On 2/22/15 12:52 PM, Timur wrote:
Hi everyone,
I have the following question regarding the defmultis of clojure:
(defmulti create-fact
(fn [item-vector] (do
(print item-vector)
(first item-vector))))
(defmethod create-fact [:a] [item-vector]
(str "a"))
(defmethod create-fact [[:a "safs"]] [item-vector]
(str "safs"))
(mapv create-fact {:a "safs"})
Dispatch function is not called in this case and return is "safs" so the
matching key is [[:a "safs"]]. I except it to be :a, why is that [[:a
"safs"]]?
First, observe:
> (seq {:a 1 :b 2})
([:a 1] [:b 2])
A map is converted into a sequence with each element being a key/value
pair. The mapv function does this under the hood so that it can operate
on the map as a sequence.
Now, when I run your code, I get an IllegalArgumentException stating
that :a is not a dispatch value. This is correct, because the two
dispatch values defined are [:a] and [[:a "safs"]]. If you change the
[:a] method to be :a, then the return value will be "a". The dispatch
values in each defmethod must be literal, and not wrapped in a vector
like when defining the arguments to a fn.
Does that clear things up?
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