From: Ross Bamford <rossrt@...>
Date: 2006-03-16T07:07:39+09:00
Subject: Re: executing a block in the context of receiver

On Thu, 2006-03-16 at 05:57 +0900, dblack@wobblini.net wrote:
> Hi --
> 
> On Wed, 15 Mar 2006, Ross Bamford wrote:
> 
> > On Wed, 2006-03-15 at 00:20 +0900, Mark Volkmann wrote:
> >> In the following code the do_this instance method of the foo object
> >> calls yield to run the block that is passed to it.
> >>
> >> foo.do_this(p1, p2) {
> >>   do_that(p3, p4)
> >> }
> >>
> >> Is there any way to make it so that do_that is executed in the context
> >> of the foo object, i.e. it acts as though it was invoked with
> >> foo.do_that(p3, p4)?  I know I could just say "foo.do_that(p3, p4)",
> >> but I was wondering if I could avoid that.
> >
> > If you're writing 'foo' then you can do:
> >
> > 	class Foo
> > 	  def do_this(&blk)
> > 	    instance_eval &blk
> > 	  end
> >
> > 	  def do_that
> > 	    puts "Doing that..."
> > 	  end
> > 	end
> >
> > 	f.do_this { do_that }
> > 	# (prints) Doing that...
> >
> > 	# can use this form, too
> > 	f.do_this { |foo| foo.do_that }
> > 	# (prints) Doing that...
> 
> But only if you yield self from do_this (just to clarify).  And
> initialize f, but I guess that was implied :-)
> 

(Oops, overzealous cut/paste job :) )

Probably irrelevant, but from what I can gather, instance_eval will pass
the new self in as a block parameter, too:

	Object.new.instance_eval { |me| self == me }
	# => true

Though this doesn't seem to be documented...?

> I like the second form better.  Just having self change mysteriously
> for the duration of a block always strikes me as a bit of an
> obfuscation.
> 

I like instance_eval for those cool DSL style things, but usually I'm
with you on that one.

-- 
Ross Bamford - rosco@roscopeco.REMOVE.co.uk