Tools: The 4 Best Features of Ruby 4.0

1. Ruby::Box (Isolated Execution Environments) ## 2. ZJIT (The Next-Gen JIT Compiler) ## 3. Ractor Improvements & Ractor::Port ## 4. Set as a Core Class ## Summary: Should you upgrade? Released on Christmas Day 2025 to mark the 30th Anniversary of the language, Ruby 4.0 is a milestone update. While the version number jump might suggest massive breaking changes, Matz (Yukihiro Matsumoto) has kept to the tradition established with Ruby 3.0: major version bumps now signify a new era of performance and capability, rather than syntax incompatibility. If you are upgrading from Ruby 3.4 or 3.5, the transition is remarkably smooth. However, under the hood, Ruby 4 introduces powerful new architectural tools. Here are the top 4 features in Ruby 4.0 that you need to know. The standout feature of Ruby 4.0 is undoubtedly Ruby::Box. For years, Rubyists have struggled with global state pollution when trying to run multiple "applications" or isolated contexts within a single process. Ruby::Box introduces a container-like isolation layer directly into the Ruby runtime. Code loaded or defined inside a "Box" does not leak into the global namespace or other Boxes. Ruby 3.x gave us YJIT (Yet Another Ruby JIT), which revolutionized Ruby performance. Ruby 4.0 introduces its successor: ZJIT. While YJIT relied on "Lazy Basic Block Versioning," ZJIT uses a more traditional method-based compilation strategy. It is built to achieve a higher performance ceiling for long-running applications and to be easier for C and Rust contributors to maintain. Ractor (Ruby Actor) was introduced in Ruby 3.0 to provide true parallel execution without the Global Interpreter Lock (GIL). In Ruby 4.0, Ractor finally feels "production-ready" with the introduction of Ractor::Port. Previously, communication between Ractors could be clunky. Ractor::Port standardizes the input/output interface, making it much easier to build complex concurrent pipelines (like producer-consumer models) without deadlocks. After years of requiring require 'set', the Set class has finally been promoted to a Core Class. While this seems like a minor convenience, it has significant performance implications. Previously, Set was implemented in pure Ruby (wrapping a Hash). In Ruby 4.0, it is rewritten in C and optimized specifically for set operations. Ruby 4.0 is a "celebration release." It honors the language's history while betting on its future with Ruby::Box and ZJIT. Because it avoids major breaking syntax changes, upgrading from Ruby 3.x is highly recommended. Templates let you quickly answer FAQs or store snippets for re-use. Are you sure you want to hide this comment? It will become hidden in your post, but will still be visible via the comment's permalink. Hide child comments as well For further actions, you may consider blocking this person and/or reporting abuse COMMAND_BLOCK: # Experimental usage (requires RUBY_BOX=1 environment variable) box = Ruby::Box.new box.eval do class MyService def call; "Hello from inside the box"; end end end # The class is defined inside the box, not globally defined?(MyService) # => nil Enter fullscreen mode Exit fullscreen mode COMMAND_BLOCK: # Experimental usage (requires RUBY_BOX=1 environment variable) box = Ruby::Box.new box.eval do class MyService def call; "Hello from inside the box"; end end end # The class is defined inside the box, not globally defined?(MyService) # => nil COMMAND_BLOCK: # Experimental usage (requires RUBY_BOX=1 environment variable) box = Ruby::Box.new box.eval do class MyService def call; "Hello from inside the box"; end end end # The class is defined inside the box, not globally defined?(MyService) # => nil COMMAND_BLOCK: # Simplified conceptual example receiver = Ractor.new do loop do msg = Ractor.receive Ractor.yield "Processed: #{msg}" end end Enter fullscreen mode Exit fullscreen mode COMMAND_BLOCK: # Simplified conceptual example receiver = Ractor.new do loop do msg = Ractor.receive Ractor.yield "Processed: #{msg}" end end COMMAND_BLOCK: # Simplified conceptual example receiver = Ractor.new do loop do msg = Ractor.receive Ractor.yield "Processed: #{msg}" end end - Testing: You can run test suites in parallel within the same process without worrying about monkey patches or global variable collisions. - Multi-tenancy: Frameworks can now load plugin code or user-defined scripts in a strictly isolated environment. - Blue-Green Deployment: You can potentially load a new version of your application code into a Box alongside the old version, warming it up before switching traffic, all without restarting the process. - Written in Rust: Like recent versions of YJIT, ZJIT requires a modern Rust compiler to build. - Status: In Ruby 4.0, ZJIT is experimental and opt-in (via --zjit). - Performance: Early benchmarks show it is significantly faster than the interpreter. While it may not beat YJIT in every micro-benchmark yet, it lays the groundwork for optimizations that were impossible in the previous architecture. - Ractor::Port: A cleaner API for sending and receiving messages between workers. - Better Garbage Collection: Ractors now handle memory reclamation more efficiently, reducing the overhead of spinning up short-lived workers. - Zero setup: No more require 'set' at the top of your files. - Speed: Operations like intersection, union, and difference are drastically faster. - Memory: The internal C implementation uses less memory than the old Hash-wrapper approach. - Stable Release: Ruby 4.0.1 (released Jan 2026) - Key Flag: Remember to enable experimental features like RUBY_BOX=1 if you want to play with the cutting edge.