From: Joel VanderWerf <vjoel@...>
Date: 2006-10-26T15:53:30+09:00
Subject: Re: What's the difference between send and instance_eval?


Putting the comments in order...

 > Joel VanderWerf wrote:
 >> Another case, showing the real point of #send:
 >>
 >> msg = :reverse
 >> a = [1,2,3]
 >> b = a.send msg
 >> p b
 >>
 >> __END__
 >>
 >> Output:
 >>
 >> [3, 2, 1]
 >>
 >> There's no easy equivalent with #instance_evaland blocks (you could
 >> mess with strings, though).


michele wrote:
> This works:
> 
> msg = :reverse
> a = [1,2,3]
> a.instance_eval(msg.to_s)


That's true, but only because I chose a bad example. Here's a better one:

h = {1=>2}
msg = [:concat, [4, 5, 6, IO, String, Kernel, h]]
a = [1,2,3]
p a.send(*msg)   # ==> [1, 2, 3, 4, 5, 6, IO, String, Kernel, {1=>2}]
p a.instance_eval(msg.to_s) # ==> NameError

To make the instance_eval work here, you would have to find a way to 
turn the argument array into a string that, when eval-ed, is that same 
array. (It's possible, but painful, and you lose the identity of the 
hash h.)

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