Oh fantastic! I was 100% wrong in literally the best way.
Thanks! On Tue, Jun 23, 2015 at 7:17 PM, Ghadi Shayban <gshay...@gmail.com> wrote: > Good question. > > Clojure's evaluation semantics dictate that the arguments are evaluated > (computed) *before* calling the function. So(set coll) is computed before > being passed to `partial`. Partial receives a function (a value) and > arguments (also values) and returns back a new function that saves those > original arguments (which happen to be stuffed away in Java final fields). > > All three of your filters return a transducer. All three of the inputs to > `remove` are a partial function. Each of the arguments to the call to > partial is a set. They are all essentially equivalent, and should perform > the same. (Except filter3 happens to create the set twice; once in the > let, and once as the arg to partial). So over a billion item collection, > the set in your examples will only be computed once, once, and twice > respectively. > > Note however that sets *are* functions that evaluate whether the argument > is in the set. This means you could remove the call to partial and shorten > to: > > (defn filter-contains1 [edn-file] > (remove (set (read-edn-file edn-file)))) > > > Tangentially: > (remove even?) > Will be faster than > (remove (fn [i] (even? i))) > because in the first case the dereference of the var 'even?' happens only > once and the value inside the var will be passed to `remove` at the > outset. In the second example the var dereference happens for every single > item (though it's very cheap). The second example is equivalent to writing > (remove > #'even?) > > On Tuesday, June 23, 2015 at 6:07:06 PM UTC-4, Sam Raker wrote: >> >> Let's say that, as part of an xf, I want to filter out everything in a >> sequence that's also in some other sequence. Here are some ways of doing >> that: >> >> (defn filter-contains1 [edn-file] (remove (partial contains? (set (read- >> edn-file edn-file))))) >> >> (defn filter-contains2 [coll] (remove (partial contains? (set coll)))) >> >> (def filter-contains3 [coll] (let [coll-as-set (set coll)] (remove ( >> partial contains? (set coll))))) >> >> I have the strong suspicion that `filter-contains3` is the best of the 3, >> and `filter-contains1` the worst. The internal mechanics of transduce are a >> bit of a mystery to me, however: if `filter-contains2` were to be used on a >> collection of, say, a million items, would `coll` be cast to a set a >> million times, or is Clojure/the JVM smarter than that? I'm also wondering >> if anyone has any "best practices" (or whatever) they can share relating to >> this kind of intersection of transducers/xfs and closures. It seems to me, >> for example, that something like >> >> (defn my-thing [coll & stuff] >> (let [s (set coll)] >> ... >> (comp >> ... >> (map foo) >> (filter bar) >> (remove (partial contains? s)) >> ... >> >> is awkward, but that a lot of limited-use transducer factory functions >> (like the ones above) aren't exactly optimal, either. >> > -- > 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 a topic in the > Google Groups "Clojure" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/clojure/H0zoF6jlY-c/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- . -- 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. For more options, visit https://groups.google.com/d/optout.
