From: David Alan Black <dblack@...>
Date: 2002-07-14T09:14:26+09:00
Subject: Re: instance_eval

Hi --

On Sun, 14 Jul 2002, John wrote:

> Hi all,
>
> I am trying to add a method dynamically.  I can do the following just fine:
>
> def create
>     str = "def hello (*args) puts 'test' end"
>     self.instance_eval (str)
> end
>
> This adds the hello method.  But I want to pass a block to test so that it
> executes the block instead of the "puts 'test'".
>
> I tried the following after searching some previous posts:
>
> def create (&block)
>     str = "def hello (*args) block.call end"
>     self.instance_eval (str)

I believe that "self.instance_eval" is essentially the same as plain
"eval", because instance_eval effectively sets the value of "self",
temporarily, to its receiver -- so setting it to "self" doesn't change
anything.  (Stay tuned for others pointing out possible subtleties
that I'm overlooking :-)

Here's a little example of switching "self" on the fly, just for
contrast:

  class Thing
    def demo
      puts "I am a #{type}"                    # => I am a Thing
      s = "string"
      s.instance_eval 'puts "I am a #{type}"'  # => I am a String
    end
  end

> end
>
> But to be honest I don't really know what I am doing there. ;-)  I realize
> that instance_eval can take a block, but what name would the method be?

I'm almost, but not quite, following your description....  Which
method do you mean?  Could you explain a little more about the
context?  I think there might be useful things you could do along the
lines of, maybe, a hash of Proc objects, or something like that, but
I'm not sure what the best fit would be.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav