You can work around this by using a symbol for the dispatch function
instead of inlining the function:
(defn create-fact-dispatch [item-vector]
(do
(print item-vector)
(first item-vector)))
(defmulti create-fact create-fact-dispatch)
When you reload in the REPL the defmulti will not be redefed, but the
dispatch function will. This is usually all you need when you are messing
around in the REPL.
There may be some performance cost, but I don't know if it is significant.
There is at least the var lookup cost. Maybe defmulti dispatch result can't
be cached? (not sure)
On Sunday, February 22, 2015 at 12:19:24 PM UTC-6, Timur wrote:
>
> Thank you all for your answers. The problem was caused by not starting the
> REPL. I did not know that defmulti had defonce semantics.
>
> On Sunday, February 22, 2015 at 7:04:58 PM UTC+1, Jeremy Heiler wrote:
>>
>> 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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