ruby4

name

ruby4

description

Enforces idiomatic Ruby 4.0 code generation. This skill should be used whenever writing, reviewing, or refactoring Ruby code in a Ruby 4.0+ project. It ensures modern idioms (hash shorthand, `it` block param, endless methods, Data.define, pattern matching) are used by default and prevents generation of removed APIs (RJIT, pipe-based open, ObjectSpace._id2ref, old Ractor API, SortedSet, full CGI) and pre-4.0 patterns that have cleaner modern equivalents.

Ruby 4.0 Idiomatic Code

Overview

Ruby 4.0 (released December 2025) establishes new language baselines. Write code that reflects 4.0 standards — not Ruby 2.x/3.0-era habits. The full change reference is in references/ruby4-reference.md. Load it at the start of any Ruby 4.0 task.

Workflow

1. Read references/ruby4-reference.md before generating or reviewing Ruby code.
2. Apply the idiom rules below as non-negotiable defaults.
3. Flag any pattern from the "Never Generate" list if found in existing code under review.

Non-Negotiable Modern Idioms

Apply unconditionally. Do not fall back to the old form under any circumstance.

Hash shorthand — always omit redundant value

# Wrong
{ user: user, project: project }
create_session(user: user)

# Right
{ user:, project: }
create_session(user:)

Anonymous block param — use it for single-param blocks

# Wrong
numbers.map { |n| n * 2 }

# Right
numbers.map { it * 2 }

Endless method — use for all single-expression methods

# Wrong
def double(x)
  x * 2
end

# Right
def double(x) = x * 2

Data.define — use instead of Struct for immutable value objects

# Wrong
Point = Struct.new(:x, :y, keyword_init: true)

# Right
Point = Data.define(:x, :y)

Set and Pathname — no require needed

# Wrong
require 'set'
require 'pathname'

# Right — they are core classes in 4.0
set = Set.new([1, 2, 3])
path = Pathname.new("/tmp")

Net::HTTP — always set Content-Type explicitly

# Wrong — Content-Type no longer auto-set in 4.0
http.post("/path", body)

# Right
http.post("/path", body, "Content-Type" => "application/x-www-form-urlencoded")

Never Generate — Removed APIs

Removed	Use Instead
--rjit / RJIT references	--yjit (stable) or --zjit (experimental)
open("|cmd") / Kernel.open("|cmd")	IO.popen, Open3.capture2
ObjectSpace._id2ref	removed; restructure code
Process::Status#& / #>>	#exitstatus, #signaled?, #stopped?
Ractor.yield	Ractor::Port#<<
Ractor#take	port.receive
Ractor#close_incoming / #close_outgoing	removed
SortedSet	add gem sorted_set
require 'cgi' for full CGI	Rack / framework; only escape helpers remain
ObjectSpace._id2ref	removed

Ractor — Use Port API

# Wrong (old API — removed)
result = ractor.take

# Right (4.0 Port API)
port = Ractor::Port.new
r = Ractor.new(port) { |p| p << compute }
result = port.receive
r.join

Key 4.0 Additions Worth Using

• Array#rfind { it > x } — reverse find, faster than .reverse_each.find
• IO.select(readers, nil, nil, Float::INFINITY) — infinite timeout now accepted
• Math.log1p(x) / Math.expm1(x) — numerically stable math
• "--text--".strip("-") — strip by character selector
• Fiber#raise(cause:) / Thread#raise(cause:) — chained exception cause
• Ruby::VERSION / Ruby::PLATFORM — top-level Ruby module constants
• Logical operators at line-start continue the previous line (no backslash needed)

Pattern Matching — Use Freely (Stable Since 3.0)

case response
in { status: 200, body: String => body }
  process(body)
in { status: 422, errors: [*, String => first, *] }
  report(first)
end

Reference

Full details, before/after examples, and edge cases: references/ruby4-reference.md