Re: cannot get blocks working

On Wed, Mar 13, 2013 at 7:58 AM, Josh Cheek <[e-mail]> wrote:

> On Wed, Mar 13, 2013 at 7:30 AM, Aashish Kiran <[e-mail]>wrote:
>
>> Hi,
>> Below is the piece of code. Can anyone help in getting expected output.
>>
>>
>> ------------------------------------------------------------
>>
>> class XmlDoc
>>   def method_missing(name, *args, &block)
>>     self.class.class_eval do
>>       define_method(name) do |*args, &block|
>>         if block_given?
>>           temp = "<#{name}#{temp}>" + block.call + "</#{name}>"
>>         else
>>           "<#{name}></#{name}>"
>>         end
>>       end
>>     end
>>     send(name, *args, &block)
>>   end
>> end
>>
>> x = XmlDoc.new
>>
>>
>> p '1111111111111111111'
>> p x.hello
>> #expected output "<hello></hello>"
>>
>> p x.hello{"dolly"}
>> #expected output   "<hello>dolly</hello>"
>>
>> -----------------------------------------------------------------------
>>
>> Thanks,
>> Aashish
>>
>> --
>> Posted via http://www.ruby-forum.com/.
>>
>>
> Change `block_given?` to `block`. The block_given? macro is checking if
> method_missing received a block. Since it didn't the first time you called
> x.hello, it never will (that scope was enclosed).
>
> Also, you should move the method definition out to a helper method,
> because the methods that this defines will capture the block sent to
> method_missing, which in turn capture their environments. Since they're
> becoming methods, they won't be garbage collected, so you run the risk of
> memory leaks. Moving it to a helper method will prevent capturing of the
> block.
>
> Also, when doing metaprogramming, it's usually best to use __send__
> instead of send (to avoid potential namespace conflicts).
>
> I would probably write it like this (well, at this point in my life, I
> probably wouldn't write anything this dynamic, but whatever)
>
>
> class XmlDoc
>   def self.define_node(name)
>     define_method name do |*args, &block|
>       block ||= lambda { '' }
>       "<#{name}>" + block.call + "</#{name}>"
>     end
>   end
>
>   def method_missing(name, *args, &block)
>     self.class.define_node name
>     __send__ name, *args, &block
>   end
> end
>
> x = XmlDoc.new
> x.hello           # => "<hello></hello>"
> x.hello{"dolly"}  # => "<hello>dolly</hello>"
>
>
> -Josh
>

I guess I would also interpolate the block.call, which would allow nil to
be returned, which would allow you to get rid of the reassignment to the
empty lambda.

    define_method name do |*args, &block|
      "<#{name}>#{block.call if block}</#{name}>"
    end