What does +@ mean as a method in ruby

Question

I was reading some code and I saw something along the lines of

module M
  def +@
    self
  end
end

I was surprised that this was legal syntax, yet when I ran ruby -c on the file (to lint) it said it was valid. -@ was also a legal method name yet when I tried *@ or d@ both of those were illegal. I was wondering what +@ means and why is it legal?

Answer 1

Ruby contains a few unary operators, including +, -, !, ~, & and *. As with other operators you can also redefine these. For ~ and ! you can simply just say def ~ and def ! as they don't have a binary counterpart (e.g. you cannot say a!b).

However for - and + there is both a unary, and a binary version (e.g. a+b and +a are both valid), so if you want to redefine the unary version you have to use def +@ and def -@.

Also note that there is a unary version of * and & as well, but they have special meanings. For * it is tied to splatting the array, and for & it is tied to converting the object to a proc, so if you want to use them you have to redefine to_a and to_proc respectively.

Here is a more complete example showing all kinds of the unary operators:

class SmileyString < String
  def +@ 
    SmileyString.new(self + " :)")
  end

  def -@ 
    SmileyString.new(self + " :(")
  end

  def ~ 
    SmileyString.new(self + " :~")
  end

  def !
    SmileyString.new(self + " :!")
  end

  def to_proc
    Proc.new { |a| SmileyString.new(self + " " + a) }
  end

  def to_a
    [SmileyString.new(":("), self]
  end
end

a = SmileyString.new("Hello")
p +a                 # => "Hello :)"
p ~a                 # => "Hello :~"
p *a                 # => [":(", "Hello"]    
p !a                 # => "Hello :!"
p +~a                # => "Hello :~ :)"
p *+!-~a             # => [":(", "Hello :~ :( :! :)"]
p %w{:) :(}.map &a   # => ["Hello :)", "Hello :("]

In your example the Module just simply defines an unary + operator, with a default value of not doing anything with the object (which is a common behaviour for the unary plus, 5 and +5 usually mean the same thing). Mixing in with any class would mean the class immediately gets support for using the unary plus operator, which would do nothing much.

For example (using ruby <=2.2):

module M
  def +@
    self
  end
end

p +"Hello"     # => NoMethodError: undefined method `+@' for "Hello":String

class String
  include M
end

p +"Hello"     # => "Hello"

Note that in this example you can clearly see from the error message that the +@ method is missing from the class

Note that the above example will be different from Ruby 2.3, as the unary minus and plus are defined for Strings since that version, and they refer to returning a frozen and unfrozen string from the original.