Ruby: Self reference in hash

Is it possible to reference one element in a hash within another element in the same hash?

# Pseudo code
foo = { :world => "World", :hello 开发者_StackOverflow中文版=> "Hello #{foo[:world]}" }
foo[:hello] # => "Hello World"

Indirectly perhaps...

foo = { :world => 'World', :hello => lambda { "Hello #{foo[:world]}" }}

puts foo[:hello].call

If you want to make values of some keys dependent on others:

foo = Hash.new{|h, k|
   case k
   when :hello; "Hello #{h[:world]}"
   when :bye; "Bye #{h[:world]}"
   end
}
foo[:world] = 'World'
foo[:hello] # => 'Hello World'
foo[:bye] # => 'Bye World'
foo[:world] = 'Heaven'
foo[:hello] # => 'Hello Heaven'
foo[:bye] # => 'Bye Heaven'

This can't be done directly because "" is strictly evaluated.

Use of a lazy-value generator (e.g. lambda/proc) with later evaluation is required.

Happy coding.

No.

At least not in one step. You could do something like:

foo = {world: "hello"}
foo[:hello] = "Hello #{foo[:world]}"

Sure you can!

options = { :deep => :stuff }
options.merge!(:options => options)

# It's the same at any depth...
options[:options][:options][:options][:options][:options]
  #=> {:deep=>:stuff, :options=>{...}}

Neat, huh? The hash object in options has the same object_id as the value assigned to :options.

Object#tap gives you a nice way to complete the Object initialisation with extra self-references.

{ foo: 1 }.tap { |obj| obj[:bar] = obj.fetch(:foo)}