readmacro magic

gistfile1.nl

;; Playing with reader macros over the weekend. 
;; Example of reader and writer being mirroring each other beautifully

;;So we need a class for IP addresses
CL-USER> (defclass ip () ((address :initarg :address) (port :initarg :port)))
#<STANDARD-CLASS IP>


;; def-hashslash-reader is a macro I wrote to create reader macros. 
;; In short this code:

CL-USER> (def-hashslash-reader (ip :space)
           (destructuring-bind (adr &optional port) (split-sequence #\: this)             
             (make-instance 'ip :address adr :port port)))

;; will take this pattern:

#/ip/somestuff:someotherstuff

;;and rewrites it as

(make-instance 'ip :address somestuff :port someotherstuff)

;;So now the reader writes the code to make new instances of ip

CL-USER> #/ip/192.168.0.1:4005
#<IP 0x3445>                      ;;<- this is an object


;; Only bummer is the writer doesn't know about this so it doesnt feel 
;; like a language primitive.
;; So we specialize print-method for the 'ip' class

CL-USER> (defmethod print-object ((object ip) stream)
           (with-slots (address port) object
             (format stream "#/ip/~a~@[:~a~]" address port))) 

;; and then..

CL-USER> #/ip/192.168.0.1:4005
#/ip/192.168.0.1:4005

;; Bingo :)