On 12/12/12 00:16, Mark Engelberg wrote:
I would much prefer to have some sort of core function that serves as
a polymorphic constructor. This way users don't need to require the
concrete implementations individually.
I faced the same sort of issue a couple of months ago. Just like you, I
wanted end-users to be able to instantiate their own records via a
polymorphic function in the core ns. In other words, I wanted a way of
instantiating records by passing only the record name (fully qualified
of course) and whatever parameters the constructor expects. If i
understand correctly this is what you're after yes?
There is a simple way of doing this but it is very very slow. Basically
you need to resort to reflection...Chris Houser had posted such a
record-factory fn on StackOverflow a while back. Here it is:
(defn record-factory [recordname]
(let [recordclass ^Class (resolve (symbol recordname))
max-arg-count (apply max (map #(count (.getParameterTypes
^java.lang.reflect.Constructor %))
(.getConstructors recordclass)))
args (map #(symbol (str "x" %)) (range (- max-arg-count 2)))]
(eval `(fn [~@args] (new ~(symbol recordname) ~@args)))))
even though i was very excited at the time i saw this, I later decided
that it's not worth the performance cost for my use-case.
Jim
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