Fun with callcc

In Ruby, foo["bar"]["baz"]["foo"] = 1 on an empty hash blows up on nil. Here I patch Hash#[], nil.[] and nil.[]= with callcc so that the final assignment builds the missing hashes after the fact.

require "continuation"

class Hash
  alias_method :_original_lookup, :[]

  def [](key)
    _original_lookup(key).tap do
      callcc { |cc| $hash_cc = [cc]; nil }&.tap do |value_from_future|
        self[key] = value_from_future
        $hash_cc.pop.call(self)
      end
    end
  end
end

def nil.[](key)
  callcc { |cc| $hash_cc << cc; nil }&.tap do |value|
    $hash_cc.pop.call(key => value)
  end
end

def nil.[]=(key, value)
  callcc { |cc|
    $hash_cc.unshift(cc)
    $hash_cc.pop.call(key => value)
  }
end

foo = {}
foo["bar"]["baz"]["foo"] = 1

puts foo

Each lookup that comes up empty saves a continuation and returns nil. When nil.[]= finally runs, it jumps back into those saved lookups one by one, handing each a freshly built hash, until the original Hash#[] stores the result and resumes after the assignment. It prints {"bar"=>{"baz"=>{"foo"=>1}}}.

Gist: timetravel.rb