common-lispclos

defgeneric with optional and keyword arguments

I want to define a generic function in CL that takes an optional and a keyword argument both of which have a default value. I tried

(defgeneric read-one (buffer &optional (sz 1) &key (signed '()))

but this throws Invalid &OPTIONAL argument specifier #1=(SZ 1)

So what is the proper way to do this sort of thing?

Solution

  • afaik you can't provide defaults in defgeneric. You would have to do this in the concrete implementation (defmethod)

    (defgeneric read-one (buffer &optional sz &key signed))

(defmethod read-one (buffer &optional (sz 1) &key (signed '()))
  (format t "~a, ~a, ~a~%" buffer sz signed))

CL-USER> (read-one (list 1 2 3) )
;; (1 2 3), 1, NIL
;; NIL

;; CL-USER> (read-one (list 1 2 3) 101 :signed t)
;; (1 2 3), 101, T
;; NIL