Creating new component types - asdf Manual
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#### 3.2.3 Creating new component types

New component types are defined by subclassing one of the existing component classes and specializing methods on the new component class.

FIXME: this should perhaps be explained more throughly, not only by example ...

As an example, suppose we have some implementation-dependent functionality that we want to isolate in one subdirectory per Lisp implementation our system supports. We create a subclass of `cl-source-file`:

         (defclass unportable-cl-source-file (cl-source-file)
             ())

A hypothetical function `system-dependent-dirname` gives us the name of the subdirectory. All that's left is to define how to calculate the pathname of an `unportable-cl-source-file`.

         (defmethod component-pathname ((component unportable-cl-source-file))
           (let ((pathname (call-next-method))
                 (name (string-downcase (system-dependent-dirname))))
             (merge-pathnames
              (make-pathname :directory (list :relative name))
              pathname)))

The new component type is used in a `defsystem` form in this way:

         (defsystem :foo
             :components
             ((:file "packages")
              ...
              (:unportable-cl-source-file "threads"
               :depends-on ("packages" ...))
              ...
             )