Which one is a Ruby deprecated proc?

Question

In the book Programming Ruby: The Pragmatic Programmers Guide by Dave Thomas with Chad Fowler and Andy Hunt, regarding the creation of Procs there is a footnote that states:

"There’s actually a third, proc, but it is effectively deprecated."

I could not find which way is this. I am aware of the following ways to create a Proc:

1

b = lambda { | msg | puts "msg: #{msg}" }
b.call("hi")

2

def create_block_object(&block)
  block
end
b = create_block_object{ |msg| puts "msg: #{msg}" }
b.call("hello")

3

b = Proc.new { |msg| puts "msg: #{msg}"}
b.call("hey")

I want to know the fourth way and would be glad if somebody would give me an answer.

Answer 1

The book you are referring to is on Ruby 1.8.

In that version of Ruby, lambda and procs are effectively aliases, while Proc is a different beast. This is obviously misleading, which is why it is not recommended that you use proc as in

prc = proc {|x, y| puts x + y}

This syntax is considered deprecated and it is recommended using lambda in this case.

This is no longer valid for later versions of Ruby, starting with 1.9.

Answer 2

This is another syntax for lambdas:

b = ->(msg) { puts "msg: #{msg}" }
b.call("hi")

Answer 3

prc = proc {|x| x*x} has been depreciated.

This syntax was process was predominantly used in Ruby-1.8.7 versions.