Why can't I pass a block to the proc in Ruby?

Question

Why can't I do something like this:

do_once = Proc.new { yield }
do_once.call { puts 1 }

irb throws LocalJumpError: no block given (yield)

Answer 1

yield applies to the block passed to the wrapping method context. In your case, I presume it is whichever method irb relies upon (lib/ruby/2.0.0/irb/workspace.rb:86 evaluate, if caller is anything to go by).

If you wrap it in a function it'll work, because you change the method context:

def do_stuff
  do_once = Proc.new { yield }
  do_once.call 
end

do_stuff { puts 1 }

Note the absence of block for do_once.call in the above: yield applies to the block passed to do_stuff, rather than to the block passed to do_once.

Alternatively, declare the block explicitly to avoid the use of yield altogether:

do_once = Proc.new { |&block| block.call }
do_once.call { puts 1 }

Answer 2

You can do:

do_once = Proc.new { |&block| block.call }
do_once.call { puts 1 }