Thought experiment to understand blocks and arguments better.

foo.rb

class Foobar
  def to_s
    "wah, wah"
  end
end
@foobar = Foobar.new

def baz who, foobar
  puts [who, foobar].join " says "
end

def foo *args, &block
  @who = args.first || "default"
  puts block_given?.inspect
  puts instance_variables.inspect
  if block_given?
    yield(@foobar)
  else
    baz @who, "NO BLOCK"
  end
end

puts "---- foo"
foo

puts "----- foo :bar"
foo :bar

puts "----- foo :bar do"
foo :bar do
  baz @who, "nada"
end

puts "----- foo do"
foo do
  baz @who, "zippo"
end

puts "----- foo :bar do |x|"
foo :bar do |x|
  baz @who, x
end

puts "----- foo do |x|"
foo do |x|
  baz @who, x
end

output.txt

---- foo
false
[:@foobar, :@who]
default says NO BLOCK
----- foo :bar
false
[:@foobar, :@who]
bar says NO BLOCK
----- foo :bar do
true
[:@foobar, :@who]
bar says nada
----- foo do
true
[:@foobar, :@who]
default says zippo
----- foo :bar do |x|
true
[:@foobar, :@who]
bar says wah, wah
----- foo do |x|
true
[:@foobar, :@who]
default says wah, wah