def one(arg)
  if block_given?
    "block given to 'one' returns #{yield}"
  else
    arg
  end
end
def two
  if block_given?
    "block given to 'two' returns #{yield}"
  else
    "no block given to 'two'"
  end
end
# ブロックはtwoに関連付けられる
result1 = one two {
  "three"
}
# ブロックはoneに関連付けられる
result2 = one two do
  "three"
end
result3 = one two
puts "With braces, result = #{result1}"
puts "With do/end, result = #{result2}"
puts "With no block, result = #{result3}"
出力結果:
With braces, result = block given to 'two' returns three
With do/end, result = block given to 'one' returns three
With no block, result = no block given to 'two'