Generic functions in Lisp

One defgeneric, two defmethod bodies, and the argument's class picks which body runs.

;; One generic function, two methods, dispatch chosen by argument class.

(defclass document () ((title :initarg :title :accessor doc-title)))
(defclass invoice (document) ((total :initarg :total :accessor total-of)))
(defclass memo (document) ((body :initarg :body :accessor body-of)))

(defgeneric summarize (item)
  (:documentation "Return a one-line summary string for ITEM."))

(defmethod summarize ((item invoice))
  (format nil "~a: ~$ due" (doc-title item) (total-of item)))

(defmethod summarize ((item memo))
  (format nil "~a: ~d characters" (doc-title item) (length (body-of item))))

(dolist (item (list (make-instance 'invoice :title "Q3" :total 240.5)
                    (make-instance 'memo :title "Standup" :body "ship it")))
  (format t "~a~%" (summarize item)))

How it works

1. invoice and memo both inherit title from the document base class.
2. defgeneric summarize declares the contract; each method specializes on (item invoice) or (item memo).
3. The closing dolist calls summarize on one of each and gets different output per class.

Keywords and builtins used here

• defclass
• defgeneric
• defmethod
• dolist
• format
• length
• list

The run, in numbers

Lines

18

Characters to type

753

Tokens

142

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 122 seconds.

Type this snippet

Step 3 of 3 in Structures & CLOS, step 21 of 27 in Language basics.

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