Eigenclass: why this syntax?

Ruby
1

Hi. Consider this syntax:

module Foo
class << self
attr_accessor :use_colours
end
end

Now, it works to do this:

Foo.use_colours = true
puts Foo.use_colours

Foo.use_colours = false
puts Foo.use_colours

My questions:

  • Why is the syntax “class << self” used? For an array, << means to
    append. I am unsure what this means for class, or why this syntax was
    used.

  • Also, in the above example, is it possible to query the @use_colours
    variable for when instance methods of module Foo are mixed into a class?
    I would like to mixin instance variables that are defined on the
    instance level of the module, inside methods defined in that module. So
    yeah, basically I want to use a module like a class, but it seems ruby
    gives obstacles to that attempt.

2

On Wed, Apr 3, 2013 at 7:36 PM, Marc H. [email protected]
wrote:

  • Also, in the above example, is it possible to query the @use_colours
    variable for when instance methods of module Foo are mixed into a class?
    I would like to mixin instance variables that are defined on the
    instance level of the module, inside methods defined in that module. So
    yeah, basically I want to use a module like a class, but it seems ruby
    gives obstacles to that attempt.

Not sure I understand the question, but it sounds like you want this:

module A
attr_accessor :whatev
end

include it in the singleton class

class B
class << self
include A
end
self.whatev = ‘something’
whatev # => “something”
end

shorthand version of the above

class C
extend A
self.whatev = ‘something’
whatev # => “something”
end

3

On Wed, Apr 3, 2013 at 7:36 PM, Marc H. [email protected]
wrote:

  • Why is the syntax “class << self” used? For an array, << means to
    append. I am unsure what this means for class, or why this syntax was
    used.

I can’t speak for why this particular notation was used, obviously,
but the ‘<<’ operation is used similarly in other places as well, not
just appending to an array. I look at it as basically a generic
append; in the case of the class’s eigenclass, you’re appending
something to it (perhaps creating the eigenclass in the first place).