What does the equal ('=') symbol do when put after the method name in a method definition?

I saw this in a screencast and was just wondering what the '=' symbol does in this case.

def express_token=(token)
...
end

I would understand if it were something like this -

def express_token(token = nil) 

The above (second code snippet) means setting nil as the default value of the tokens parameter. However, in the first code snippet, '=' is outside the brackets.

That snippet defines a Virtual Attribute (or a "setter" method) so that "express_token" looks like an attribute, even though it's just the name of the method. For example:

class Foo
  def foo=(x)
    puts "OK: x=#{x}"
  end
end
f = Foo.new
f.foo = 123 # => 123
# OK: x=123

Note that the object "f" has no attribute or instance variable named "foo" (nor does it need one), so the "foo=" method is just syntactic sugar for allowing a method call that looks like an assignment. Note also that such setter methods always return their argument, regardless of any return statement or final value.

If you're defining a top-level setter method, for example, in "irb", then the behavior may be a little confusing because of the implicit addition of methods to the Object class. For example:

def bar=(y)
  puts "OK: y=#{y}"
end
bar = 123 # => 123, sets the variable "bar".
bar # => 123
Object.new.bar = 123 # => 123, calls our method
# OK: y=123
Object.public_methods.grep /bar/ # => ["bar="]

Those methods let you set instance vars in a more indirect way: imagine you have a class Person

class Person < ActiveRecord::Base
  attr_accessible :first_name, :last_name

  def full_name
    [@first_name, @last_name].join
  end

  def full_name=(name)
    @first_name, @last_name = name.split(" ")
  end
end

Then you can do something like this:

p = Person.new
p.full_name = "John Doe"
p.first_name # => "John"
p.last_name # => "Doe"
p.full_name # => "John Doe"

Let's take a look at the following example:

class NewDog  
  def initialize(breed)  
    @breed = breed  
  end

  # create reader only
  attr_reader :breed, :name 

  # setter method  
  def set_name(nm)  
    @name = nm  
  end  
end  

nd = NewDog.new('Doberman')  
nd.set_name('Benzy')  
puts nd.name  

If you refactor the setter method to this:

def name=(nm)  
  @name = nm  
end 

other programmers know that the name= method behaves as a setter method. Also as show by @maerics it behaves like a virtual attribute.

The result looks like this:

class NewDog  
  def initialize(breed)  
    @breed = breed  
  end  

  # create reader only  
  attr_reader :breed, :name 
  
  # setter method  
  def name=(nm)  
    @name = nm  
  end  
end  
  
nd = NewDog.new('Doberman')  
nd.name = 'Benzy'  
puts nd.name 

Its actually part of the name of the function. So it's a setter, in case you need separate functionality than default for getters and setters.