From: John <nojgoalbyspam@...>
Date: 2002-07-14T14:21:17+09:00
Subject: Re: instance_eval

"David Alan Black" <dblack@candle.superlink.net> wrote in message
news:Pine.LNX.4.30.0207131952450.9723-100000@candle.superlink.net...
> 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
>
>

I am sorry, my examples were a little off. These should show it better.

Original attempt that works but without a block parameter:

  def create (methodName)
      str = "def " + methodName + " (*args) puts 'test' end"
      self.instance_eval (str)
  end

My attempt with a block:

  def create (methodName, &block)
      str = "def " + methodName + " (*args) block.call end"
      self.instance_eval (str)
  end

I want to call create something like this:

  obj.create "newMethod" { puts "I am a new method" }
  obj.create "anotherMethod"  { puts "I am another method" }

and when I call newMethod it outputs the text "I am a new method".
and when I call anotherMethod it outputs the text "I am another method".

I could do this with a hash of proc objects like you suggested, but I would
prefer to do it using the instance_eval method as I think its cleaner (less
member variables), and I also want to learn if there is a way to associated
a block with a new method and name it.

I would also rather not pass a string for the method in to the create call.
I want to pass a block, mainly cause I want to see how to make use of the
block feature.

Make sense?

John.