It's shorthand for
foo is an object with a
to_proc method, then you can pass it to a method as
&foo, which will call
foo.to_proc and use that as the method's block.
Symbol#to_proc method was originally added by ActiveSupport but has been integrated into Ruby 1.8.7. This is its implementation:
Proc.new do |obj, *args|
obj.send self, *args