Error: can't use variable name 'some_name' inside assignment to variable 'some_name'

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It works!

  def keys
    keys = [] of String
    _keys = uninitialized (Array(String), Node?) ->

    _keys = ->(arr : Array(String), node : Node?) do
      return if node.nil?

      _keys.call(arr, node.left)
      arr << node.key unless node.key == ""
      _keys.call(arr, node.right)
    end

    _keys.call(keys, @root)

    keys
  end

Although, i have to use return if node.nil? instead next if node.nil?(in ruby), return return to the _keys.call(...), not the method keys, this is expected, right?

Then, how to return to the keys method?