From: Logan Capaldo <logancapaldo@...>
Date: 2007-07-05T12:21:26+09:00
Subject: Re: Isn't hash default value behavior just a little bizarre???

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On 7/4/07, Farhad Farzaneh <ff@onebeat.com> wrote:
>
> Try this:
>
> >> x = Hash.new([])
> => {}
> >> x[:test]
> => []
> >> x[:test] << 'this'
> => ["this"]
> >> x[:bar]
> => ["this"]
>
> Huh?  It seems that when I index a hash with a new key, it returns the
> Hash's default value object, not a copy of it.  So if you modify that,
> it modifies the default value, which then effects the default value for
> every other new key.
>
> I don't know, I wouldn't call this the "principle of least surprise"...
> I was plenty surprised!
>
> --
> Posted via http://www.ruby-forum.com/.
>
>
Copying is an interesting thing. For many objects there is a sane, obvious
way to make a copy. But for many others it is not so clear. Consider for
instance, Hash.new($stdin). What should it mean, in this context, to "copy"
$stdin? Or consider Hash.new(some_very_large_object). Do you want a copy all
the time? Luckily, Hash does have a mechanism to acheive what you want:

>> x = Hash.new { |h,k| h[k] = [] }
=> {}
>> x[:test]
=> []
>> x[:test] << 'this'
=> ["this"]
>> x[:bar]
=> []

POLS also is matz.'s POLS.

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