Ruby 定数

大文字で始まるものは定数

module M
  MyConstant = "外側"
  class MyClass
    MyConstant = "内側"
  end
end
p M.constants #=> [:MyConstant, :MyClass]。大文字で始まるものはクラス名であろうがとりあえず全部定数。
p ::M::MyConstant #=> "外側"
p ::M::MyClass::MyConstant #=> "内側"
Hoge = ::M::MyClass::MyConstant
p Hoge #=> "内側"

Module.constantsはトップレベルの定数を全部出力

Module #=> [:Object, :Module, :Class, :BasicObject, :Kernel, :NilClass, :NIL, :Data, :TrueClass, :TRUE, :FalseClass, :FALSE, :Encoding, :Comparable, :Enumerable, :String, :Symbol, :Exception, :SystemExit, :SignalException, :Interrupt, :StandardError, :TypeError, :ArgumentError, :IndexError, :KeyError, :RangeError, :ScriptError, :SyntaxError, :LoadError, :NotImplementedError, :NameError, :NoMethodError, :RuntimeError, :SecurityError, :NoMemoryError, :EncodingError, :SystemCallError, :Errno, :ZeroDivisionError, :FloatDomainError, :Numeric, :Integer, :Fixnum, :Float, :Bignum, :Array, :Hash, :ENV, :Struct, :RegexpError, :Regexp, :MatchData, :Marshal, :Range, :IOError, :EOFError, :IO, :STDIN, :STDOUT, :STDERR, :ARGF, :FileTest, :File, :Dir, :Time, :Random, :Signal, :Process, :Proc, :LocalJumpError, :SystemStackError, :Method, :UnboundMethod, :Binding, :Math, :GC, :ObjectSpace, :Enumerator, :StopIteration, :RubyVM, :Thread, :TOPLEVEL_BINDING, :ThreadGroup, :Mutex, :ThreadError, :Fiber, :FiberError, :Rational, :Complex, :RUBY_VERSION, :RUBY_RELEASE_DATE, :RUBY_PLATFORM, :RUBY_PATCHLEVEL, :RUBY_REVISION, :RUBY_DESCRIPTION, :RUBY_COPYRIGHT, :RUBY_ENGINE, :TracePoint, :ARGV, :Gem, :RbConfig, :Config, :CROSS_COMPILING, :ConditionVariable, :Queue, :SizedQueue, :StringIO, :MonitorMixin, :Monitor, :Exception2MessageMapper, :IRB, :RubyToken, :RubyLex, :Readline, :RUBYGEMS_ACTIVATION_MONITOR]