The defrunonce macro below works, creating a function that runs only
once and tracking its run status in metadata.
Now, how do I write it without using eval?
(defn runonce
"Create a function that will only run once, given a function. Returns
a vector containing the function and the reference that tracks
whether
the function has been run."
[function]
(let [has-run (ref false)]
[(fn [& args]
(or @has-run
; TODO: think through semantics for parallel target invocation
(do
(apply function args)
(dosync (ref-set has-run true)))))
has-run]))
(defmacro defrunonce [sym doc & forms]
"Defines a function with runonce semantics. Curren run status
is kept in a reference under the :has-run metadata key."
(let [[function has-run] (runonce (eval (concat (list 'fn [])
forms)))]
`(def ~(with-meta sym {:has-run has-run}) ~function)))
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