From: Rick DeNatale <rick.denatale@...>
Date: 2010-02-04T23:10:58+09:00
Subject: Re: hooking subscript operations in a hash

On Thu, Feb 4, 2010 at 8:54 AM, Marnen Laibow-Koser <marnen@marnen.org> wrote:
> Ralph Shnelvar wrote:
>> RN> Is env['X'] = 'Y' the same as env[]=('X','Y') ?
>> RN> Since you defined a method with def []= so I think env[]=('X','Y')
>> is
>> RN> easier for understanding.
>>
>> First ... I think you meant
>>   env[]={'X', 'Y'}
>> (braces instead of parenthesis)
>
> But you are wrong.  It's just like any other method call:
> object.method(args)
> so
> env.[]=('X', 'Y')
>
> But never do it this way.  env['X'] = 'Y' is far clearer.
>
>>
>> The first form
>>   env[]={'X', 'Y'}
>> means
>>   Create
>
> No.  It won't create env.
>
>> and/or replace a hash table
>
> Hash, not hash table.
>
>> called "env" and stuff it with an
>> index of 'X' and a value of 'Y'
>>
>> The second form
>>   env['X'] = 'Y'
>> means
>>   Take an existing hash table called "env" and add/replace the index 'X'
>> in that table with the value 'Y'
>
> No.  The two forms are 100% equivalent.

Actually more like only 99 44/100% equivalent.

In cases where a 'setter' method returns a value other than the
argument there's a subtle diference:

class MyHash
  def []=(key, value)
    "Hey Look, No Hands!"
  end
end

mh = MyHash.new

a = mh[1] = 2
a # => 2

b = mh.[]=(1, 2)
b # => "Hey Look, No Hands!"

Ruby sees mh[index]=value as an assignment, and compiles it to the equivalent of

mh,[]=(index, value)
a = value


-- 
Rick DeNatale

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