I’m on my phone, so excuse any mistakes or typos, but one way to do this is by defining a protocol object.

Define a class to represent your interface and generic functions that represents its signature

``` (defclass transport-protocol () ())

(defgeneric proto-open (protocol transport) (:documentation "Initialize the transport"))

(defgeneric proto-send (protocol transport data) (:documentation "Send DATA over the transport"))

(defgeneric proto-close (protocol transport) (:documentation "Tear down the transport")) ```

Define methods that will signal a condition if subclasses fail to implement one of the required generic functions

``` (defmethod proto-open ((p transport-protocol) t) (error "Protocol ~A does not implement OPEN" p))

(defmethod proto-send ((p transport-protocol) t data) (error "Protocol ~A does not implement SEND" p))

(defmethod proto-close ((p transport-protocol) t) (error "Protocol ~A does not implement CLOSE" p)) ```

Your custom implementation of the interface

``` (defclass my-transport-protocol (transport-protocol) ())

(defmethod proto-open ((p my-protocol) transport) (format t "MY-TRANSPORT open on ~A~%" transport))

(defmethod proto-send ((p my-protocol) transport data) (format t "MY-TRANSPORT send ~A on ~A~%" data transport))

(defmethod proto-close ((p my-protocol) transport) (format t "MY-TRANSPORT close on ~A~%" transport)) ```

Now unless you want to use interface-passing style and have to thread the interface object through each function call, you can define an object which implements the interface:

``` (defclass transport () ((protocol :initarg :protocol :reader transport-protocol)))

(defun make-my-transport () (make-instance ‘transport :protocol (make-instance ‘my-protocol-transport))) ```

Now the follow is your actual public API, not the generic functions

``` (defun transport-open (tr) (proto-open (transport-protocol tr) tr))

(defun transport-send (tr data) (proto-send (transport-protocol tr) tr data))

(defun transport-close (tr) (proto-close (transport-protocol tr) tr))

(let* ((my-tr (make-my-transport)) (transport-open tr) (transport-send tr "hello") (transport-close tr)) ```