Hypothetical example of nested methods in Ruby. Really just intended to be an example, the actual syntax and implications would need more thought than I've put into it.

1_with_nested_methods.rb

# hypothetical implementation of nested methods
module Tetris
  extend self
  
  def add_block(board, block_definition, x, y=nil)
    block_height = block_definition.height_for_rotation(0)
    y ||= board.height + block_height
    
    is_within_board_width!
    is_not_below_board!

    board.dup { |new_board| new_board.current_block = block_placed_on_board }
    
    def is_within_board_width!
      board_bounds = 0...board.width
      return if board_bounds.include? x
      raise ArgumentError, "#{x.inspect} not in board width: #{board_bounds.inspect}"
    end
    
    def is_not_below_board!
      return unless (y - block_height + 1) < 0
      raise ArgumentError, "#{y.inspect} for block of height #{block_height} goes below the bottom of the board."
    end
    
    def block_placed_on_board
      BlockOnBoard.new do |block|
        block.block_definition = block_definition
        block.rotation         = 0
        block.x                = x
        block.y                = y
      end
    end
  end
end

Tetris.instance_methods.include? :add_block      # => true
Tetris.instance_methods.include? :block_on_board # => false

Tetris.instance_method(:block_on_board).nested_methods # => [:is_within_board_width!, :is_not_below_board!, :block_placed_on_board]

2_with_classes.rb

# This is how this would currently be done (there's a number of ways, this is actually just one of them)

module Tetris
  class AddBlock
    def self.call(*args)
      new(*args).call
    end
    
    def initialize(board, block_definition, x, y=nil)
      self.board            = board
      self.block_definition = block_definition
      self.x                = x
      self.block_height     = block_definition.height_for_rotation(0)
      self.y                = y || (board.height + block_height)
    end
    
    def call
      is_within_board_width!
      is_not_below_board!
      board.dup { |new_board| new_board.current_block = block_placed_on_board }
    end
    
    private
    
    attr_accessor :board, :block_definition, :block_height, :x, :y
    
    def is_within_board_width!
      board_bounds = 0...board.width
      return if board_bounds.include? x
      raise ArgumentError, "#{x.inspect} not in board width: #{board_bounds.inspect}"
    end
    
    def is_not_below_board!
      return unless (y - block_height + 1) < 0
      raise ArgumentError, "#{y.inspect} for block of height #{block_height} goes below the bottom of the board."
    end
    
    def block_placed_on_board
      BlockOnBoard.new do |block|
        block.block_definition = block_definition
        block.rotation         = 0
        block.x                = x
        block.y                = y
      end
    end
  end
end