Except that doesn't work, since the var is de-reffed when handed to the
defmulti. You can var quote it, and that should work:
(defmulti create-fact #'create-fact-dispatch)
Timothy
On Mon, Feb 23, 2015 at 8:44 AM, Francis Avila <fav...@breezeehr.com> wrote:
> 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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