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.
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