Re: method-let

nallen05@xxxxxxxxx writes:

Has anyone ever seen something implimented like a METHOD-LET operator,
that defines a method for a generic function but only within the scope
of it's form, like FLET for methods?

(defclass wheat ())

(defclass bread (wheat) ())

(defclass toast (bread) ())

(defgeneric eat (x))

(defmethod eat ((x wheat))
(print "yuck"))

(defmethod eat ((x toast))
(print "yum"))

(method-let ((foo ((x bread))
(print "will you put this in the toaster?")))
(mapc #'foo (list (make-instance 'wheat)
(make-instance 'bread)
(make-instance 'toast))))
"will you put this in the toaster?"

You can do it with a normal flet:

C/USER[615]> (flet ((eat (x)
(if (eq (class-of x) (find-class 'bread))
(print "will you put this in the toaster?")
(eat x))))
(mapc (function eat) (list (make-instance 'wheat)
(make-instance 'bread)
(make-instance 'toast)))

"will you put this in the toaster?"

I think this would be super usefull for say... making PRINT-OBJECT
behave differently in different situations (like printing to XML or in
a GUI) but not losing the semactics of the type-hierarchy and the
method dispatch...

This is not a good idea. PRINT-OBJECT, being a standard method has
some restrictions that may become unfortunate. For example, it's not
necessarily called when specialized on a standard type or standard

Note that:

"The function print-object is called by the Lisp printer; it
should not be called by the user."


If you want to format differently depending on the "device", you
should use your own generic function:

(defgeneric render (object device))

(defmethod render ((object integer) (device xml-device))
(format (device-stream device) "<integer>~D</integer>" object))

(defmethod render ((object date) (device text-device))
(format (device-stream device) "~4,'0D-~2,'0D-~2,'0D"
(year date) (month date) (day date)))


__Pascal Bourguignon__

Relevant Pages

  • Re: method-let
    ... (defclass bread (wheat) ... generic function to be "sealed" and that no more methods can be defined ...
  • Re: Subways Meatball Marinara
    ... It's a WELL-KNOWN fact that Subway bakes there own bread FRESH right there ... 9-GRAIN WHEAT Enriched wheat flour (wheat flour, barley malt, niacin, ... flavor (maltodextrin, natural flavor, silicon dioxide, lactic acid). ...
  • Re: Sausage and peppers
    ... Daughter can't have wheat either. ... We use pasta made of rice, ... Another issue is the boiling of the pasta and the draining. ... Ener-G brand gluten free bread is more flavorless than sawdust, ...
  • Re: whither the price of bread?
    ... Even bought a much better bread making machine, and even with that, I have been successful at times and not successful at times. ... I made two loaves of part whole wheat and part white bread ... (limits were 90 cents for spring wheat, ... means that the March futures contract will no longer have a limit. ...
  • Re: Freezing bread
    ... I guess freezing won't change the ... Or should I just eat wheat ... Santa Rosa wheat Tortilla ... Fresh French bread which I eventually put in the fridge ...