Why is a string key for a hash frozen?

Question

According to the specification, strings that are used as a key to a hash are duplicated and frozen. Other mutable objects do not seem to have such special consideration. For exa

Answer 1

You are askin 2 different questions: theoretical and practical. Lain was the first to answer, but I would like to provide what I consider a proper, lazier solution to your practical question:

Hash.new { |hsh, key| # this block get's called only if a key is absent
  downcased = key.to_s.downcase
  unless downcased == key # if downcasing makes a difference
    hsh[key] = hsh[downcased] if hsh.has_key? downcased # define a new hash pair
  end # (otherways just return nil)
}

The block used with Hash.new constructor is only invoked for those missing keys, that are actually requested. The above solution also accepts symbols.