pass_by_reference_example.rb

# these two methods appear to be the same but
# because one accepts memo as an argument
# it hackily gets away with the actual
# problem which is that `+` doesn't mutate memo
# since Ruby passes args by reference it
# works, ngl this took me sometime to find
# why one worked and the other didn't

# works

def keys_for_deep_find(hash, memo: [], &block)
  hash.each do |k, v|
    if v.is_a?(Hash)
      memo + keys_for_deep_find(v, memo: memo, &block)
    else
      if block.call(k, v)
        memo.push(k)
      end
    end
  end

  memo
end

# does not work

def keys_for_deep_find(hash, &block)
  hash.each_with_object([]) do |(k, v), memo|
    if v.is_a?(Hash)
      memo + keys_for_deep_find(v, &block)
    else
      if block.call(k, v)
        memo.push(k)
      end
    end
  end
end