Defining a generic function and implementing specialised methods

Question

Emacs provides support for polymorphism, supporting "generic functions", like in CLOS.
According to Emacs manual:

A generic function specifies an abstract operation, by defining its name and list of arguments, but (usually) no implementation. The actual implementation for several specific classes of arguments is provided by "methods", which should be defined separately [...] by “specializing” the arguments defined by the generic function.

Unfortunately the manual does not give practical examples of a methods implementation once a generic function is defined. Can you give such an example, implementing the standard OO paradigms through cl-defgeneric/cl-defmethod?

Answer 1

An attempt...

(require 'cl-generic)

;; Checking accounts can have a negative balance
;; Savings accounts can't
(cl-defstruct checking-account balance)
(cl-defstruct savings-account balance)

;; A generic `withdraw' method
(cl-defgeneric withdraw (account amount)    
  (:documentation "Withdraw AMOUNT from ACCOUNT balance.
Give an error for savings accounts where balance < AMOUNT"))

;; Implementation of `withdraw' for checking accounts
(cl-defmethod withdraw ((account checking-account) amount)
  (cl-decf (checking-account-balance account) amount))

;; Implementation of `withdraw' for savings accounts
(cl-defmethod withdraw ((account savings-account) amount)
  (when (< (savings-account-balance account) amount)
    (error "Account overdrawn."))
  (cl-decf (savings-account-balance account) amount))

;; Account instantiations, i.e. creation of account objects  
(setq x (make-checking-account :balance 50))
(setq y (make-savings-account :balance 50))

;; Withdraw amounts from accounts 
(withdraw x 60) ; => - 10
(withdraw y 60) ; => (error "Account overdrawn.")

See gigamonkeys.com for Common Lisp.