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So Method#to_proc is basically totally bankrupt. It is now the most severely mocked typecast in the book. From RailsConf:

 [10, 20, 30].map &4.method(:+)
 ary.each &(hsh={}).method(:store)
 def initialize(hsh)
 hsh.each &method(:instance_variable_set)
 end

Clearly, fun stuff. But, you know, bankrupt.

If you don’t mind drawing a bit more blood, did you know you can use this stuff to kill (\w+)_with_index?

 %w[joe tim francis].map &with_index { |x| [x, index] }
 #=> [["joe", 0], ["tim", 1], ["francis", 2]]

Here’s how it works: the index accessor is added to the Proc. And each time we loop, the accessor is incremented.

It takes some glue, though. Proc#bind (from Rails, originally cloaker) is used to turn the block into a method of itself! Then with_index adds an incrementing wrapper over it. The Method#to_proc happens when you amp it into each.

 # from activesupport
 class Proc
 def bind(object)
 block, time = self, Time.now
 (class << object; self end).class_eval do
 method_name = "__bind_#{time.to_i}_#{time.usec}" 
 define_method(method_name, &block)
 method = instance_method(method_name)
 remove_method(method_name)
 method
 end.bind(object)
 end
 end
 # there are three interesting variables here:
 # blk:: the block you've passed in.
 # bound:: the same block, but now a method of itself.
 # wrap:: the wrapper proc which does the counting.
 def with_index(&blk)
 class << blk
 attr_reader :index
 def even; @index % 2 0 end
 def odd; @index % 2 1 end
 end
 i = 0
 bound = blk.bind(blk)
 wrap = proc do |a|
 blk.instance_variable_set(’@index’, i)
 x = bound[a]
 i += 1
 x
 end
 wrap
 end

Now, for some exercises:

 >> %w[joe tim francis].map &with_index { |x| [x, index] }
 => [["joe", 0], ["tim", 1], ["francis", 2]]
 >> %w[badger goat mule eagle shark].select &with_index { even }
 => ["badger", "mule", "shark"]

However, 1.9’s Enumerator#with_index is definitely less boggling and sloppy. If you have instance variables inside the block, they’ll be lookin in the block object for a home.