What is this Hash-like/Tree-like Construct Called?

Question

I want to create a "Config" class that acts somewhere between a hash and a tree. It's just for storing global values, which can have a context.

Here's how I use it:

Config.get("root.parent.child_b") #=> "value"

Here's what the class might look like:

class Construct

  def get(path)
    # split path by "."
    # search tree for nodes
  end

  def set(key, value)
    # split path by "."
    # create tree node if necessary
    # set tree value
  end

  def tree
    {
      :root => {
        :parent => {
          :child_a => "value",
          :child_b => "another value"
        },
        :another_parent => {
          :something => {
            :nesting => "goes on and on"
          }
        }
      }
    }
  end

end

Is there a name for this kind of thing, somewhere between Hash and Tree (not a Computer Science major)? Basically a hash-like interface to a tree.

Something that outputs like this:

t = TreeHash.new
t.set("root.parent.child_a", "value")
t.set("root.parent.child_b", "another value")

desired output format:

t.get("root.parent.child_a") #=> "value"
t.get("root") #=> {"parent" => {"child_a" => "value", "child_b" => "another value"}}

instead of this:

t.get("root") #=> nil

or this (which you get the value from by calling {}.value)

t.get("root") #=> {"parent" => {"child_a" => {}, "child_b" => {}}}

Answer 1

You can implement one in no-time:

class TreeHash < Hash
  attr_accessor :value

  def initialize
    block = Proc.new {|h,k| h[k] = TreeHash.new(&block)}
    super &block
  end

  def get(path)
    find_node(path).value
  end

  def set(path, value)
    find_node(path).value = value
  end

private

  def find_node(path)
    path.split('.').inject(self){|h,k| h[k]}
  end
end

You could improve implementation by setting unneeded Hash methods as a private ones, but it already works the way you wanted it. Data is stored in hash, so you can easily convert it to yaml.

---

EDIT:

To meet further expectations (and, convert to_yaml by default properly) you should use modified version:

class TreeHash < Hash
  def initialize
    block = Proc.new {|h,k| h[k] = TreeHash.new(&block)}
    super &block
  end

  def get(path)
    path.split('.').inject(self){|h,k| h[k]}
  end

  def set(path, value)
    path = path.split('.')
    leaf = path.pop
    path.inject(self){|h,k| h[k]}[leaf] = value
  end
end

This version is slight trade-off, as you cannot store values in non-leaf nodes.

Answer 2

I think the name for the structure is really a nested hash, and the code in the question is a reinvention of javascript's dictionaries. Since a dictionary in JS (or Python or ...) can be nested, each value can be another dictionary, which has its own key/val pairs. In javascript, that's all an object is.

And the best bit is being able to use JSON to define it neatly, and pass it around:

tree : {
  'root' : {
    'parent' : {
      'child_a' : "value",
      'child_b' : "another value"
    },
    'another_parent' : {
      'something' : {
        'nesting' : "goes on and on"
      }
    }
  }
};

In JS you can then do tree.root.parent.child_a.

This answer to another question suggests using the Hashie gem to convert JSON objects into Ruby objects.