What do `def +@` and `def -@` mean?

Question

In this Haskell-like comprehensions implementation in Ruby there's some code I've never seen in Ruby:

class Array
  def +@
    # implementation
  end

  def -@
    # implementation
  end
end

What do def +@ and def -@ mean? Where to find (semi-)official informations about them?

Answer 1

They are unary + and - methods. They are called when you write -object or +object. The syntax +x, for example, is replaced with x.+@.

Consider this:

class Foo
  def +(other_foo)
    puts 'binary +'
  end

  def +@
    puts 'unary +'
  end
end

f = Foo.new
g = Foo.new

+ f   
# unary +

f + g 
# binary +

f + (+ g) 
# unary +
# binary +

Another less contrived example:

class Array
  def -@
    map(&:-@)
  end
end

- [1, 2, -3]
# => [-1, -2, 3]

They are mentioned here and there's an article about how to define them here.