ruby hash 默认值的问题

参考：http://stackoverflow.com/questions/16159370/ruby-hash-default-value-behavior

使用ruby hash 默认值为空数组，向key 对应的value 追加值然后去get一个不存在的key 时候发现value为 一个非空的arry，不是默认值[]

具体使用示例如下：

 One default Array with mutation

 hsh = Hash.new([])

 hsh[:one] << 'one'
 hsh[:two] << 'two'

 hsh[:nonexistent]
 # => ['one', 'two']
 # Because we mutated the default value, nonexistent keys return the changed value

 hsh
 # => {}
 # But we never mutated the hash itself, therefore it is still empty!
 One default Array without mutation

 hsh = Hash.new([])

 hsh[:one] += ['one']
 hsh[:two] += ['two']
 # This is syntactic sugar for hsh[:two] = hsh[:two] + ['two']

 hsh[:nonexistant]
 # => []
 # We didn't mutate the default value, it is still an empty array

 hsh
 # => { :one => ['one'], :two => ['two'] }
 # This time, we *did* mutate the hash.
 A new, different Array every time with mutation

 hsh = Hash.new { [] }
 # This time, instead of a default *value*, we use a default *block*

 hsh[:one] << 'one'
 hsh[:two] << 'two'

 hsh[:nonexistent]
 # => []
 # We *did* mutate the default value, but it was a fresh one every time.

 hsh
 # => {}
 # But we never mutated the hash itself, therefore it is still empty!

47 hsh = Hash.new {|hsh, key| hsh[key] = [] }
 # This time, instead of a default *value*, we use a default *block*
 # And the block not only *returns* the default value, it also *assigns* it

 hsh[:one] << 'one'
 hsh[:two] << 'two'

 hsh[:nonexistent]
 # => []
 # We *did* mutate the default value, but it was a fresh one every time.

 hsh
 # => { :one => ['one'], :two => ['two'], :nonexistent => [] }