Hi all - I've recently encouraged a friend to start learning Clojure,
and he has written some basic Markov chaining code as a learning
exercise. His code works fine with small sets of input data, but
larger inputs have been causing a StackOverflowError.
I've taken a look at the code and suspect that the error may be caused
by recurrent calls to filter occupying increasing amounts of stack
space due to filter leaving closed-over locals hanging around. That
being said, this is only a suspicion, I'm still learning the language
myself, so I could be totally wrong. Wrapping the filter calls in
doall seems to prevent the problem from happening, but performance is
abysmal in that case.
I'm using Clojure 1.1.0-alpha-SNAPSHOT for testing.
(defn generate-chain [source]
(loop [the-list (map #(list (first (split-at 2 %)) (last %))
(partition 3 1 (.split (.replace source "\n" "
") " ")))
res (hash-map)]
(if (empty? the-list)
res
(recur (filter #(not= (first (first the-list))
(first %)) the-list)
(assoc res (first (first the-list))
(map #(last %)
(filter #(= (first (first the-list))
(first %)) the-list)))))))
(println (generate-chain (slurp "big-file.txt")))
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