Hi all,
probably I don't see the forest for trees, but anyhow. In my graph
transformation lib I have these functions for applying rules:
--8<---------------cut here---------------start------------->8---
(defn iteratively
"Applies the function f with args as long as it returns logical true.
Returns the number of successful applications or nil if it couldn't be
applied at least once."
[r & args]
(loop [val (apply r args), i 0]
(if val
(recur (apply r args) (inc i))
(if (zero? i) nil i))))
(defn choose
"Chooses non-deterministically one of the given funs and applies it.
Returns the value of applying the chosen fun.
Returns nil if none of the funs was applicable."
[& funs]
(when (seq funs)
(let [fun (rand-nth funs)
val (fun)]
(or val (recur (remove #(= fun %) funs))))))
(defn fold-constants
[g]
(let [start-block (the (vseq g 'StartBlock))]
(iteratively pull-up-consts g)
(iteratively replace-not g start-block)
(iteratively replace-binary g start-block)))
(defn fold-constants-with-choose
[g]
(let [start-block (the (vseq g 'StartBlock))]
(iteratively (fn []
(choose #(pull-up-consts g)
#(replace-not g start-block)
#(replace-binary g start-block))))))
;; Takes 3.5 secs
(time (fold-constants g))
;; Takes 450 times longer!
(time (fold-constants-with-choose g))
--8<---------------cut here---------------end--------------->8---
Why does the variant with `choose' take 450 times longer than without?
The individual rules match totally disjunctive structures and there's a
fixed number of places where a rule can be applied. So even if choose
always selects the single applicable rule at last I wouldn't expect a
slowdown of more than 3.
Any hints?
Bye,
Tassilo
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