call-protocol.rb

class Foo
  def bar(arg)
    arg.call
  end
end

class X
  def call
    "Normal class"
  end
end

Y = Class.new do
  def call
    "Class using Class.new"
  end
end

Z = Proc.new do
  "Proc!"
end

class MethodObject
  def foo
    "No call defined!"
  end
end

puts Foo.new.bar(X.new)
"Normal class"

puts Foo.new.bar(Y.new)
"Class using Class.new"

puts Foo.new.bar(Z)
"Proc!"

puts Foo.new.bar(lambda { "Lambda!" })
"Lambda!"

puts Foo.new.bar(-> { "Stabby lambda!" })
"Stabby lambda!"

puts Foo.new.bar(MethodObject.new.method(:foo))
"No call defined!"