hash keys

hash_keys.rb

#!/usr/bin/env ruby

=begin
[30] pry(main)> a.class.ancestors
=> [Twitter::NullObject,
 Twitter::NullObject::Customizations,
 Comparable,
 Twitter::NullObject::GeneratedMethods,
 Naught::NullObjectTag,
 Naught::Conversions,
 Naught::BasicObject,
 BasicObject]
=end

hash = {}

class NotHasHashMethod < BasicObject
end

class HasHashMethod < BasicObject
  def hash
    :dummy   # not has to_int
  end
end

hash[NotHasHashMethod.new] = 100 # => undefined method `hash' for #<NotHasHashMethod:0x007f8201886c80> (NoMethodError)
hash[HasHashMethod.new]    = 200 # => no implicit conversion of Symbol into Integer (TypeError)