From: David Alan Black <dblack@...>
Date: 2002-01-20T21:03:01+09:00
Subject: Re: $_ as default parameter for a function

Hello --

On Sun, 20 Jan 2002, Thomas wrote:

> Could you, or someone else explain whats going on below?
[...]
> Mathieu Bouchard <matju@sympatico.ca> wrote in message
>
> > Yes, $_ is a local, and yes, built-in methods have access to the $_ of the
> > caller. Here's an excerpt from MetaRuby 0.7 in module HollowString:
> >
> > 	# it's your own fault for using ~
> > 	def ~
> > 		# setting up quantum superposition
> > 		r,w = IO.pipe
> > 		if not fork then
> > 			set_trace_func(proc{|*a|
> > 				w.puts(eval("$_", a[4]).inspect)

[...]

The basic idea is that #set_trace_func takes a proc as an argument,
and to that proc it provides an array of parameters when certain
run-time events occur, such as returning from a method call.  The
fifth parameter (at [4]) is the binding at the time of the event.  The
idea is to eval $_ in the context of that binding (using the binding
as the second argument to eval).

Here's a little self-contained program that demonstrates this:

  def show_me
    my_proc = proc do |*params|
      print "Here's the $_ right after the 'event': "
      puts eval("$_.inspect",params[4])
    end

    puts "Here's my $_: #{$_.inspect}"
    set_trace_func(my_proc)

    # Now the 'return' event (returning from this method) will
    # trigger the proc we've defined for #set_trace_func.

  end

  $_ = 100
  show_me

  # Output:
  #   Here's my $_: nil
  #   Here's the $_ right after the 'event': 100


David

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