On Wed, Oct 21, 2009 at 2:49 PM, Shel <skap...@gmail.com> wrote:
>
> Hi, I'm having a bit of trouble with the supposed use of isa? as a
> part of multimethod dispatch.
> Consider the following example:
>
> (defmulti area (fn [s m] [(:shape s) (:measure m)]))
> (defmethod area[:rectangle :side] [s m] (* (:side m) (:side m)))
> (defmethod area[:rectangle :sides] [s m] (* (:side1 m) (:side2 m)))
> (derive ::square ::rectangle)
>
> user=> (isa? ::square ::rectangle)
> true
> user=> (area {:shape :square} {:measure :side :side 2})
> java.lang.IllegalArgumentException: No method in multimethod 'area'
> for dispatch value: [:square :side] (NO_SOURCE_FILE:0)
:square is not the same as ::square
user=> (isa? :square ::square)
false
user=> (isa? :square :rectangle)
false
Try this instead. Note the consistent use of :: keywords for
values that participate in the 'isa?' test:
(defmulti area (fn [s m] [(:shape s) (:measure m)]))
(defmethod area [::rectangle ::side] [s m] (* (:side m) (:side m)))
(defmethod area [::rectangle ::sides] [s m] (* (:side1 m) (:side2 m)))
(derive ::square ::rectangle)
user=> (area {:shape ::square} {:measure ::side :side 2})
4
--Chouser
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