Some mind blown examples: ```rb [1] pry(main)> list = Array.new(5, "foo") # => ["foo", "foo", "foo", "foo", "foo"] [2] pry(main)> list[4].clear # => "" [3] pry(main)> list # => ["", "", "", "", ""] ``` 因為每個 default foo 都是同一個 object 以及 ```rb [1] pry(main)> master_list = Hash.new([]) # => {} [2] pry(main)> master_list[:evens] << 2 # => [2] [3] pry(main)> master_list[:odds] << 3 # => [2, 3] [4] pry(main)> master_list # => {} [5] pry(main)> master_list[:evens] <<= 4 # => [2, 3, 4] [6] pry(main)> master_list # => {:evens=>[2, 3, 4]} [7] pry(main)> master_list[:odds] <<= 5 # => [2, 3, 4, 5] [8] pry(main)> master_list # => {:evens=>[2, 3, 4, 5], :odds=>[2, 3, 4, 5]} ``` 一樣的原理，每個 default array 都是同一個 object 一個避免這樣的做法是用 proc 確保每個 default 值是不同 object，例如： ```rb Array.new(5) { "foo" } Hash.new { [] } ``` ex: ```rb [1] pry(main)> Array.new(5) { "foo" } => ["foo", "foo", "foo", "foo", "foo"] [2] pry(main)> a = Array.new(5) { "foo" } => ["foo", "foo", "foo", "foo", "foo"] [3] pry(main)> a[2].clear => "" [4] pry(main)> a => ["foo", "foo", "", "foo", "foo"] [5] pry(main)> ``` 另外還有更有趣的「以你為 freeze 了是吧？Boom~!」 ```rb [1] pry(main)> CONSTANT = ["foo"] # => ["foo"] [2] pry(main)> CONSTANT << "bar" # => ["foo", "bar"] [3] pry(main)> CONSTANT.freeze # => ["foo", "bar"] [4] pry(main)> CONSTANT << "baz" # RuntimeError: can't modify frozen Array # from (pry):4:in `__pry__' [5] pry(main)> CONSTANT[1] << "baz" # => "barbaz" [6] pry(main)> CONSTANT # => ["foo", "barbaz"] ``` 這是因為 freeze 是 freeze CONSTANT 這個 Array 的 object，沒有 freeze 裡面的 string，所以 string 還是 mutable (Ice Nine gem for deep freezing) 為了拯救 developers 於水深火熱的加班 debug 地獄中，不需要 mutate 的時候就不要 mutate，寫每個 function 都想一下「這真的有必要 mutate 嗎？」

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