Constants in Ruby

Constants are names starting with an uppercase letter. Reassigning a constant emits a warning (not an error) to discourage changes.

Basics

Use constants for configuration and fixed values. They are looked up lexically.

PI = 3.14159
PI = 3.14   # warning: already initialized constant PI

Namespacing

Organize constants under modules/classes for clarity; access with the :: operator.

module App
  VERSION = "1.0.0"
  class Config
    DEFAULTS = { debug: false }
  end
end

App::VERSION
App::Config::DEFAULTS

Lookup and reflection

Constant lookup prefers lexical scope, then ancestors. Reflect with helpers.

Module.nesting          # current lexical nesting
Object.const_defined?(:PI)
Object.const_get(:PI)

Immutability of referenced objects

Constants reference objects, which can still be mutable. Freeze objects to prevent accidental changes.

TOKEN = "abc".freeze      # freeze object to avoid mutation
SET = [1,2,3].freeze

Style notes

• Use SCREAMING_SNAKE_CASE for global constants, CamelCase for classes/modules
• Group related constants inside modules/classes for discoverability

Summary

• Uppercase names define constants; reassignment emits a warning
• Lexical lookup first, then ancestors; use reflection when needed
• Freeze referenced objects if you want true immutability