In Ruby, there is always an implicit receiver for all method calls. The language keeps a reference to the current implicit receiver stored in the variable self. Certain language keywords like class and module will change what self points to. Understanding these behaviors is very helpful in mastering the language.

For example, when you first open irb

irb(main):001:0> self
=> main

In this case the main object is the implicit receiver (see http://stackoverflow.com/a/917842/417872 (opens new window) for more about main).

You can define methods on the implicit receiver using the def keyword. For example:

irb(main):001:0> def foo(arg)
irb(main):002:1> arg.to_s
irb(main):003:1> end
=> :foo
irb(main):004:0> foo 1
=> "1"

This has defined the method foo on the instance of main object running in your repl.

Note that local variables are looked up before method names, so that if you define a local variable with the same name, its reference will supersede the method reference. Continuing from the previous example:

irb(main):005:0> defined? foo
=> "method"
irb(main):006:0> foo = 1
=> 1
irb(main):007:0> defined? foo
=> "local-variable"
irb(main):008:0> foo
=> 1
irb(main):009:0> method :foo
=> #<Method: Object#foo>

The method method can still find the foo method because it doesn't check for local variables, while the normal reference foo does.

When you define a class or module, the implicit receiver becomes a reference to the class itself. For example:

puts "I am #{self}"
class Example
  puts "I am #{self}"
end

Executing the above code will print:

"I am main"
"I am Example"

Most Ruby code utilizes the implicit receiver, so programmers who are new to Ruby are often confused about when to use self. The practical answer is that self is used in two major ways:

1. To change the receiver.

Ordinarily the behavior of def inside a class or module is to create instance methods. Self can be used to define methods on the class instead.

class Foo
  def bar
    1
  end

  def self.bar
    2
  end
end

Foo.new.bar #=> 1
Foo.bar #=> 2

2. To disambiguate the receiver

When local variables may have the same name as a method an explicit receiver may be required to disambiguate.

Examples:

class Example
  def foo
    1
  end

  def bar
    foo + 1
  end

  def baz(foo)
    self.foo + foo # self.foo is the method, foo is the local variable
  end

  def qux
    bar = 2
    self.bar + bar # self.bar is the method, bar is the local variable
  end 
end

Example.new.foo    #=> 1
Example.new.bar    #=> 2
Example.new.baz(2) #=> 3
Example.new.qux    #=> 4

The other common case requiring disambiguation involves methods that end in the equals sign. For instance:

class Example
  def foo=(input)
    @foo = input
  end

  def get_foo
    @foo
  end

  def bar(input)
    foo = input # will create a local variable
  end

  def baz(input)
    self.foo = input # will call the method
  end
end

e = Example.new
e.get_foo #=> nil
e.foo = 1
e.get_foo #=> 1
e.bar(2)
e.get_foo #=> 1
e.baz(2)
e.get_foo #=> 2