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)
But :square "isa" :rectangle. What am I doing wrong?
Do I have to define a method for each node in the "derive" hierarchy,
like this?
user=> (defmethod area [:square :side] [s m] (* (:side m) (:side m)))
#<MultiFn clojure.lang.mult...@4dbb9a58>
user=> (area {:shape :square} {:measure :side :side 2})
4
If this is what I have to do, I'm bummed. I hope I'm doing something
wrong... can you tell me what it is?
(I'm not looking for a critique of the :measure thing - that doesn't
really play out in this example but
it is useful in other cases not shown here. Let's stick to the isa?
dispatch question).
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