Pretty sure what you’re looking for is either https://clojuredocs.org/clojure.core/reductions or something close. This idea is called a “scanLeft” in some functional languages, so that should give you another search term to use. Good luck!
On Sun, Nov 27, 2022, at 12:00 PM, Jules wrote: > Guys, > > I've found myself needing a function that I am sure cannot be an original but > I'm not aware of it existing anywhere... > > It is a cross between 'map', 'reduce' and 'iterate'... > > Given a function 'f' and a sequence 's' it would return you a sequence of : > ``` > [(f s[0]) (f (f s[0]) s[1]) (f (f (f s[0]) s[1]) s[2]) ...] > ``` > > or, more concretely, e.g.: > ``` > util-test> (stateful-map + [0 1 2 3 4 5]) > [0 1 3 6 10 15] > util-test> > ``` > > I have a couple of approaches to it - one using reduce: > > ``` > (defn stateful-map-1 [f [h & t]] > (reduce > (fn [acc v] > (conj acc (f (last acc) v))) > [h] > t)) > ``` > > and another, mapping using a stateful function: > > ``` > (let [secret (Object.)] > (defn stateful-mapper [f] > (let [state (volatile! secret)] (fn [v] (vswap! state (fn [old new] (if > (= secret old) (f new) (f old new))) v))))) > > (defn stateful-map-2 [f s] > (mapv (stateful-mapper f) s)) > ``` > > The former feels more idiomatic whereas the latter (although uglier) is more > efficient and has the added benefit of being able to be used for general > map-ing which is important as I want to use this approach to transduce a > clojure.async.channel. > > It could, of course, be expressed directly as a transducer but it feels like > something simpler that should only be lifted to a transducer as and when > needed (see test below) ... > > Here is my working testsuite: > > ``` > (deftest stateful-map-test > (testing "reduction" > (is > (= > [0 1 3 6 10 15] > (stateful-map-1 + [0 1 2 3 4 5])))) > (testing "mapping stateful function" > (is > (= > [0 1 3 6 10 15] > (stateful-map-2 + [0 1 2 3 4 5])))) > (testing "transduction" > (is > (= > [0 1 3 6 10 15] > (sequence (map (stateful-mapper +)) [0 1 2 3 4 5]))))) > ``` > > Am I missing a standard way of doing this in Clojure ? Or is a stateful > function the best answer ? > > Interested in your thoughts, > > > Jules > > > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with your > first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/clojure/f1ada73f-8249-431a-9f4d-580aea12bdefn%40googlegroups.com > > <https://groups.google.com/d/msgid/clojure/f1ada73f-8249-431a-9f4d-580aea12bdefn%40googlegroups.com?utm_medium=email&utm_source=footer>. -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/clojure/798d8cc2-b3b0-4036-b3a2-5cbf43751c98%40app.fastmail.com.
