From: Gregory Brown <gregory.t.brown@...>
Date: 2007-06-24T07:12:20+09:00
Subject: Ruby 1.9 : What is the use for Proc#yield

I'm reading through Mauricio's list of changes[0] to Ruby 1.9 and
having trouble with understanding the point of Proc#yield

---

Invokes the block, setting the block's parameters to the values in
 params in the same manner the yield statement does.

    a_proc.yield(9, 1, 2, 3)   #=> [9, 18, 27]
    a_proc.yield([9, 1, 2, 3]) #=> [9, 18, 27]
    a_proc = Proc.new {|a,b| a}
    a_proc.yield(1,2,3)	  # => [1]

---

It seems like this is the same as Proc#[] or Proc#call

>> VERSION
=> "1.8.4"
>>  a_proc = Proc.new {|a, *b| b.collect {|i| i*a }}
=> #<Proc:0x00317930@(irb):8>
>> a_proc[9,1,2,3]
=> [9, 18, 27]
>> a_proc[[9,1,2,3]]
=> [9, 18, 27]
>> a_proc = Proc.new { |a,b| a }
=> #<Proc:0x0030a44c@(irb):11>
>> a_proc[1,2,3]
=> 1

Mauricio says "Proc#yield was added (also NilClass#yield which raises
a LocalJumpError so you can use it on &block)."

The best 'feature' I can think of is that this lets you do

def something(&block)
   block.yield(1,2,3)
rescue LocalJumpError
   puts "was expected a block"
end

But I'm not sure I'm convinced of how cool that is.  Am I missing something?