCop

Since Ruby Next provides support for features not available in RuboCop yet, you need to add a patch for compatibility. In you .rubocop.yml add the following:

require:
  - ruby-next/rubocop

You must set TargetRubyVersion: next to make RuboCop use a Ruby Next parser.

Alternatively, you can load the patch from the command line by running: rubocop -r ruby-next/rubocop ....

We recommend using the latest RuboCop version, 'cause it has support for new nodes built-in.

Also, when pre-transpiling source code with ruby-next nextify, we suggest ignoring the transpiled files:

AllCops:
  Exclude:
    - 'lib/.rbnext/**/*'

NOTE: you need ruby-next gem available in the environment where you run RuboCop (having ruby-next-core is not enough).

IRB

Ruby Next supports IRB. In order to enable edge Ruby features for your REPL, add the following line to your .irbrc:

require "ruby-next/irb"

Alternatively, you can require it at startup:

irb -r ruby-next/irb
# or
irb -ruby-next/irb

Pry

Ruby Next supports Pry. In order to enable edge Ruby features for your REPL, add the following line to your .pryrc:

require "ruby-next/pry"

Alternatively, you can require it at startup:

pry -r ruby-next/pry
# or
pry -ruby-next/pry

Using with EOL Rubies

We currently provide support for Ruby 2.2, 2.3 and 2.4.

NOTE: By "support" here we mean using ruby-next CLI and runtime transpiling. Transpiled code may run on Ruby 2.0+.

Ruby Next itself relies on 2.5 features and contains polyfills only for version 2.5+ (and that won't change). Thus, to make it work with <2.5 we need to backport some APIs ourselves.

The recommended way of doing this is to use backports gem. You need to load backports before Ruby Next.

When using runtime features, you should do the following:

# first, require backports upto 2.5
require "backports/2.5"
# then, load Ruby Next
require "ruby-next"
# if you need 2.6+ APIs, add Ruby Next core_ext
require "ruby-next/core_ext"
# then, load runtime transpiling
require "ruby-next/language/runtime"
# or
require "ruby-next/language/bootsnap"

To load backports while using ruby-next nextify command, you must configure the environment variable:

RUBY_NEXT_CORE_STRATEGY=backports ruby-next nextify lib/

NOTE: Make sure you have backports gem installed globally or added to your bundle (if you're using bundle exec ruby-next ...).

NOTE: For Ruby 2.2, safe navigation operator (&.) and squiggly heredocs (<<~TXT) support is provided.

IMPORTANT: Unparser ~> 0.4.8 is required to run the transpiler on Ruby <2.4.

Proposed and edge features

Ruby Next aims to bring edge and proposed features to Ruby community before they (hopefully) reach an official Ruby release. This includes:

• Features already merged to master (edge)
• Features proposed in Ruby bug tracker (proposed)
• Features once merged to master but got reverted.

These features are disabled by default, you must opt-in in one of the following ways:

• Add --edge or --proposed option to nextify command when using CLI.
• Enable programmatically when using a runtime mode:

# It's important to load language module first
require "ruby-next/language"

require "ruby-next/language/rewriters/edge"
# or
require "ruby-next/language/rewriters/proposed"

# and then activate the runtime mode
require "ruby-next/language/runtime"
# or require "ruby-next/language/bootsnap"

• Set RUBY_NEXT_EDGE=1 or RUBY_NEXT_PROPOSED=1 environment variable.

Supported edge features

It's too early, Ruby 3.1 has just been released. See its features in the supported features list.

Supported proposed features

• Method reference operator (.:) (#13581).
• Binding non-local variables in pattern matching (42 => @v) (#18408).

Known limitations

Ruby Next aims to be reasonably compatible with MRI. That means, some edge cases could be uncovered. Below is the list of known limitations.

For gem authors, we recommend testing against all supported versions on CI to make sure you're not hit by edge cases.

Enumerable methods

Using refinements (using RubyNext) for modules could lead to unexpected behaviour in case there is also a prepend for the same module in Ruby <2.7. To eliminate this, we also refine Array (when appropriate), but other enumerables could be affected.

See this issue for details.

Refinement#import_methods

• Doesn't support importing methods generated with eval.
• Doesn't support aliases (both alias and alias_method).
• In JRuby, importing attribute accessors/readers/writers is not supported.
• When using AST transpiling in runtime, likely fails to import methods from a transpiled files (due to the updated source location).

See the original PR for more details.

Other

See Parser's known issues.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/ruby-next/ruby-next.

See also the development guide.

License

The gem is available as open source under the terms of the MIT License.