duplicate_entry_catching_hash.rb

# JSON hashes of course allow multiple values for the same key.
# Only one is kept
# E.g.
# > h = JSON.parse '{"a":1, "a":2}'
# => {"a"=>2}
#
# If you don't like that, you can use this :

require 'json'
class DuplicateEntryCatchingHash < Hash
  def []=(k,v)
    @a ||= []
    raise JSON::ParserError, "Duplicate Hash Entry #{k}" if  @a.include?(k.to_s)
    @a << k
    super k,v
  end
end

if __FILE__ == $0
  def show_example(s)
    puts "> JSON.parse '#{s}', :object_class => DuplicateEntryCatchingHash"
    begin
      h = JSON.parse s, :object_class => DuplicateEntryCatchingHash
      puts "=> #{h.inspect}"
    rescue JSON::ParserError => e
      puts "=> JSON::ParserError #{e}"
    end
    puts
  end

  show_example '{"a": 1}'
  show_example '{"a": 1, "a": 2}'
  show_example '{"a": 1, "a": {"b": 2}}'
  show_example '{"a": 1, "a": {"b": 2, "b": 3}}'
end

Example run:

$ ruby duplicate_entry_catching_hash.rb
> JSON.parse '{"a": 1}', :object_class => DuplicateEntryCatchingHash
=> {"a"=>1}

> JSON.parse '{"a": 1, "a": 2}', :object_class => DuplicateEntryCatchingHash
=> JSON::ParserError Duplicate Hash Entry a

> JSON.parse '{"a": 1, "a": {"b": 2}}', :object_class => DuplicateEntryCatchingHash
=> JSON::ParserError Duplicate Hash Entry a

> JSON.parse '{"a": 1, "a": {"b": 2, "b": 3}}', :object_class => DuplicateEntryCatchingHash
=> JSON::ParserError Duplicate Hash Entry b