notifier_4.rb

class Foo
  def call
    'hi'
  end
end

foo = proc { 'hi' }

Foo.new.call # 'hi'
foo.call # 'hi'

class Sayer
  def initialize(word_giver)
    @word_giver = word_giver
  end

  def say
    puts @word_giver.call
  end
end

# They're the same!
Sayer.new(Foo.new).say
Sayer.new(foo).say