Prevent loading if dependencies are missing

While the :after keyword delays loading until the dependencies are loaded, the somewhat simpler :requires keyword never loads the package if the dependencies are not available when the use-package declaration is evaluated. In this context, “available” means that foo is available if (featurep 'foo) evaluates to a non-nil value. For example:

(use-package abbrev
  :requires foo)

This is the same as:

(use-package abbrev
  :if (featurep 'foo))

As a convenience, a list of such packages may be specified:

(use-package abbrev
  :requires (foo bar baz))

For more complex logic, such as that supported by :after, simply use :if and the appropriate Lisp expression.