An "append-only" / "write-only" hash in Ruby

Question

I'm looking for a kind of "append-only" hash where keys may only be set once.

For example:

capitals = AppendOnlyHash.new
capitals['france'] = 'paris'
capitals['japan'] = 'tokyo'
capitals['france'] = 'nice' # raises immutable exception

Any library recommendations or ideas how to achieve this?

(Use case is a logging type object which will be passed to numerouis loosely connected classes, and wanting to detect if any use the same key.)

Answer 1

There are 10 methods, directly mutating the hash:

Hash.instance_methods.grep(/.+!\z/) << %i|[]= delete keep_if|
#⇒ [:select!, :filter!, :reject!, :compact!, delete, keep_if,
#   :transform_keys!, :transform_values!, :merge!, :[]=]

Also, there is a possibility to mutate values themselves (capitals['france'] << ' and Lyon',) so we are to prevent this as well.

class MyHash < Hash; end

MyHash.prepend(
  Module.new do
    (Hash.instance_methods.grep(/.+!\z/) | %i|delete keep_if|).each do |method|
      define_method(method) do |*args|
        raise "Method #{method} is restricted since it is mutating"
      end
    end
    def []=(key, val)
      raise "This hash is immutable" if key?(key)
      super(key, val.freeze) # to prevent inplace mutations
    end
  end
)

One needs to derive from Hash because otherwise we are to break all the hashes.

I did not test this code but it should work out of the box, (if not, the idea should be clear.)