I kind of hate Ruby for this

wat.rb

irb(main):001:0> wat = Hash.new {|h, k| h[k] = "Wat #{k}?"}
=> {}
irb(main):002:0> wat.keys
=> []
irb(main):003:0> wat.class
=> Hash
irb(main):004:0> puts 'wat?' if wat['foo']
wat?
=> nil
irb(main):005:0> wat.keys
=> ["foo"]
irb(main):006:0> wat
=> {"foo"=>"Wat foo?"}