From: Joel VanderWerf <vjoel@...>
Date: 2006-10-26T06:38:41+09:00
Subject: Re: Pass block instead of here document?

James Edward Gray II wrote:
> On Oct 25, 2006, at 2:54 PM, dblack@wobblini.net wrote:
> 
>> It's easy, but it can also be a bit obfuscating.  For example:
>>
>>   class C
>>     def initialize(thing)
>>       @thing = thing
>>     end
>>
>>     def tell(&block)
>>       instance_eval(&block)
>>     end
>>   end
>>
>>   c = C.new("Hi")
>>
>>   @thing = "Hello"
>>   c.tell do
>>     puts @thing        # Hi
>>   end
>>
>> So you get some perhaps unwanted variable-shadowing, and similarly
>> with method calls.
> 
> Greg Brown and I were playing around with a solution for the method call 
> issue at RubyConf.  Here's the code we came up with:
> 
> class C
>   def initialize(thing)
>     @thing = thing
>   end
> 
>   attr_reader :thing
> 
>   def tell(&block)
>     if block and block.arity == 1
>       block[self]
>     else
>       instance_eval(&block)
>     end
>   end
> end
> 
> def thing; "Hello" end
> 
> c = C.new("Hi")
> 
> c.tell do
>   thing       # => "Hi"
> end
> 
> c.tell do |obj|
>   thing       # => "Hello"
>   obj.thing   # => "Hi"
> end
> 
> I know people frown on the instance_eval() trick, but this seems to be 
> less of a problem.  You can just choose to use the variable when you 
> need it.

I dunno... the presence of "|obj|" changes the semantics of the rest of 
the block. It's so easy to comment out the |obj| and then suddenly 
methods are handled in a surprising way:

c.tell do |obj|
   thing       # => "Hello"
end

c.tell do #|obj|
   thing       # => "Hi"
end

Visually, it doesn't look like anything important has changed.

Seems like a bug magnet.

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407