Deep Fetch

deep_fetch.rb

module DeepFetch
  def deep_fetch(*keys, &fetch_default)
    throw_fetch_default = fetch_default && lambda {|key, coll|
      args = [key, coll]
      # only provide extra block args if requested
      args = args.slice(0, fetch_default.arity) if fetch_default.arity >= 0
      # If we need the default, we need to stop processing the loop immediately
      throw :df_value, fetch_default.call(*args)
    }
    catch(:df_value){
      keys.inject(self){|value,key|
        block = throw_fetch_default && lambda{|*args|
          # sneak the current collection in as an extra block arg
          args << value
          throw_fetch_default.call(*args)
        }
        value.fetch(key, &block)
      }
    }
  end

  # Overload [] to work with multiple keys
  def [](*keys)
    case keys.size
    when 1 then super
    else deep_fetch(*keys){|key, coll| coll[key]}
    end
  end

end

h = {
  :foo => {
    :bar => [:x, :y, :z],
    :baz => Hash.new{ "missing value in :bar" }
  }
}
h.extend(DeepFetch)
h[:foo]                         # => {:baz=>{}, :bar=>[:x, :y, :z]}
h[:foo, :bar]                   # => [:x, :y, :z]
h[:foo, :bar, 1]                # => :y
h[:foo, :bar, 5]                # => nil
h[:foo, :baz]                   # => {}
h[:foo, :baz, :fuz]             # => "missing value in :bar"
h.deep_fetch(:foo, :bar, 0)     # => :x
h.deep_fetch(:buz) { :default_value } # => :default_value
h.deep_fetch(:foo, :bar, 5) { :default_value } # => :default_value