hash_keys_fun.rb

hash = {
  "one"     => "two",
  :one      => "two",
  Class.new => :foo,
  1         => :foo,
  1.555     => :foo,
  true      => :foo, 
  false     => :blah, 
  nil       => :nil,
  lambda {} => :lambda
}
hash.keys.each {|k| puts "#{k.inspect} is #{k.frozen? ? 'frozen' : 'not frozen'}"}

"one" is frozen
:one is not frozen
#<Class:0x007fdda39a7530> is not frozen
1 is not frozen
1.555 is not frozen
true is not frozen
false is not frozen
nil is not frozen
#<Proc:0x007fdda39a7238@hash.rb:8 (lambda)> is not frozen