From: Ross Bamford <rosco@...>
Date: 2006-01-03T05:32:57+09:00
Subject: Implicit block parameter?

Hi,

Probably this has been considered before, but I'll ask anyway.

Before I used Ruby, I used Groovy. In some ways they are (well, were  
anyway) quite similar, and one feature that I found cool in Groovy that I  
often miss in Ruby is the implicit (/default/whatever?) block parameter,  
'it':

	[1,2,3].each { puts it }

AFAIR this was only provided when no 'it' existed in scope, and the block  
had a single argument passed when none were declared. Maybe this would  
interfere with Ruby's warnings about block parameter mismatch, or maybe  
the implementation doesn't allow for it, but I just wondered if it might  
be possible, because I notice I do:

	[1,2,3].each { |it| puts it }

and it bugs me a little bit :D

I did try hacking it from the Ruby side and came up with a halfway  
solution using instance variables and the Binding extension from Rubyforge:

	require 'rubygems'
	require 'extensions/binding'

	def itproc(&blk)
	  class << blk
	    def [](*args)
	      if args.length == 1
	        begin
	          old = binding[:@it]
	          binding[:@it] = args[0]
	          super
	        ensure
	          binding[:@it] = old
	        end
	      else
	        super
	      end
	    end

	    alias :call :[]
	  end
	  blk
	end

But of course this doesn't work with regular procs (doing it on Proc  
causes a segfault here, I guess because extensions uses procs itself to do  
the binding stuff?) and of course it doesn't happen with yielded blocks,  
even when passed from procs:

	pr = itproc { puts "@it = #{@it.inspect}" }
	pr2 = itproc { |one| puts "@it = #{@it.inspect}; one = #{one.inspect}" }
	pr3 = itproc { |a, b| puts "@it = #{@it.inspect}; a = #{a.inspect}; b =  
#{b}" }

	# Works
	puts "Call"
	pr.call('Hello')
	pr2.call('Hello')
	pr3.call('He','llo')

	# Works
	puts "\n[]"
	pr['Bye']
	pr2['Bye']
	pr3['Bye','Bye']

	# Doesn't work through yield though :(
	puts "\nYield"
	[1,2,3].each &pr
	[1,2,3].each &pr2
	[1,2,3].each &pr3

Anyway, it's a bit of an abuse of instance vars I guess, and obviously  
doesn't do the job properly - I wonder if anyone else has thought about  
this, and whether it's something that doesn't already exist in Ruby itself  
for a reason?

Cheers,

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