From: Rick DeNatale <rick.denatale@...>
Date: 2009-05-26T03:13:01+09:00
Subject: Re: hash assignment [:x] vs ['x']

On Mon, May 25, 2009 at 1:56 PM, Bertram Scharpf
<lists@bertram-scharpf.de> wrote:
> Hi,
>
> Am Dienstag, 26. Mai 2009, 02:19:29 +0900 schrieb Mk 27:
>> bob['phone']
>> bob[:phone]
>>
>> That seems slightly weird, I had assumed the :form is just a shorthand
>> like %w(  ).  So -- is there anything more that I need to understand
>> about this distinction?
>
>  $ irb
>  irb(main):001:0> :phone.class
>  => Symbol
>  irb(main):002:0> 'phone'.class
>  => String
>
> and
>
>  irb(main):003:0> :phone.methods.sort - 'phone'.methods
>  => ["id2name", "to_int"]
>  irb(main):004:0> String.instance_methods.sort - Symbol.instance_methods
>  => [ ... (loads)]
>
> :phone.methods  ==  Symbol.instance_methods

Strings are mutable, Symbols are immutable.

There is only one instance of a Symbol with a given string 'value' so

(hash1 == hash2) iff (hash1.object_id == hash2.object_id)

This is the main property of a Symbol, it's used for keys in Hashes
where lookup performance is important (like the method hashes used
internally by Ruby), since it means that Hash#== is O(1) and doesn't
depend on the 'length'

The downside is that Symbols can't be garbage collected (at least for
the MRI implementation) in order to preserve the id/value invariant a
list of all string values which have been 'interned' when a Hash has
been created needs to be kept.

In Rails, it's common to see Symbol and String keys used
'interchangeably', but this is only possible because Rails provides a
HashWithIndifferentAccess class which actually uses string keys
internally (because of the GC issue) but allows either to be used as
key arguments to methods.

-- 
Rick DeNatale

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