How do I iterate until a fixed point in Clojure?

Question

I'm frequently in the position that my code reads like so:

(iterate improve x)

And I'm looking for the first value that no longer is an improvement over the previous. Neither filter nor take-while lend themselves to an obvious solution. However, I'm hesitant to write out:

(loop [current x
       next (improve x)]
  (if (= current next)
    current
    (recur next (improve next))))

or:

(let [improvements (iterate improve x)]
  (->> (map vector improvements (rest improvements))
    (filter (partial apply =))
    (ffirst)))

Because at some point this is becoming repetitive and surely fixed point iteration is such a basic task that there must be some kind of library support somewhere, right?

ez121sl · Accepted Answer · 2017-06-10 23:02:15Z

12

You can use reduce and reduced to stop when necessary. reduced wraps the argument in a special object, which reduce is designed to look for and stop processing immediately returning the wrapped value.

(def vals (iterate improve x))

(reduce #(if (= %1 %2) (reduced %1) %2) vals)

edited Jun 10, 2017 at 23:02

answered Jun 10, 2017 at 22:56

ez121sl

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Chris Murphy · Accepted Answer · 2017-06-10 22:56:41Z

2

You could use drop-while then first:

(defn still-improving? [[x y]]
  ...)

(->> st
     (iterate transition)
     (partition 2 1)
     (drop-while still-improving?)
     ffirst)

edited Jun 10, 2017 at 22:56

answered Jun 10, 2017 at 22:36

Chris Murphy

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