Add new Lint/EmptyClass cop

[fatkodima]

This cop checks for metaclasses without a body.

  # bad
  class Foo
    class << self
    end
  end

  class << obj
  end

  # good
  class Foo
    class << self
      attr_reader :bar
    end
  end

  class << obj
    attr_reader :bar
  end

[bbatsov]

I'm wondering if this shouldn't be part of a more generic EmptyClass cop which can check for empty metaclass and an empty class that doesn't inherit from anything.

[fatkodima]

Ha, I was 100% sure that there is already a cop for empty classes 😄
Then it should be also extended for modules, I think?

Empty modules and metaclasses are always useless, while empty classes are sometimes used and useful (for example, empty classes for some results class ResultSuccess; end, etc).

Maybe there is a better name for the cop then, if we will combine checks for classes, modules and metaclasses?

[marcandre]

Empty modules and metaclasses are always useless,

Metaclasses: yes. Modules: not necessarily:

# main.rb:
module MyNameSpace
end

require_relative 'foo'
require_relative 'bar'
# ...

# foo.rb:
class MyNameSpace::Foo
end

# same with bar

[bbatsov]

Still, I've never seen a namespace module created empty, but I guess someone might be using them in this way. We can always have a flag for this.

[marcandre]

Still, I've never seen a namespace module created empty

Here's one I wrote myself.

I have an issue with cops that flag code that is either valid or innocuous, especially when rare. The extremely negative feelings it might generate can completely outweigh the (potentially negligible) gain. Remember, you've never seen an empty module, so clearly it's not a common mistake that gains much from being flagged and corrected.

[fatkodima]

Modules: not necessarily:

Ah, forgot about this case. I have seen a couple of examples like this.

Thanks for the link to keynote, haven't seen that. It was fun 😄 Personally, I find Lint and spacing-like cops the most useful for everyone. Style, Naming, etc cops can not satisfy everyone, so projects configure them for their specific needs.

The cops I absolutely hate, when enabled, are Style/GuardClause and much-much less than previous -Style/IfUnlessModifier.

The first of them leads to code like

return ... if ...
return ... unless ...
3 more times return ...
do_something

Even, forcing me to write something like

def method
  return unless condition

  do_something
end

drives me mad a little.

[fatkodima]

Changed the cop's name to EmptyClass. And now this cop checks for classes in addition to metaclasses. Decided against checking for empty modules, at least for now.

[bbatsov]

Simple is good. :-) Thanks!