Ruby

Quick Reference

Topic	File
Mutable strings, object equality	objects.md
Proc vs lambda, return behavior	blocks.md
Visibility, method_missing	methods.md
Array/hash mutation traps	collections.md
define_method, eval traps	metaprogramming.md
ActiveRecord, N+1, callbacks	rails.md

Critical Rules

• Strings are mutable — s = "hi"; s << "!"; t = s means t also has "!"
• == vs equal? vs eql? — == value, equal? identity, eql? hash equality
• Default hash value is shared — Hash.new([]) shares same array, use block form
• return in proc returns from enclosing method — use lambda for local return
• Block variable shadows outer scope — x = 1; [2].each { |x| }; x is still 1 (3.0+)
• method_missing without respond_to_missing? — breaks respond_to? checks
• private in Ruby is per-object — self.private_method fails, implicit receiver works
• ||= doesn't work for false/nil distinction — false ||= true replaces false
• Frozen string literals — # frozen_string_literal: true makes strings immutable
• Symbol#to_proc — &:method_name only works with no-argument methods
• rescue => e without type — catches StandardError, not Exception
• ensure always runs — even after return, use for cleanup