On Aug 31, 8:48 pm, Parth Malwankar <[EMAIL PROTECTED]> wrote:
> Hello,
>
> I have a situation in which different multi-methods
> need similar behavior with different arguments.
>
> So they call the same function (one-or-two below)
> with the arguments. E.g.
>
> (defmulti what-num (fn [x] x))
>
> (defn one-or-two [x] (println "saw one or two: " x))
>
> (defmethod what-num :one [x] (one-or-two x))
> (defmethod what-num :two [x] (one-or-two x))
> (defmethod what-num :three [x] (println "saw 3"))
> (defmethod what-num :four [x] (println "saw 4"))
>
> user=> (what-num :one)
> saw one or two: :one
> nil
> user=> (what-num :two)
> saw one or two: :two
> nil
> user=> (what-num :three)
> saw 3
>
> This works fine. However, I was wondering if its at all
> possible to return duplication of creating the
> methods 'what-num :one [x]' and 'what-num :two [x]'.
>
> Can we have something to the effect of
> 'what-num (or :one :two) [x]'? This would help get
> rid of one-or-two and I would be able to define a single
> what-num for this.
>
> I could have 'defmulti what-num' conditionally return
> something like :one-or-two but I am not sure if thats such
> a good idea as I might end up with a big 'cond' over
> time.
>
> What would be a good way to do something like this?
I don't know if it is a "good" way, but here is what I used for a
similar problem:
(defmacro defmethod-for-values
[multifn dispatch-values-seq & fn-tail]
`(let [pvar# (var ~multifn)
method# (fn [EMAIL PROTECTED])]
(doseq dispatch-val# ~dispatch-values-seq
(. pvar# (commuteRoot (fn [#^clojure.lang.MultiFn mf#]
(. mf#
(assoc dispatch-val# method#))))))))
Which is just a wrapper around the defmethod macro that allows you to
specify a list of values instead of a single value that will be mapped
to the method.
This would allow you to define your above example with:
(defmulti what-num (fn [x] x))
(defmethod-for-values what-num '(:one :two) [x] (println "saw one or
two: " x))
(defmethod what-num :three [x] (println "saw 3"))
(defmethod what-num :four [x] (println "saw 4"))
See this thread for more details:
http://groups.google.com/group/clojure/browse_thread/thread/eed9015c2456e336/8822ecd0d7f54b8b
Hope this helps,
--Darren
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